Precise ages of largest number of stars hosting planets ever measured

Credit: IAC

New study lead by Aarhus astronomer Víctor Silva Aguirre to be published in MNRAS: 33 Kepler stars have been selected for their solar like oscillations and a set of basic parameters have been determined with high precision showing that even stars older than 11 billion years have Earth-like planets.

A new study of 33 Kepler stars with solar-like oscillations to be published in Monthly Notices of the Royal Astronomical Society. According to lead author of the article Víctor Silva Aguirre from the Stellar Astrophysics Centre at Aarhus University, Denmark: " Our team has determined ages for individual host stars before with similar levels of accuracy, but this constitutes the best characterised set of exoplanet host stars currently available." 

Measuring the ages of stars is one of the very tough problems that contemporary astronomers are faced with. Up to now only the age of the Sun has been determined with high precision (it is 4.57 billion years, with a precision of 10 million years to each side). The international group of astronomers have determined ages, diameters, densities, masses and distances for 33 stars better than ever before. As an extra, all of these stars have earth-like planets, giving us a clear indication that such planets have formed in our Milky Way Galaxy long before the Earth and are still being formed out there. 

The 33 stars have been carefully selected from the more than 1 200 stars with planets around them that have been observed with the highly successful Kepler satellite. The stars have to be sufficiently bright to give a good statistical basis for the results, and they have to show some of the same characteristics similar to the Sun to make them comparable. 

Stars pulsate, vibrate and resonate just like sound waves in a musical instrument. The advanced technique of measuring these starry tunes is called asteroseismology - a method quite similar to the one used by geologists to sound out the composition of the interior of the Earth by means of earthquakes. 

The NASA-launched Kepler satellite has constantly measured tiny variations in the light from some 145 000 stars over a period of a little more than four years. Analyzing these variations over time gives the periods of the many simultaneous pulsations in each star, and from that the scientists can derive the important basic properties of the individual stars. 

Why is it important

 

Knowing the ages, sizes and other basic parameters of the stars, apart from being interesting in itself, is important if one wants to study the large scale development of our galaxy and the Universe as a whole - a relatively new discipline named "galactic archeology". We all wish to know where we came from! On a more practical level the stars function in largely the same way as a fusion reactor. Precise knowledge of the internal machines in stars might help in future energy production here on Earth. 

It is not the first time that precise ages of individual stars have been determined. But using a large sample and studying them with the same instrument - the Kepler satellite - and the same theoretical and statistical methods gives us a much higher confidence in the precision of the results. Comparing the stars may also reveal unusual and so far unknown stellar properties. 

With a large, and hopefully growing, set of well-studied stars it will be possible to expand our knowledge even to stars which are too faint to obtain asteroseismological values for. The precise knowledge especially of stellar ages can be related to the properties of the light; the spectra, from the same stars. This gives us a set of well-known calibration stars and thus it enables us to work backwards from spectroscopy of faint stars to their ages. 

How is it done

 

The 33 stars selected for the study are not all similar to the Sun, but they behave in much the same way as the Sun does. They are what technically is called "solar-like oscillators". Víctor Silva Aguirre explains: "The term solar-like oscillators means that the stars exhibit pulsations excited by the same mechanism as in the Sun: gas bubbles moving up and down. These bubbles produce sound waves that travel across the interior of stars, bouncing back and forth between the deep interior and the surface producing tiny variations in the stellar brightness." 

How precise?

 

The new study gives us values for the selected stars with uprecedented precision. On the average stellar properties are better than the percentages below. If a star e.g. has a calculated age of 5 billion years, the 14% means that it's  true age lies between 4.3 and 5.7 billion years:

1.2% (radius),

1.7% (density),

3.3% (mass),

4.4% (distance),

and 14% (age). 

Are they representative?

 

All the stars studied by the Kepler satellite lie in a small area of the sky, close to the constellation of Cygnus. The 33 stars in this study span distances between 100 and 1600 light-years from the Sun. With such a small area of the Milky Way Galaxy studied over such relatively short time, one could wonder if the stars selected for the study are at all representative for the more than 300 billion stars in our galaxy. The answer is a qualified "yes". Certainly the astronomers would like to study many more stars for much longer time, but for the time being and compared to what was previously known this is a large first step. In the future we will be able to study larger samples of stars, selected from a larger area of sky with the current Kepler2 project and from 2017 on hopefully from all over the sky with the TESS-satellite. Even better results are expected from the PLATO-satellite due to be launched by the European Space Agency in the mid-2020'ies. 

What about those planets

 

The Kepler satellite is able to provide two very different types of results with the same sort of measurements. From the small variations in the intensity of starlight, one can both deduce asteroseismic values of the stars and also discover any exoplanets circling the stars. Determining the exact properties of these exoplanets is only possible if we also know the basics of the host stars, and these come from asteroseismology. The two fields of astronomy are closely connected. Assistant Professor Silva Aguirre sums up: 

"One of the biggest questions in astrophysics is: does life exists beyond earth? To even begin answering this, we need to know how many planets like ours exist out there, and when they formed. However determining ages of stars (and thus of their orbiting planets) is extremely difficult; precise ages are only available for a handful of host stars thanks to asteroseismic observations made with the Kepler satellite. 

Our study provides the first sample of homogeneously determined ages for tens of exoplanet host stars with a high level of precision. The stars we studied harbour exoplanets of size comparable to earth (between 0.3 and 15 earth radii), and our results reveal a wide range of ages for these host stars, both younger (down to half the solar age) and older (up to 2.5 times the solar age) than the Sun. This is regardless of the size of the exoplanets in the system or multiplicity, showing that formation of exoplanets similar in size to earth has occurred throughout the history of our Galaxy (and is still taking place!). Actually some of these planets were of the same age as the Earth is now, at the time when the Earth itself formed. This in itself is a remarkable finding." 

The title of the study is: "Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology". The full article can be found in arXiv here. 

Contact:

 

Víctor Silva Aguirre

victor@phys.au.dk

0045 87155635

Public/media, Staff, Students

 Source: Aarhus University/Stellar AstrophysicscCentre

Unexpectedly Little Black-hole Monsters Rapidly Suck up Surrounding Matter

Using the Subaru Telescope, researchers at the Special Astrophysical Observatory in Russia and Kyoto University in Japan have found evidence that enigmatic objects in nearby galaxies – called ultra-luminous X-ray sources (ULXs) – exhibit strong outflows that are created as matter falls onto their black holes at unexpectedly high rates. The strong outflows suggest that the black holes in these ULXs must be much smaller than expected. Curiously, these objects appear to be "cousins" of SS 433, one of the most exotic objects in our own Milky Way Galaxy. The team's observations help shed light on the nature of ULXs, and impact our understanding of how supermassive black holes in galactic centers are formed and how matter rapidly falls onto those black holes (Figure 1).

Figure 1: Multi-color optical image around the ULX "X-1" (indicated by the arrow) in the dwarf galaxy Holmberg II, located in the direction of the constellation Ursa Major, at a distance of 11 million light-years. The image size corresponds to 1,100 × 900 light-years at the galaxy. The red color represents spectral line emission from hydrogen atoms. (Credit: Special Astrophysical Observatory/Hubble Space Telescope)


X-ray observations of nearby galaxies have revealed these exceptionally luminous sources at off-nuclear positions that radiate about million times higher power than the Sun. The origins of ULXs have been a subject of heated debate for a long time. The basic idea is that a ULX is a close binary system consisting of a black hole and a star. As matter from the star falls onto the black hole, an accretion disk forms around the black hole. As the gravitational energy of the material is released, the innermost part of the disk is heated up to a temperature higher than 10 million degrees, which causes it to emit strong X-rays.

The unsolved key question about these objects asks: what is the mass of the black hole in these bright objects? ULXs are typically more than a hundred times more luminous than known black hole binaries in the Milky Way, whose black hole masses are at most 20 times the mass of the Sun.

There are two different black hole scenarios proposed to explain these objects: (1) they contain very "big" black holes that could be more than a thousand times more massive than the Sun (Note 1), or (2) they are relatively small black holes, "little monsters" with masses no more than a hundred times that of the Sun, that shine at luminosities exceeding theoretical limits for standard accretion (called "supercritical (or super-Eddington) accretion," Note 2). Such supercritical accretion is expected to produce powerful outflow in a form of a dense disk wind.

To understand which scenario explains the observed ULXs researchers observed four objects: Holmberg II X-1, Holmberg IX X-1, NGC 4559 X-7, NGC 5204 X-1, and took high-quality spectra with the FOCAS instrument on Subaru Telescope for four nights. Figure 1 shows an optical multi-color image toward Holmberg II X-1 as observed with Hubble Space Telescope. The object X-1, indicated by the arrow, is surrounded by a nebula (colored in red), which is most likely the gas heated by strong radiation from the ULX.

The team discovered a prominent feature in the optical spectra of all the ULXs observed (Figure 2). It is a broad emission line from helium ions, which indicates the presence of gas heated to temperatures of several tens of thousands of degrees in the system. In addition, they found that the width of the hydrogen line, which is emitted from cooler gas (with a temperature of about 10,000 K), is broader than the helium line. The width of a spectral line reflects velocity dispersion of the gas and shows up due to the Doppler effect caused by a distribution of the velocities of gas molecules. These findings suggest that the gas must be accelerated outward as a wind from either the disk or the companion star and that it is cooling down as it escapes.

Figure 2: Optical spectra of the four ULXs observed with the Subaru Telescope (from upper to lower, Holmberg II X-1, Holmberg IX X-1, NGC 4559 X-7, NGC 5204 X-1). He II and Hα denote the spectral lines from helium ions and from hydrogen atoms, respectively. (Credit: Kyoto University)

Distant ULXs and a Similar Mysterious Object in the Milky Way

The activity of these ULXs in distant galaxies is very similar to a mysterious object in our own Milky Way. The team noticed that the same line features are also observed at SS 433, a close binary consisting of an A-type star and most probably a black hole with a mass less than 10 times that of the Sun. SS 433 is famous for its persistent jets with a velocity of 0.26 times the speed of light. It is the only confirmed system that shows supercritical accretion (that is, an excessive amount of accretion that results in a very powerful outflow). By contrast, such features have not been observed from "normal" black hole X-ray binaries in the Milky Way where sub-critical accretion takes place.

After carefully examining several possibilities, the team concluded that huge amounts of gas are rapidly falling onto "little monster" black holes in each of these ULXs, which produces a dense disk wind flowing away from the supercritical accretion disk. They suggest that "bona-fide" ULXs with luminosities of about million times that of the Sun must belong to a homogeneous class of objects, and SS 433 is an extreme case of the same population. In these, even though the black hole is small, very luminous X-ray radiation is emitted as the surrounding gas falls onto the disk at a huge rate.

Figure 3 is a schematic view of the ULXs (upper side) and SS 433 (lower side). If the system is observed from a vertical direction, it's clear that the central part of the accretion disk emits intense X-rays. If SS 433 were observed in the same direction, it would be recognized as the brightest X-ray source in the Milky Way. In reality, since we are looking at SS 433 almost along the disk plane, our line-of-sight view towards the inner disk is blocked by the outer disk. The accretion rate is inferred to be much larger in SS 433 than in the ULXs, which could explain the presence of persistent jets in SS 433.

Figure 3: Schematic view of ULXs (looking from upper side) and SS 433 (looking from left side). Strong X-rays are emitted from the inner region of the supercritical accretion disk. Powerful winds are launched from the disk, which eventually emit spectral lines of helium ions and hydrogen atoms. (Credit: Kyoto University)


Such "supercritical accretion" is thought to be a possible mechanism in the formation of supermassive black holes at galactic centers in very short time periods (which are observed very early in cosmic time). The discovery of these phenomena in the nearby universe has significant impacts on our understanding of how supermassive black holes are formed and how matter rapidly falls onto them.

There are still some remaining questions: What are the typical mass ranges of the black holes in ULXs? In what conditions can steady baryonic jets as observed in SS 433 be produced? Dr. Yoshihiro Ueda, a core member of the team, expresses his enthusiasm for future research in this area. "We would like to tackle these unresolved problems by using the new X-ray observations by ASTRO-H, planned to be launched early next year, and by more sensitive future X-ray satellites, together with multi-wavelength observations of ULXs and SS 433," he said.

This work has been published online in Nature Physics on 2015 June 1 (Fabrika et al. 2015, "Supercritical Accretion Discs in Ultraluminous X-ray Sources and SS 433", 10.1038/nphys3348). The research was supported by the Japan Society for the Promotion of Science's KAKENHI Grant number 26400228. 

Authors:

• Sergei Fabrika (Special Astrophysical Observatory, Russia; Kazan Federal University, Russia)
• Yoshihiro Ueda (Department of Astronomy, Kyoto University, Japan)
• Alexander Vinokurov (Special Astrophysical Observatory, Russia)
• Olga Sholukhova (Special Astrophysical Observatory, Russia)
• Megumi Shidatsu (Department of Astronomy, Kyoto University, Japan)

Notes:

1. Generally, black holes with masses between about 100 and about 100,000 times that of the Sun are called "intermediate-mass black holes," although there is no strict definition for the mass range.
2. In a spherically symmetric case, matter cannot fall onto a central object when the radiation pressure exceeds the gravity. This luminosity is called the Eddington limit, which is proportional to the mass of the central object. When matter is accreted at rates higher than that corresponding to the Eddington limit, it is called "supercritical (or super-Eddington) accretion." In the case of non-spherical geometry, such as disk accretion, supercritical accretion may happen.

Source: Subaru Telescope

Discovering a New Stage in the Galactic Lifecycle

Using ALMA, astronomers surveyed an array of normal galaxies seen when the Universe was only 1 billion years old. They detected the glow of ionized carbon filling the space between the stars, indicating these galaxies were fully formed but chemically immature, when compared to similar galaxies a few billion years later. The ALMA data for four of these galaxies is show in relation to objects in the COSMOS field taken with the Hubble Space Telescope. Credit: ALMA (NRAO/ESO/NAOJ), P. Capak; B. Saxton (NRAO/AUI/NSF), NASA/ESA Hubble

On its own, dust seems fairly unremarkable. However, by observing the clouds of gas and dust within a galaxy, astronomers can determine important information about the history of star formation and the evolution of galaxies. Now thanks to the unprecedented sensitivity of the telescope at the Atacama Large Millimeter Array (ALMA) in Chile, a Caltech-led team has been able to observe the dust contents of galaxies as seen just 1 billion years after the Big Bang—a time period known as redshift 5-6. These are the earliest average-sized galaxies to ever be directly observed and characterized in this way.

The work is published in the June 25 edition of the journal Nature.

Dust in galaxies is created by the elements released during the formation and collapse of stars. Although the most abundant elements in the universe—hydrogen and helium—were created by the Big Bang, stars are responsible for making all of the heavier elements in the universe, such as carbon, oxygen, nitrogen, and iron. And because young, distant galaxies have had less time to make stars, these galaxies should contain less dust. Previous observations had suggested this, but until now nobody could directly measure the dust in these faraway galaxies.

"Before we started this study, we knew that stars formed out of these clouds of gas and dust, and we knew that star formation was probably somehow different in the early universe, where dust is likely less common. But the previous information only really hinted that the properties of the gas and the dust in earlier galaxies were different than in galaxies we see around us today. We wanted to find data that showed that," says Peter Capak, a staff scientist at the Infrared Processing and Analysis Center (IPAC) at Caltech and the first author of the study.

Armed with the high sensitivity of ALMA, Capak and his colleagues set out to perform a direct analysis of the dust in these very early galaxies.

Young, faraway galaxies are often difficult to observe because they appear very dim from Earth. Previous observations of these young galaxies, which formed just 1 billion years after the Big Bang, were made with the Hubble Space Telescope and the W. M. Keck Observatory—both of which detect light in the near-infrared and visible bands of the electromagnetic spectrum. The color of these galaxies at these wavelengths can be used to make inferences about the dust—for example, galaxies that appear bluer in color tend to have less dust, while those that are red have more dust. However, other effects like the age of the stars and our distance from the galaxy can mimic the effects of dust, making it difficult to understand exactly what the color means.

The researchers began their observations by first analyzing these early galaxies with the Keck Observatory. 

Keck confirmed the distance from the galaxies as redshift greater than 5—verifying that the galaxies were at least as young as they previously had been thought to be. The researchers then observed the same galaxies using ALMA to detect light at the longer millimeter and submillimeter wavelengths of light. The ALMA readings provided a wealth of information that could not be seen with visible-light telescopes, including details about the dust and gas content of these very early galaxies.

Capak and his colleagues were able to use ALMA to—for the first time—directly view the dust and gas clouds of nine average-sized galaxies during this epoch. Specifically, they focused on a feature called the carbon II spectral line, which comes from carbon atoms in the gas around newly formed stars. The carbon line itself traces this gas, while the data collected around the carbon line traces a so-called continuum emission, which provides a measurement of the dust. The researchers knew that the carbon line was bright enough to be seen in mature, dust-filled nearby galaxies, so they reasoned that the line would be even brighter if there was indeed less dust in the young faraway galaxies.

Using the carbon line, their results confirmed what had previously been suggested by the data from Hubble and Keck: these older galaxies contained, on average, 12 times less dust than galaxies from 2 billion years later (at a redshift of approximately 4).

"In galaxies like our Milky Way or nearby Andromeda, all of the stars form in very dusty environments, so more than half of the light that is observed from young stars is absorbed by the dust," Capak says. "But in these faraway galaxies we observed with ALMA, less than 20 percent of the light is being absorbed. In the local universe, only very young galaxies and very odd ones look like that. So what we're showing is that the normal galaxy at these very high redshifts doesn't look like the normal galaxy today. Clearly there is something different going on."

That "something different" gives astronomers like Capak a peek into the lifecycle of galaxies. Galaxies form because gas and dust are present and eventually turn into stars—which then die, creating even more gas and dust, and releasing energy. Because it is impossible to watch this evolution from young galaxy to old galaxy happen in real time on the scale of a human lifespan, the researchers use telescopes like ALMA to take a survey of galaxies at different evolutionary stages. Capak and his colleagues believe that this lack of dust in early galaxies signifies a never-before-seen evolutionary stage for galaxies.

"This result is really exciting. It's the first time that we're seeing the gas that the stars are forming out of in the early universe. We are starting to see the transition from just gas to the first generation of galaxies to more mature systems like those around us today. Furthermore, because the carbon line is so bright, we can now easily find even more distant galaxies that formed even longer ago, sooner after the Big Bang," Capak says.

Lin Yan, a staff scientist at IPAC and coauthor on the paper, says that their results are also especially important because they represent typical early galaxies. "Galaxies come in different sizes. Earlier observations could only spot the largest or the brightest galaxies, and those tend to be very special—they actually appear very rarely in the population," she says. "Our findings tell you something about a typical galaxy in that early epoch, so they're results can be observed as a whole, not just as special cases."

Yan says that their ability to analyze the properties of these and earlier galaxies will only expand with ALMA's newly completed capabilities. During the study, ALMA was operating with only a portion of its antennas, 20 at the time; the capabilities to see and analyze distant galaxies will be further improved now that the array is complete with 66 antennas, Yan adds.

"This is just an initial observation, and we've only just started to peek into this really distant universe at redshift of a little over 5. An astronomer's dream is basically to go as far distant as we can. And when it's complete, we should be able to see all the distant galaxies that we've only ever dreamed of seeing," she says.

The findings are published in a paper titled, "Systematically low dust content and high [CII] emission in galaxies at redshifts 5-6." The work was supported by funds from NASA and the European Union's Seventh Framework Program. Nick Scoville, the Francis L. Moseley Professor of Astronomy, was an additional coauthor on this paper. In addition to Keck, Hubble, and ALMA data, observations from the Spitzer Space Telescope were used to measure the stellar mass and age of the galaxies in this study. Coauthors and collaborators from other institutions include C. Carilli, G. Jones, C.M. Casey, D. Riechers, K. Sheth, C.M. Corollo, O. Ilbert, A. Karim, O. LeFevre, S. Lilly, and V. Smolcic. 

Written by Jessica Stoller-Conrad

Contact: 

Deborah Williams-Hedges

(626) 395-3227
Email: debwms@caltech.edu

Source: Caltech

Star Formation Near Supermassive Black Holes

The bright radio galaxy 3C219. The blue object at the center is its active nucleus powered by a supermassive black hole; red shows the extent of the radio emission. Infrared observations of a complete set of similar galaxies dating from about seven billion years ago find that although star formation is active in these objects, the nuclear activity dominates the luminosity. Credit: NRAO and Parijskij et al.

Most if not all galaxies are thought to host a supermassive black hole in their nuclei, a finding that is both one the most important and amazing in modern astronomy. A supermassive black hole grows by accreting mass, and while growing its feeding frenzy is not hidden from our view -- it generates large amounts of energy. 

During the evolutionary phase in which it is most active, the object is known as an active galactic nucleus (AGN). Although there is a difference of a factor of about one billion in physical size scales between the black hole’s accreting environment and its host galaxy, the two sizes are found to be closely correlated, suggesting that there is some kind of feedback between the growth of the black hole and that of its host galaxy. Understanding what the feedback mechanisms are, and how they affect the growth of the galaxy (in particular its star formation), are of paramount importance for our understanding galaxy formation and evolution. Both processes are thought to peak in activity when the universe was only a few billion years old. Neither is particularly well understood.

CfA astronomers Belinda Wilkes, Joanna Kuraszkiewicz, Steve Willner, Matt Ashby, and Giovanni Fazio, along with their colleagues, used the Herschel Space Telescope to study the infrared emission from sixty-four bright, radio and X-ray emitting galaxies with AGN nuclei, and which contain more than one hundred billion solar-masses of stars. Their set is a complete sample of objects of a well-defined class dating from about seven billion years ago, and includes some of the most powerful quasars known. All the objects have large bipolar jets that were driven into intergalactic space by the AGN. The scientists set out to determine how much of the luminosity in these powerful galaxies was due to the AGN and how much was due to star formation activity. The infrared is emitted by dust heated by these two processes, and details of the emission (its typical temperature for example) can help sort out the relative contributions of the two processes.

The astronomers conclude that the star formation rates in these monsters run into the hundreds of solar-masses per year, and therefore reject suggestions that the AGN outflows will quench the star formation in such galaxies. Whatever the details of the growth feedback mechanism, therefore, they do not suppress the star formation. Nevertheless, despite the active star formation underway, the majority of the luminosity is due to the AGN, even during periods when the star formation is most active. Their paper is also significant because it can explain the principal observational differences between the galaxies in this set simply by the orientation of their disk to our line-of-sight, with the large, double-lobed jet sources being seen edge on and the quasars being seen more face-on.

Reference(s):

"Star Formation in z > 1 3CR Host Galaxies as Seen by Herschel," Podigachoski, P.; Barthel, P. D.; Haas, M.; Leipski, C.; Wilkes, B.; Kuraszkiewicz, J.; Westhues, C.; Willner, S. P.; Ashby, M. L. N.; Chini, R.; Clements, D. L.; Fazio, G. G.; Labiano, A.; Lawrence, C.; Meisenheimer, K.; Peletier, R. F.; Siebenmorgen, R.; Verdoes Kleijn, G., A&A, 575, 80, 2015.

Source: Smithsonian Astrophysical Observatory (SAO)

Can Planets Be Rejuvenated Around Dead Stars?

This artist's concept shows a hypothetical "rejuvenated" planet -- a gas giant that has reclaimed its youthful infrared glow. NASA's Spitzer Space Telescope found tentative evidence for one such planet around a dead star, or white dwarf, called PG 0010+280 (depicted as white dot in illustration). Image credit: NASA/JPL-Caltech.  › Full image and caption

For a planet, this would be like a day at the spa. After years of growing old, a massive planet could, in theory, brighten up with a radiant, youthful glow. Rejuvenated planets, as they are nicknamed, are only hypothetical. But new research from NASA's Spitzer Space Telescope has identified one such candidate, seemingly looking billions of years younger than its actual age.

"When planets are young, they still glow with infrared light from their formation," said Michael Jura of UCLA, coauthor of a new paper on the results in the June 10 issue of the Astrophysical Journal Letters. "But as they get older and cooler, you can't see them anymore. Rejuvenated planets would be visible again."

How might a planet reclaim the essence of its youth? Years ago, astronomers predicted that some massive, Jupiter-like planets might accumulate mass from their dying stars. As stars like our sun age, they puff up into red giants and then gradually lose about half or more of their mass, shrinking into skeletons of stars, called white dwarfs. The dying stars blow winds of material outward that could fall onto giant planets that might be orbiting in the outer reaches of the star system.

Thus, a giant planet might swell in mass, and heat up due to friction felt by the falling material. This older planet, having cooled off over billions of years, would once again radiate a warm, infrared glow.

The new study describes a dead star, or white dwarf, called PG 0010+280. An undergraduate student on the project, Blake Pantoja, then at UCLA, serendipitously discovered unexpected infrared light around this star while searching through data from NASA's Wide-field Infrared Survey Explorer, or WISE. Follow-up research led them to Spitzer observations of the star, taken back in 2006, which also showed the excess of infrared light.

At first, the team thought the extra infrared light was probably coming from a disk of material around the white dwarf. In the last decade or so, more and more disks around these dead stars have been discovered -- around 40 so far. The disks are thought to have formed when asteroids wandered too close to the white dwarfs, becoming chewed up by the white dwarfs' intense, shearing gravitational forces.

Other evidence for white dwarfs shredding asteroids comes from observations of the elements in white dwarfs. White dwarfs should contain only hydrogen and helium in their atmospheres, but researchers have found signs of heavier elements -- such as oxygen, magnesium, silicon and iron -- in about 100 systems to date. The elements are thought to be leftover bits of crushed asteroids, polluting the white dwarf atmospheres.

But the Spitzer data for the white dwarf PG 0010+280 did not fit well with models for asteroid disks, leading the team to look at other possibilities. Perhaps the infrared light is coming from a companion small "failed" star, called a brown dwarf -- or more intriguingly, from a rejuvenated planet.

"I find the most exciting part of this research is that this infrared excess could potentially come from a giant planet, though we need more work to prove it," said Siyi Xu of UCLA and the European Southern Observatory in Germany. "If confirmed, it would directly tell us that some planets can survive the red giant stage of stars and be present around white dwarfs."

In the future, NASA's upcoming James Webb Space Telescope could possibly help distinguish between a glowing disk or a planet around the dead star, solving the mystery. But for now, the search for rejuvenated planets -- much like humanity's own quest for a fountain of youth -- endures.

JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

For more information on Spitzer, visit:  http://spitzer.caltech.edu - http://www.nasa.gov/spitzer


Media Contact

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673 
Email:  whitney.clavin@jpl.nasa.gov

Source:  JPL-Caltech/News

Monster black hole wakes up after 26 years

Black hole with stellar companion

Copyright: ESA/ATG medialab

Integral image before and after the outburst

Copyright: ESA/Integral/IBIS/ISDC

Integral light curve 
Copyright: ESA/Integral/IBIS/ISDC

Over the past week, ESA's Integral satellite has been observing an exceptional outburst of high-energy light produced by a black hole that is devouring material from its stellar companion.

X-rays and gamma rays point to some of the most extreme phenomena in the Universe, such as stellar explosions, powerful outbursts and black holes feasting on their surroundings.

In contrast to the peaceful view of the night sky we see with our eyes, the high-energy sky is a dynamic light show, from flickering sources that change their brightness dramatically in a few minutes to others that vary on timescales spanning years or even decades.

On 15 June 2015, a long-time acquaintance of X-ray and gamma ray astronomers made its comeback to the cosmic stage: V404 Cygni, a system comprising a black hole and a star orbiting one another. It is located in our Milky Way galaxy, almost 8000 light-years away in the constellation Cygnus, the Swan. In this type of binary system, material flows from the star towards the black hole and gathers in a disc, where it is heated up, shining brightly at optical, ultraviolet and X-ray wavelengths before spiralling into the black hole.

First signs of renewed activity in V404 Cygni were spotted by the Burst Alert Telescope on NASA's Swift satellite, detecting a sudden burst of gamma rays, and then triggering observations with its X-ray telescope. Soon after, MAXI (Monitor of All-sky X-ray Image), part of the Japanese Experiment Module on the International Space Station, observed an X-ray flare from the same patch of the sky.

These first detections triggered a massive campaign of observations from ground-based telescopes and from space-based observatories, to monitor V404 Cygni at many different wavelengths across the electromagnetic spectrum. As part of this worldwide effort, ESA's Integral gamma-ray observatory started monitoring the out-bursting black hole on 17 June.

“The behaviour of this source is extraordinary at the moment, with repeated bright flashes of light on time scales shorter than an hour, something rarely seen in other black hole systems,” comments Erik Kuulkers, Integral project scientist at ESA.

“In these moments, it becomes the brightest object in the X-ray sky – up to fifty times brighter than the Crab Nebula, normally one of the brightest sources in the high-energy sky.”

The V404 Cygni black hole system has not been this bright and active since 1989, when it was observed with the Japanese X-ray satellite Ginga and high-energy instruments on board the Mir space station.

“The community couldn't be more thrilled: many of us weren't yet professional astronomers back then, and the instruments and facilities available at the time can’t compare with the fleet of space telescopes and the vast network of ground-based observatories we can use today. It is definitely a 'once in a professional lifetime' opportunity,” adds Kuulkers.

The 1989 outburst of V404 Cygni was crucial in the study of black holes. Until then, astronomers knew only a handful of objects that they thought could be black holes, and V404 Cygni was one of the most convincing candidates.

A couple of years after the 1989 outburst, once the source had returned to a quieter state, the astronomers were able to see its companion star, which had been outshone by the extreme activity. The star is about half as massive as the Sun, and by studying the relative motion of the two objects in the binary system, it was determined that the companion must be a black hole, about twelve times more massive than the Sun.

At the time, the astronomers also looked back at archival data from optical telescopes over the twentieth century, finding two previous outbursts, one in 1938 and another one in 1956.

These peaks of activity, which occur every two to three decades, are likely caused by material slowly piling up in the disc surrounding the black hole, until eventually reaching a tipping point that dramatically changes the black hole's feeding routine for a short period.

“Now that this extreme object has woken up again, we are all eager to learn more about the engine that powers the outburst we are observing,” says Carlo Ferrigno from the Integral Science Data Centre at the University of Geneva, Switzerland.

“As coordinators of Integral operations, Enrico Bozzo and I received a text message at 01:30 am on 18 June from our burst alert system, which is designed to detect gamma-ray bursts in the Integral data. In this case, it turned out to be 'only' an exceptional flare since Integral was observing this incredible black hole: definitely a good reason to be woken up in the middle of the night!”

Since the first outburst detection on 15 June by the Swift satellite, V404 Cygni has remained very active, keeping astronomers extremely busy. Over the past week, several teams around the world published over twenty Astronomical Telegrams and other official communications, sharing the progress of the observations at different wavelengths.

This exciting outburst has also been discussed by astronomers attending the European Week of Astronomy and Space Science conference this week in Tenerife, sharing information on observations that have been made in the past few days.

Integral too has been observing this object continuously since 17 June, except for some short periods when it was not possible for operational reasons. The X-ray data show huge variability, with intense flares lasting only a couple of minutes, as well as longer outbursts over time scales of a few hours. Integral also recorded a huge emission of gamma rays from this frenzied black hole.

Because different components of a black-hole binary system emit radiation at different wavelengths across the spectrum, astronomers are combining high-energy observations with those made at optical and radio wavelengths in order to get a complete view of what is happening in this unique object. “We have been observing V404 Cygni with the Gran Telescopio Canarias, which has the largest mirror currently available for optical astronomy,” explains Teo Muñoz-Darias from the Instituto de Astrofísica de Canarias in Tenerife, Spain.

Using this 10.4-m telescope located on La Palma, the astronomers can quickly obtain high quality spectra, thus probing what happens around the black hole on short time scales.

“There are many features in our spectra, showing signs of massive outflows of material in the black hole's environment. We are looking forward to testing our current understanding of black holes and their feeding habits with these rich data,” adds Muñoz-Darias.

Radio astronomers all over the world are also joining in this extraordinary observing campaign. The first detection at these long wavelengths was made shortly after the first Swift alert on 15 June with the Arcminute Microkelvin Imager from the Mullard Radio Astronomy Observatory near Cambridge, in the UK, thanks to the robotic mode of this telescope.

Like the data at other wavelengths, these radio observations also exhibit a continuous series of extremely bright flares. Astronomers will exploit them to investigate the mechanisms that give rise to powerful jets of particles, moving away at velocities close to the speed of light, from the black hole's accretion disc.

There are only a handful of black-hole binary systems for which data have been collected simultaneously at many wavelengths, and the current outburst of V404 Cygni offers the rare chance to gather more observations of this kind. Back in space, Integral has a full-time job watching the events unfold.

“We have been devoting all of Integral's time to observe this exciting source for the past week, and we will keep doing so at least until early July,” comments Peter Kretschmar, ESA Integral mission manager.

“The observations will soon be made available publicly, so that astronomers across the world can exploit them to learn more about this unique object. It will also be possible to use Integral data to try and detect polarisation of the X-ray and gamma ray emission, which could reveal more details about the geometry of the black hole accretion process. This is definitely material for the astrophysics textbooks for the coming years.”

Note for Editors 

The International Gamma-ray Astrophysics Laboratory Integral was launched on 17 October 2002. It is an ESA project with the instruments and a science data centre funded by ESA Member States (especially the Principal Investigator countries: Denmark, France, Germany, Italy, Spain and Switzerland), and with the participation of Russia and the USA. The mission is dedicated to spectroscopy (E/∆E = 500) and imaging (angular resolution: 12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV with concurrent source monitoring in the X-ray (3–35 keV) and optical (V-band, 550 nm) wavelengths. 

For further information, please contact:

Markus Bauer















ESA Science and Robotic Exploration Communication Officer
















Tel: +31 71 565 6799
















Mob: +31 61 594 3 954








Email: markus.bauer@esa.int

Source: ESA

New NASA Supercomputer Model Shows Planet Making Waves in Nearby Debris Disk

Erika Nesvold and Marc Kuchner discuss how their new supercomputer simulation helps astronomers understand Beta Pictoris.
Credits: NASA's Goddard Space Flight Center
 Download this video in HD formats from NASA Goddard's Scientific Visualization Studio

These images compare a view of Beta Pictoris in scattered light as seen by the Hubble Space Telescope (top) with a similar view constructed from data in the SMACK simulation (red overlay, bottom). The X pattern in the Hubble image forms as a result of a faint secondary dust disk inclined to the main debris disk. Previous simulations were unable to reproduce this feature, but the SMACK model replicates the overall pattern because it captures the three-dimensional distribution of the collisions responsible for making the dust.  Credits: Top, NASA/ESA and D. Golimowski (Johns Hopkins Univ.); bottom, NASA Goddard/E. Nesvold and M. Kuchner.  Labeled Image - Unlabeled image

A new NASA supercomputer simulation of the planet and debris disk around the nearby star Beta Pictoris reveals that the planet's motion drives spiral waves throughout the disk, a phenomenon that causes collisions among the orbiting debris. Patterns in the collisions and the resulting dust appear to account for many observed features that previous research has been unable to fully explain.

"We essentially created a virtual Beta Pictoris in the computer and watched it evolve over millions of years," said Erika Nesvold, an astrophysicist at the University of Maryland, Baltimore County, who co-developed the simulation. "This is the first full 3-D model of a debris disk where we can watch the development of asymmetric features formed by planets, like warps and eccentric rings, and also track collisions among the particles at the same time."

In 1984, Beta Pictoris became the second star known to be surrounded by a bright disk of dust and debris. Located only 63 light-years away, Beta Pictoris is an estimated 21 million years old, or less than 1 percent the age of our solar system. It offers astronomers a front-row seat to the evolution of a young planetary system and it remains one of the closest, youngest and best-studied examples today. The disk, which we see edge on, contains rock and ice fragments ranging in size from objects larger than houses to grains as small as smoke particles. It's a younger version of the Kuiper belt at the fringes of our own planetary system.

Nesvold and her colleague Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, presented the findings Thursday during the "In the Spirit of Lyot 2015" conference in Montreal, which focuses on the direct detection of planets and disks around distant stars. A paper describing the research has been submitted to The Astrophysical Journal.

In 2009, astronomers confirmed the existence of Beta Pictoris b, a planet with an estimated mass of about nine times Jupiter's, in the debris disk around Beta Pictoris. Traveling along a tilted and slightly elongated 20-year orbit, the planet stays about as far away from its star as Saturn does from our sun.

Astronomers have struggled to explain various features seen in the disk, including a warp apparent at submillimeter wavelengths, an X-shaped pattern visible in scattered light, and vast clumps of carbon monoxide gas. A common ingredient in comets, carbon monoxide molecules are destroyed by ultraviolet starlight in a few hundred years. To explain why the gas is clumped, previous researchers suggested the clumps could be evidence of icy debris being corralled by a second as-yet-unseen planet, resulting in an unusually high number of collisions that produce carbon monoxide. Or perhaps the gas was the aftermath of an extraordinary crash of icy worlds as large as Mars.

"Our simulation suggests many of these features can be readily explained by a pair of colliding spiral waves excited in the disk by the motion and gravity of Beta Pictoris b," Kuchner said. "Much like someone doing a cannonball in a swimming pool, the planet drove huge changes in the debris disk once it reached its present orbit."

Keeping tabs on thousands of fragmenting particles over millions of years is a computationally difficult task. Existing models either weren't stable over a sufficiently long time or contained approximations that could mask some of the structure Nesvold and Kuchner were looking for.

Working with Margaret Pan and Hanno Rein, both now at the University of Toronto, they developed a method where each particle in the simulation represents a cluster of bodies with a range of sizes and similar motions. By tracking how these "superparticles" interact, they could see how collisions among trillions of fragments produce dust and, combined with other forces in the disk, shape it into the kinds of patterns seen by telescopes. The technique, called the Superparticle-Method Algorithm for Collisions in Kuiper belts (SMACK), also greatly reduces the time required to run such a complex computation.

Using the Discover supercomputer operated by the NASA Center for Climate Simulation at Goddard, the SMACK-driven Beta Pictoris model ran for 11 days and tracked the evolution of 100,000 superparticles over the lifetime of the disk.

As the planet moves along its tilted path, it passes vertically through the disk twice each orbit. Its gravity excites a vertical spiral wave in the disk. Debris concentrates in the crests and troughs of the waves and collides most often there, which explains the X-shaped pattern seen in the dust and may help explain the carbon monoxide clumps. The planet's orbit also is slightly eccentric, which means its distance from the star varies a little every orbit. This motion stirs up the debris and drives a second spiral wave across the face of the disk. This wave increases collisions in the inner regions of the disk, which removes larger fragments by grinding them away. In the real disk, astronomers report a similar clearing out of large debris close to the star.

"One of the nagging questions about Beta Pictoris is how the planet ended up in such an odd orbit," Nesvold explained. "Our simulation suggests it arrived there about 10 million years ago, possibly after interacting with other planets orbiting the star that we haven't detected yet."

Related Links

• Download high-resolution images and video in HD formats from NASA Goddard's Scientific Visualization Studio
• Paper: A SMACK Model of Colliding Planetesimals and Dust in the Beta Pictoris Debris Disk: Thermal Radiation and Scattered Light
• Disk Detective: Help astronomers find other systems like Beta Pictoris
• Hubble Gets Best View of Circumstellar Debris Disk Distorted by Planet
• Nearby Star's Icy Debris Suggests 'Shepherd' Planet

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.

Source: NASA/Distant Planets

A nitrogen-rich nebula

NGC 6153

Credit: ESA/Hubble & NASA
Acknowledgement: Matej Novak

This NASA/ESA Hubble Space Telescope image shows a planetary nebula named NGC 6153, located about 4000 light-years away in the southern constellation of Scorpius (The Scorpion). The faint blue haze across the frame shows what remains of a star like the Sun after it has depleted most of its fuel. When this happens, the outer layers of the star are ejected, and get excited and ionised by the energetic ultraviolet light emitted by the bright hot core of the star, forming the nebula.

NGC 6153 is a planetary nebula that is elliptical in shape, with an extremely rich network of loops and filaments, shown clearly in this Hubble image. However, this is not what makes this planetary nebula so interesting for astronomers.

Measurements show that NGC 6153 contains large amounts of neon, argon, oxygen, carbon and chlorine — up to three times more than can be found in the Solar System. The nebula contains a whopping five times more nitrogen than the Sun! Although it may be that the star developed higher levels of these elements as it grew and evolved, it is more likely that the star originally formed from a cloud of material that already contained lots more of these elements.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Matej Novak.

Links

• Matej Novak’s image on Flickr

Source: ESA/Hubble - Space Telescope

Giant Galaxy is Still Growing

PR Image eso1525a

The halo of galaxy Messier 87

PR Image eso1525b

Planetary nebulae in galaxy Messier 87

PR Image eso1525c

Messier 87 in the constellation of Virgo

Messier 87 has swallowed an entire galaxy in the last billion years

New observations with ESO’s Very Large Telescope have revealed that the giant elliptical galaxy Messier 87 has swallowed an entire medium-sized galaxy over the last billion years. For the first time a team of astronomers has been able to track the motions of 300 glowing planetary nebulae to find clear evidence of this event and also found evidence of excess light coming from the remains of the totally disrupted victim.

Astronomers expect that galaxies grow by swallowing smaller galaxies. But the evidence is usually not easy to see — just as the remains of the water thrown from a glass into a pond will quickly merge with the pond water, the stars in the infalling galaxy merge in with the very similar stars of the bigger galaxy leaving no trace.

But now a team of astronomers led by PhD student Alessia Longobardi at the Max-Planck-Institut für extraterrestrische Physik, Garching, Germany has applied a clever observational trick to clearly show that the nearby giant elliptical galaxy Messier 87 merged with a smaller spiral galaxy in the last billion years.

"This result shows directly that large, luminous structures in the Universe are still growing in a substantial way — galaxies are not finished yet!" says Alessia Longobardi. "A large sector of Messier 87's outer halo now appears twice as bright as it would if the collision had not taken place."

Messier 87 lies at the centre of the Virgo Cluster of galaxies. It is a vast ball of stars with a total mass more than a million million times that of the Sun, lying about 50 million light-years away.

Rather than try to look at all the stars in Messier 87 — there are literally billions and they are too faint and numerous be studied individually — the team looked at planetary nebulae, the glowing shells around ageing stars [1]. Because these objects shine very brightly in a specific hue of aquamarine green, they can be distinguished from the surrounding stars. Careful observation of the light from the nebulae using a powerful spectrograph can also reveal their motions [2].

Just as the water from a glass is not visible once thrown into the pond — but may have caused ripples and other disturbances that can be seen if there are particles of mud in the water — the motions of the planetary nebulae, measured using the FLAMES spectrograph on the Very Large Telescope, provide clues to the past merger.

"We are witnessing a single recent accretion event where a medium-sized galaxy fell through the centre of Messier 87, and as a consequence of the enormous gravitational tidal forces, its stars are now scattered over a region that is 100 times larger than the original galaxy!" adds Ortwin Gerhard, head of the dynamics group at the Max-Planck-Institut für extraterrestrische Physik, Garching, Germany, and a co-author of the new study.

The team also looked very carefully at the light distribution in the outer parts of Messier 87 and found evidence of extra light coming from the stars in the galaxy that had been pulled in and disrupted. These observations have also shown that the disrupted galaxy has added younger, bluer stars to Messier 87, and so it was probably a star-forming spiral galaxy before its merger.

"It is very exciting to be able to identify stars that have been scattered around hundreds of thousands of light-years in the halo of this galaxy — but still to be able to see from their velocities that they belong to a common structure. The green planetary nebulae are the needles in a haystack of golden stars. But these rare needles hold the clues to what happened to the stars," concludes co-author Magda Arnaboldi (ESO, Garching, Germany).

Notes

[1] Planetary nebulae form as Sun-like stars reach the ends of their lives, and they emit a large fraction of their energy in just a few spectral lines, the brightest of which is in the green part of the spectrum. Because of this, they are the only single stars whose motions can be measured at Messier 87's distance of 50 million light-years from Earth. They behave like beacons of green light and as such they tell us where they are and at what velocity they are travelling.

[2] These planetary nebulae are still very faint and need the full power of the Very Large Telescope to study them: the light emitted by a typical planetary nebula in the halo of the Messier 87 galaxy is equivalent to two 60-watt light bulbs on Venus as seen from Earth.

The motions of the planetary nebulae along the line of sight towards or away from Earth lead to shifts in the spectral lines, as a result of the Doppler effect. These shifts can be measured accurately using a sensitive spectrograph and the velocity of the nebulae deduced.

More Information

This research was presented in a paper entitled “The build-up of the cD halo of M87 — evidence for accretion in the last Gyr”, by A. Longobardi et al., to appear in the journal Astronomy & Astrophysics Letters on 25 June 2015.

This work was also presented at the annual conference of the European Astronomical Society, EWASS 2015, which is being held in La Laguna, Tenerife, at the same time.

The team is composed of A. Longobardi (Max-Planck-Institut für extraterrestrische Physik, Garching, Germany), M. Arnaboldi (ESO, Garching, Germany), O. Gerhard (Max-Planck-Institut für extraterrestrische Physik, Garching, Germany) and J.C. Mihos (Case Western University, Cleveland, Ohio, USA).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become "the world’s biggest eye on the sky".

Links

• Research paper in Astronomy & Astrophysics
• Photos of the VLT

Contacts:

Alessia Longobardi
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30000 3022
Email: alongobardi@mpe.mpg.de

Magda Arnaboldi
ESO
Garching bei München, Germany
Tel: +49 89 3200 6599
Email: marnabol@eso.org

Ortwin Gerhard
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30000 3539
Email: gerhard@mpe.mpg.de

Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

Source: ESO

Hubble Sees a 'Behemoth' Bleeding Atmosphere Around a Warm Neptune-Sized Exoplanet Artist's Concept of Extasolar Planet GJ 436b

 Artist's Concept of Extasolar Planet GJ 436b

This artist's concept shows "The Behemoth," an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. Also depicted is the parent star, which is a faint red dwarf named GJ 436. The hydrogen is evaporating from the planet due to extreme radiation from the star. A phenomenon this large has never before been seen around any exoplanet. Credit: NASA, ESA, and G. Bacon (STScI)

Polar View of GJ 436b System

This artist's diagram shows a polar view of the GJ 436 system. The warm, Neptune-sized exoplanet GJ 436b resides very close to its star — less than 3 million miles — and whips around it in just 2.6 Earth days. A huge, comet-like cloud of hydrogen nicknamed "The Behemoth" is shown bleeding off of the planet and trailing it like the tail of a comet.The planet is just 30 light-years from Earth. Credit: NASA, ESA, and A. Feild (STScI)

Photometry of Transiting Planet GJ 436b

This artist's diagram shows the unusual light curve produced when the exoplanet GJ 436b and the huge, comet-like hydrogen cloud nicknamed "The Behemoth" pass in front of the parent star. Because the planet's orbit is tilted nearly edge-on to our view from Earth, the planet and cloud can be seen eclipsing its star. Astronomers see the extended dip in the light caused by the enormous cloud. That dip trails off slowly due to the cloud's comet-like tail. Credit: NASA, ESA, and A. Feild (STScI). 

Released Images

Astronomers using NASA's Hubble Space Telescope have discovered an immense cloud of hydrogen dubbed "The Behemoth" bleeding off a planet orbiting a nearby star. The enormous, comet-like feature is about 50 times the size of the parent star. The hydrogen is evaporating from a warm, Neptune-sized planet, due to extreme radiation from the star. A phenomenon this large has never before been seen around any exoplanet. Given this planet's small size, it may offer clues to how Hot Super-Earths — massive, rocky, hot versions of Earth — are born around other stars through the evaporation of their outer layers of hydrogen. 

"This cloud is very spectacular, though the evaporation rate does not threaten the planet right now," explains the study's leader, David Ehrenreich of the Observatory of the University of Geneva in Switzerland. "But we know that in the past, the star, which is a faint red dwarf, was more active. This means that the planet evaporated faster during its first billion years of existence. Overall, we estimate that it may have lost up to 10 percent of its atmosphere." The planet, named GJ 436b, is considered to be a "Warm Neptune," because of its size and it is much closer to its star than Neptune is to our sun. Although it is in no danger of having its atmosphere completely evaporated and being stripped down to a rocky core, this planet could explain the existence of so-called Hot Super-Earths that are very close to their stars.

These hot, rocky worlds were discovered by the Convection Rotation and Planetary Transits (CoRoT) spacecraft (led by the French Space Agency (CNES) in collaboration with ESA (the European Space Agency), and several other international partners), and NASA's Kepler space telescope. Hot Super-Earths could be the remnants of more massive planets that completely lost their thick, gaseous atmospheres to the same type of evaporation.

Because Earth's atmosphere blocks most ultraviolet light, astronomers needed a space telescope with Hubble's ultraviolet capability and exquisite precision to find "The Behemoth."

"You would have to have Hubble's eyes," says Ehrenreich. "You would not see it in visible wavelengths. But when you turn the ultraviolet eye of Hubble onto the system, it's really kind of a transformation, because the planet turns into a monstrous thing."

Because the planet's orbit is tilted nearly edge-on to our view from Earth, the planet can be seen passing in front of its star. Astronomers also saw the star eclipsed by "The Behemoth" hydrogen cloud around the planet.

Ehrenreich and his team think that such a huge cloud of gas can exist around this planet because the cloud is not rapidly heated and swept away by the radiation pressure from the relatively cool red dwarf star. This allows the cloud to stick around for a longer time. The team's findings will be published in the June 25 edition of the journal Nature.

Evaporation such as this may have happened in the earlier stages of our own solar system, when Earth had a hydrogen-rich atmosphere that dissipated over 100 million to 500 million years. If so, Earth may previously have sported a comet-like tail. It's also possible it could happen to Earth's atmosphere at the end of our planet's life, when the sun swells up to become a red giant and boils off our remaining atmosphere, before engulfing our planet completely.

GJ 436b resides very close to its star — less than 3 million miles — and whips around it in just 2.6 Earth days. (In comparison, Earth is 93 million miles from our sun and orbits it every 365.24 days.) This exoplanet is at least 6 billion years old, and may even be twice that age. It has a mass of around 23 Earths. At just 30 light-years from Earth, it's one of the closest known extrasolar planets.

Finding "The Behemoth" could be a game-changer for characterizing atmospheres of the whole population of Neptune-sized planets and Super-Earths in ultraviolet observations. In the coming years, Ehrenreich expects that astronomers will find thousands of this kind of planet.

The ultraviolet technique used in this study also may spot the signature of oceans evaporating on smaller, more Earth-like planets. It will be extremely challenging for astronomers to directly see water vapor on those worlds, because it's too low in the atmosphere and shielded from telescopes. However, when water molecules are broken by the stellar radiation into hydrogen and oxygen, the relatively light hydrogen atoms can escape the planet. If scientists could spot this hydrogen evaporating from a planet that is a bit more temperate and little less massive than GJ 436b, that is a good sign of an ocean on the surface.

Other related links:

•  Science Paper by: D. Ehrenreich et al. (PDF document)
• Hubble-Europe's Release
• NASA's Hubble Portal

Contact:

Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov

Ann Jenkins / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu

David Ehrenreich
University of Geneva, Geneva, Switzerland
011-41-22-379-2390
david.ehrenreich@unige.ch

Source: HubbleSite