A Day at the Beach for Life on Other Worlds

Artist’s impression of the molten surface of a young planet reacting with its atmosphere to form water vapor.
Credit: Tadahiro Kimura) Original size (1.2MB)

Science

New simulations show that truly Earth-like exoplanets with oceans and continents, and beaches along the boundaries, may be much more common around red dwarfs than previously expected. This means ongoing and future exoplanet survey missions can expect to find multiple Earth-analogs for further study before the end of the decade.

The “habitable zone” is defined as the range of orbits around a star where the temperature would be right for an exoplanet to have liquid water on its surface. This doesn’t necessarily mean that there is life or even water on the planet. In fact, for most exoplanets in the habitable zone, life on the planet would be “no day at the beach.” On Earth, both the oceans and the continents play vital roles in the geochemical carbon cycle which helps maintain a temperate climate where liquid water and life can exist. So to look for potentially habitable Earth-like planets, exactly what we need is “a day at the beach,” where the land and sea can coexist.

Previous research had warned that such beach-friendly planets could be extremely rare, even in the habitable zones around the most common types of stars (namely red dwarfs). This is because there is a distinct difference in the water content of rocky materials found in the inner and outer parts of a protoplanetary disk where planets form, leading to the formation of planets with either too much or too little water in most cases. But new numerical simulations conducted by Tadahiro Kimura from the University of Tokyo and Masahiro Ikoma from the National Astronomical Observatory of Japan provide a sunnier view. By taking into consideration water produced from interactions between the still molten surface of a young planet and its primordial atmosphere, the team found that a wide range in final water content is expected. And within that range, several percent of roughly-Earth-sized planets in habitable zones should have appropriate amounts of water for a temperate climate. This is a high enough percentage that ongoing and future exoplanet survey missions like TESS and PLATO can expect to find multiple examples of truly Earth-like exoplanets with beaches in the 2020s.

These results appeared as Kimura and Ikoma “Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs” in Nature Astronomy on September 29, 2022.

Related Links

• New theory predicts Earth-like aqua planets exist around red dwarfs (Division of Science)
• New theory predicts Earth-like aqua planets exist around red dwarfs (The University of Tokyo)

Source: National Astronomical Observatory of Japan (NAOJ)/News

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More Clues That Earth-Like Exoplanets Are Indeed Earth-Like

This artist's concept depicts Kepler-186f

NASA Ames/JPL-Caltech/T. Pyle

Low Resolution (jpg)

Cambridge, MA -A new study provides new clues indicating that an exoplanet 500 light-years away is much like Earth.

Kepler-186f is the first identified Earth-sized planet outside the Solar System orbiting a star in the habitable zone. This means it's the proper distance from its host star for liquid water to pool on the surface.

The Georgia Tech study used simulations to analyze and identify the exoplanet's spin axis dynamics. Those dynamics determine how much a planet tilts on its axis and how that tilt angle evolves over time. Axial tilt contributes to seasons and climate because it affects how sunlight strikes the planet's surface.

The researchers suggest that Kepler-186f's axial tilt is very stable, much like the Earth, making it likely that it has regular seasons and a stable climate. The Georgia Tech team thinks the same is true for Kepler-62f, a super-Earth-sized planet orbiting around a star about 1,200 light-years away from us.

How important is axial tilt for climate? Large variability in axial tilt could be a key reason why Mars transformed from a watery landscape billions of years ago to today's barren desert.

"Mars is in the habitable zone in our solar system, but its axial tilt has been very unstable -- varying from 0 to 60 degrees," said Georgia Tech Assistant Professor Gongjie Li, who led the study together with graduate student Yutong Shan from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. "That instability probably contributed to the decay of the Martian atmosphere and the evaporation of surface water."

As a comparison, Earth's axial tilt oscillates more mildly -- between 22.1 and 24.5 degrees, going from one extreme to the other every 10,000 or so years.

The orientation angle of a planet's orbit around its host star can be made to oscillate by gravitational interaction with other planets in the same system. If the orbit were to oscillate at the same speed as the precession of the planet's spin axis (akin to the circular motion exhibited by the rotation axis of a top or gyroscope), the spin axis would also wobble back and forth, sometimes dramatically.

Mars and Earth interact strongly with each other, as well as with Mercury and Venus. As a result, by themselves, their spin axes would precess with the same rate as the orbital oscillation, which may cause large variations in their axial tilt. Fortunately, the Moon keeps Earth's variations in check. The Moon increases our planet's spin axis precession rate and makes it differ from the orbital oscillation rate. Mars, on the other hand, doesn't have a large enough satellite to stabilize its axial tilt.

"It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets," said Li, a faculty member in the School of Physics.

"We don't know whether they possess moons, but our calculations show that even without satellites, the spin axes of Kepler-186f and 62f would have remained constant over tens of millions of years."

Kepler-186f is less than 10 percent larger in radius than Earth, but its mass, composition, and density remain a mystery. It orbits its host star every 130 days. According to NASA, the brightness of that star at high noon, while standing on 186f, would appear as bright as the sun just before sunset here on Earth. Kepler-186f is located in the constellation Cygnus as part of a five-planet star system.

Kepler-62f was the most Earth-like exoplanet until scientists noticed 186f in 2014. It's about 40 percent larger than our planet and is likely a terrestrial or ocean-covered world. It's in the constellation Lyra and is the outermost planet among five exoplanets orbiting a single star.

That's not to say either exoplanet has water, let alone life. But both are relatively good candidates.
"Our study is among the first to investigate climate stability of exoplanets and adds to the growing understanding of these potentially habitable nearby worlds," said Li.

"I don't think we understand enough about the origin of life to rule out the possibility of its presence on planets with irregular seasons," added the CfA’s Shan. "Even on Earth, life is remarkably diverse and has shown incredible resilience in extraordinarily hostile environments.

"But a climatically stable planet might be a more comfortable place to start."

A paper describing these results appeared in the May 17, 2018 issue of The Astronomical Journal.
(This release was originally issued by Georgia Tech.)

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Megan Watzke
Harvard-Smithsonian Center for Astrophysics
+1 617-496-7998
mwatzke@cfa.harvard.edu

Peter Edmonds
Harvard-Smithsonian Center for Astrophysics
+1 617-571-7279
pedmonds@cfa.harvard.edu

Source:  Harvard-Smithsonian Center for Astrophysics (CfA)

Between a Rock and a Hard Place: Can Garnet Planets Be Habitable?

Artist rendition of interior compositions of planets around the stars Kepler 102 and Kepler 407.

The picture shows what minerals are likely to occur several different depths. Kepler 102 is Earth-like, dominated by olivine minerals, whereas Kepler 407 is dominated by garnet, so less likely to have plate tectonics.Click here for a larger version. Image Credit: Robin Dienel, Carnegie DTM

What makes a rocky planet Earth-like?

Astronomers and geoscientists have joined forces using data from the Sloan Digital Sky Survey (SDSS) to study the mix of elements in exoplanet host stars, and to consider what this reveals about their planets.

In results presented today at the American Astronomical Society (AAS) meeting in Grapevine, Texas, astronomer Johanna Teske explained, “our study combines new observations of stars with new models of planetary interiors. We want to better understand the diversity of small, rocky exoplanet composition and structure — how likely are they to have plate tectonics or magnetic fields?”

Earth-sized planets have been found around many stars — but Earth-sized does not necessarily mean Earth-like. Some of these Earth-sized planets have been found orbiting stars with chemical compositions quite different from our Sun, and those differences in chemistry could have important consequences.

Astronomers in the Sloan Digital Sky Survey have made these observations using the APOGEE (Apache Point Observatory Galactic Evolution Experiment) spectrograph on the 2.5m Sloan Foundation Telescope at Apache Point Observatory in New Mexico. This instrument collects light in the near-infrared part of the electromagnetic spectrum and disperses it, like a prism, to reveal signatures of different elements in the atmospheres of stars. A fraction of the almost 200,000 stars surveyed by APOGEE overlap with the sample of stars targeted by the NASA Kepler mission, which was designed to find potentially Earth-like planets. The work presented today focuses on ninety Kepler stars that show evidence of hosting rocky planets, and which have also been surveyed by APOGEE. 

In particular, Teske and colleagues presented solar systems around the stars Kepler 102 and Kepler 407. Kepler 102 is slightly less luminous than the Sun and has five known planets; Kepler 407 is a star almost identical in mass to the Sun and hosts at least two planets, one with a mass less than 3 Earth masses.

“Looking at these two exoplanet systems in particular,” Teske explains, “we determined that Kepler 102 is like the Sun, but Kepler 407 has a lot more silicon.”

To understand what a lot more silicon might mean for the planets around Kepler 407, astronomers turned to geophysicists for help. Cayman Unterborn of Arizona State University ran computer models of planet formation. “We took the star compositions found by APOGEE and modeled how the elements condensed into planets in our models. We found that the planet around Kepler 407, which we called ‘Janet,” would likely be rich in the mineral garnet. The planet around Kepler 102, which we called ‘Olive,’ is probably rich in olivine, like Earth.”

That seemingly-small difference in minerals might have major consequences for Janet and Olive. Garnet is a stiffer mineral than olivine, so it flows more slowly. Unterborn explains that this means that a garnet planet like Janet would be much less likely to have long-term plate tectonics. “To sustain plate tectonics over geologic timescales, a planet must have the right mineral composition,” Unterborn says.

Plate tectonics is believed to be essential for life on Earth, because of how volcanoes and ocean ridges recycle elements between Earth’s crust and mantle. This recycling regulates the composition of our atmosphere. Wendy Panero of the School of Earth Sciences at The Ohio State University says that “without these geological processes, life may not have had the chance to evolve on Earth.” 

Determining the likelihood of such geological processes on other planets will help distinguish which ones are the best targets for future missions searching for signs of life. “If we’re looking for a needle,” Panero says, “why not start in the sewing box?”

The next step in the team’s research is to extend this study to all of the stars observed by APOGEE that host small planets. That extension would allow astronomers to map out a wider range of planet compositions and structures to find those most likely to be Earth-like in their mineral content. Teske concludes, “As we’ve learned more about the Earth, we have learned about how many pieces come together to make it habitable. How often will exoplanets get that lucky?”

 Source: Sloan Digital Sky Survey (SDSS)

Cloudy Days on Exoplanets May Hide Atmospheric Water

Hot Jupiters, exoplanets around the same size as Jupiter that orbit very closely to their stars, often have cloud or haze layers in their atmospheres. This may prevent space telescopes from detecting atmospheric water that lies beneath the clouds, according to a study in the Astrophysical Journal. Image credit: NASA/JPL-Caltech.  › Full image and caption

Water is a hot topic in the study of exoplanets, including "hot Jupiters," whose masses are similar to that of Jupiter, but which are much closer to their parent star than Jupiter is to the sun. They can reach a scorching 2,000 degrees Fahrenheit (1,100 degrees Celsius), meaning any water they host would take the form of water vapor.

Astronomers have found many hot Jupiters with water in their atmospheres, but others appear to have none. Scientists at NASA's Jet Propulsion Laboratory, Pasadena, California, wanted to find out what the atmospheres of these giant worlds have in common.

Researchers focused on a collection of hot Jupiters studied by NASA's Hubble Space Telescope. They found that the atmospheres of about half of the planets were blocked by clouds or haze.

"The motivation of our study was to see what these planets would be like if they were grouped together, and to see whether they share any atmospheric properties," said Aishwarya Iyer, a JPL intern and master's degree candidate at California State University, Northridge, who led the study.

The new study, published in the June 1 issue of the Astrophysical Journal, suggests that clouds or haze layers could be preventing a substantial amount of atmospheric water from being detected by space telescopes.

The clouds themselves are likely not made of water, as the planets in this sample are too hot for water-based clouds.

"Clouds or haze seem to be on almost every planet we studied," Iyer said. "You have to be careful to take clouds or haze into account, or else you could underestimate the amount of water in an exoplanet's atmosphere by a factor of two."

In the study, scientists looked at a set of 19 hot Jupiters previously observed by Hubble. The telescope's Wide Field Camera 3 had detected water vapor in the atmospheres of 10 of these planets, and no water on the other nine. But that information was spread across more than a dozen studies. The methods of analyzing and interpretation varied because the studies were conducted separately. There had not been one overarching analysis of all these planets.

To compare the planets and look for patterns, the JPL team had to standardize the data: Researchers combined the datasets for all 19 hot Jupiters to create an average overall light spectrum for the group of planets. They then compared these data to models of clear, cloud-free atmospheres and those with various cloud thicknesses.

The scientists determined that, for almost every planet they studied, haze or clouds were blocking half of the atmosphere, on average.

"In some of these planets, you can see water peeking its head up above the clouds or haze, and there could still be more water below," Iyer said.

Scientists do not yet know the nature of these clouds or hazes, including what they are they made of.

"Clouds or haze being on almost all these planets is pretty surprising," said Robert Zellem, a postdoctoral fellow at JPL and co-author of the study.

The implications of this result agree with findings published in the Dec. 14, 2015, issue of the journal Nature. 

The Nature study used data from NASA's Hubble and Spitzer Space Telescopes to suggest that clouds or haze could be hiding undetected water in hot Jupiters. This new study uses exoplanet data from a single instrument on Hubble to uniformly characterize a larger group of hot Jupiters, and is the first to quantify how much of the atmosphere would be shielded as a result of clouds or haze.

The new research could have implications for follow-up studies with future space observatories, such as NASA's James Webb Space Telescope. Exoplanets with thick cloud covers blocking the detection of water and other substances may be less desirable targets for more extensive study.

These results are also important for figuring out how planets form, scientists say.

"Did these planets form in their current positions or migrate toward their host stars from farther out? Understanding the abundances of molecules such as water helps us answer those questions," Zellem said.

"This paper is an exciting step forward for the study of exoplanets and comparing their properties," said Mark Swain, study co-author and group supervisor for the exoplanet discovery and science group at JPL.

Michael Line of the University of California, Santa Cruz, also contributed to the study. Other co-authors from JPL included Gael Roudier, Graca Rocha and John Livingston.

 

For more information about the Hubble Space Telescope, visit: www.nasa.gov/hubble

News Media Contact

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
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Source: JPL-Caltech

Eight New Planets Found in "Goldilocks" Zone

This artist’s conception depicts an Earth-like planet orbiting an evolved star that has formed a stunning "planetary nebula." Earlier in its life, this planet may have been like one of the eight newly discovered worlds orbiting in the habitable zones of their stars. Credit: David A. Aguilar (CfA). High Resolution (jpg) - Low Resolution (jpg)

Cambridge, MA -Astronomers announced today that they have found eight new planets in the "Goldilocks" zone of their stars, orbiting at a distance where liquid water can exist on the planet's surface. This doubles the number of small planets (less than twice the diameter of Earth) believed to be in the habitable zone of their parent stars. Among these eight, the team identified two that are the most similar to Earth of any known exoplanets to date.

"Most of these planets have a good chance of being rocky, like Earth," says lead author Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics (CfA).

These findings were announced today in a press conference at a meeting of the American Astronomical Society.

The two most Earth-like planets of the group are Kepler-438b and Kepler-442b. Both orbit red dwarf stars that are smaller and cooler than our Sun. Kepler-438b circles its star every 35 days, while Kepler-442b completes one orbit every 112 days.

With a diameter just 12 percent bigger than Earth, Kepler-438b has a 70-percent chance of being rocky, according to the team's calculations. Kepler-442b is about one-third larger than Earth, but still has a 60-percent chance of being rocky.

To be in the habitable zone, an exoplanet must receive about as much sunlight as Earth. Too much, and any water would boil away as steam. Too little, and water will freeze solid.

"For our calculations we chose to adopt the broadest possible limits that can plausibly lead to suitable conditions for life," says Torres.

Kepler-438b receives about 40 percent more light than Earth. (In comparison, Venus gets twice as much solar radiation as Earth.) As a result, the team calculates it has a 70 percent likelihood of being in the habitable zone of its star.

Kepler-442b get about two-thirds as much light as Earth. The scientists give it a 97 percent chance of being in the habitable zone.

"We don't know for sure whether any of the planets in our sample are truly habitable," explains second author David Kipping of the CfA. "All we can say is that they're promising candidates."

Prior to this, the two most Earth-like planets known were Kepler-186f, which is 1.1 times the size of Earth and receives 32 percent as much light, and Kepler-62f, which is 1.4 times the size of Earth and gets 41 percent as much light.

The team studied planetary candidates first identified by NASA's Kepler mission. All of the planets were too small to confirm by measuring their masses. Instead, the team validated them by using a computer program called BLENDER to determine that they are statistically likely to be planets. BLENDER was developed by Torres and colleague Francois Fressin, and runs on the Pleaides supercomputer at NASA Ames. This is the same method that has been used previously to validate some of Kepler's most iconic finds, including the first two Earth-size planets around a Sun-like star and the first exoplanet smaller than Mercury.

After the BLENDER analysis, the team spent another year gathering follow-up observations in the form of high-resolution spectroscopy, adaptive optics imaging, and speckle interferometry to thoroughly characterize the systems.

Those follow-up observations also revealed that four of the newly validated planets are in multiple-star systems. However, the companion stars are distant and don't significantly influence the planets.

As with many Kepler discoveries, the newly found planets are distant enough to make additional observations challenging. Kepler-438b is located 470 light-years from Earth while the more distant Kepler-442b is 1,100 light-years away.

The paper reporting these results has been accepted for publication in The Astrophysical Journal and is available online.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source:  Harvard-Smithsonian Center for Astrophysics

 

Ground-Based Detection of Super-Earth Transit Paves Way to Remote Sensing of Small Exoplanets

This artist's conception shows the super-Earth 55 Cancri e (right) compared to the Earth (left). Astronomers using a ground-based telescope have measure the transit of 55 Cancri e for the first time. It is the shallowest transit ever detected from the ground. Credit: NASA/JPL. High Resolution (jpg)  -  Low Resolution (jpg)

Astronomers have measured the passing of a super-Earth in front of a bright, nearby Sun-like star using a ground-based telescope for the first time. The transit of the exoplanet 55 Cancri e is the shallowest detected from the ground yet. Since detecting a transit is the first step in analyzing a planet's atmosphere, this success bodes well for characterizing the many small planets that upcoming space missions are expected to discover in the next few years.

The international research team used the 2.5-meter Nordic Optical Telescope on the island of La Palma, Spain, a moderate-sized facility by today's standards but equipped with state-of-the-art instruments, to make the detection. Previous observations of this planet transit had to rely on space-borne telescopes.

The host star, 55 Cancri, is located just 40 light-years away from us and is visible to the naked eye. During its transit, the planet crosses 55 Cancri and blocks a tiny fraction of the starlight, dimming the star by 1/2000th (or 0.05%) for almost two hours. This shows that the planet is about twice the size of Earth, or 16,000 miles in diameter.

"Our observations show that we can detect the transits of small planets around Sun-like stars using ground-based telescopes," says Ernst de Mooij of Queen's University Belfast in the United Kingdom, lead author of the study.

He continues, "This is especially important because upcoming space missions such as TESS and PLATO should find many small planets around bright stars and we will want to follow up the discoveries with ground-based instruments."

TESS is a NASA mission scheduled for launch in 2017, while PLATO is to be launched in 2024 by the European Space Agency; both will search for transiting terrestrial planets around nearby bright stars.

"With this result we are also closing in on the detection of the atmospheres of small planets with ground-based telescopes," says co-author Mercedes Lopez-Morales of the Harvard-Smithsonian Center for Astrophysics (CfA). "We are slowly paving the way toward the detection of bio-signatures in Earth-like planets around nearby stars."

"It's remarkable what we can do by pushing the limits of existing telescopes and instruments, despite the complications posed by the Earth's own turbulent atmosphere," says study co-author Ray Jayawardhana of York Univerity in Canada. "Remote sensing across tens of light-years isn't easy, but it can be done with the right technique and a bit of ingenuity."

The planet 55 Cancri e is about twice as big and eight times as massive as Earth. With a period of 18 hours, it is the innermost of five planets in the system. Because of its proximity to the host star, the planet's dayside temperature reaches over 3100° Fahrenheit (1700° Celsius), hot enough to melt metal, with conditions far from hospitable to life. Initially identified a decade ago through radial velocity measurements, it was later confirmed through transit observations with the MOST and Spitzer space telescopes.

Until now, the transits of only one other super-Earth, GJ 1214b circling a red dwarf, had been observed with ground-based telescopes. The Earth's roiling air makes such observations extremely difficult. But the team's success with 55 Cancri e raises the prospects of characterizing dozens of super-Earths likely to be revealed by upcoming surveys.

"We expect these surveys to find so many nearby, terrestrial worlds that space telescopes simply won't be able to follow up on all of them. Future ground-based instrumentation will be key, and this study shows it can be done," adds Lopez-Morales.

The research team also includes Raine Karjalainen and Marie Hrudkova of the Isaac Newton Group of Telescopes. Their findings appear in a paper to be published in The Astrophysical Journal Letters.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar 
Director of Public Affairs 
Harvard-Smithsonian Center for Astrophysics 
617-495-7462 
daguilar@cfa.harvard.edu

Christine Pulliam 
Public Affairs Specialist 
Harvard-Smithsonian Center for Astrophysics 
617-495-7463 
cpulliam@cfa.harvard.edu

Source: Harvard-Smithsonian Center for Astrophysics

Astronomers Find a New Type of Planet: The "Mega-Earth"

The newly discovered "mega-Earth" Kepler-10c dominates the foreground in this artist's conception. Its sibling, the lava world Kepler-10b, is in the background. Both orbit a sunlike star. Kepler-10c has a diameter of about 18,000 miles, 2.3 times as large as Earth, and weighs 17 times as much. Therefore it is all solids, although it may possess a thin atmosphere shown here as wispy clouds. Credit: David A. Aguilar (CfA). High Resolution (jpg)  -  Low Resolution (jpg)

 

Cambridge, MA -Astronomers announced today that they have discovered a new type of planet - a rocky world weighing 17 times as much as Earth. Theorists believed such a world couldn't form because anything so hefty would grab hydrogen gas as it grew and become a Jupiter-like gas giant. This planet, though, is all solids and much bigger than previously discovered "super-Earths," making it a "mega-Earth."

"We were very surprised when we realized what we had found," says astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the data analysis and made the discovery.

"This is the Godzilla of Earths!" adds CfA researcher Dimitar Sasselov, director of the Harvard Origins of Life Initiative. "But unlike the movie monster, Kepler-10c has positive implications for life."

The team's finding was presented today in a press conference at a meeting of the American Astronomical Society (AAS).

The newfound mega-Earth, Kepler-10c, circles a sunlike star once every 45 days. It is located about 560 light-years from Earth in the constellation Draco. The system also hosts a 3-Earth-mass "lava world," Kepler-10b, in a remarkably fast, 20-hour orbit.

Kepler-10c was originally spotted by NASA's Kepler spacecraft. Kepler finds planets using the transit method, looking for a star that dims when a planet passes in front of it. By measuring the amount of dimming, astronomers can calculate the planet's physical size or diameter. However, Kepler can't tell whether a planet is rocky or gassy.

Kepler-10c was known to have a diameter of about 18,000 miles, 2.3 times as large as Earth. This suggested it fell into a category of planets known as mini-Neptunes, which have thick, gaseous envelopes.

The team used the HARPS-North instrument on the Telescopio Nazionale Galileo (TNG) in the Canary Islands to measure the mass of Kepler-10c. They found that it weighed 17 times as much as Earth - far more than expected. This showed that Kepler-10c must have a dense composition of rocks and other solids.
"Kepler-10c didn't lose its atmosphere over time. It's massive enough to have held onto one if it ever had it," explains Dumusque. "It must have formed the way we see it now."

Planet formation theories have a difficult time explaining how such a large, rocky world could develop. However, a new observational study suggests that it is not alone.

Also presenting at AAS, CfA astronomer Lars A. Buchhave found a correlation between the period of a planet (how long it takes to orbit its star) and the size at which a planet transitions from rocky to gaseous. This suggests that more mega-Earths will be found as planet hunters extend their data to longer-period orbits.
The discovery that Kepler-10c is a mega-Earth also has profound implications for the history of the universe and the possibility of life. The Kepler-10 system is about 11 billion years old, which means it formed less than 3 billion years after the Big Bang.

The early universe contained only hydrogen and helium. Heavier elements needed to make rocky planets, like silicon and iron, had to be created in the first generations of stars. When those stars exploded, they scattered these crucial ingredients through space, which then could be incorporated into later generations of stars and planets.

This process should have taken billions of years. However, Kepler-10c shows that the universe was able to form such huge rocks even during the time when heavy elements were scarce.

"Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life," says Sasselov.

This research implies that astronomers shouldn't rule out old stars when they search for Earth-like planets. And if old stars can host rocky Earths too, then we have a better chance of locating potentially habitable worlds in our cosmic neighborhood.

The HARPS-N project is led by the Astronomical Observatory of the Geneva University (Switzerland). The National Institute for Astrophysics (INAF, Italy) has agreed to provide 80 observing nights per year over five years to use HARPS-N coupled to the TNG. The U.S. partners are the Harvard-Smithsonian Center for Astrophysics and the Harvard University Origins of Life Initiative; and the UK partners are the Universities of St. Andrews and Edinburgh, and the Queens University of Belfast.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source:  Harvard-Smithsonian Center for Astrophysics

Four white dwarf stars caught in the act of consuming 'earth-like' exoplanets

Rocky material in orbit around a white dwarf star (centre). Collisions turn larger material into dust, some of which then rains down on to the white dwarf. Credit: © Mark A. Garlick / space-art.co.uk / University of Warwick

University of Warwick astrophysicists have pinpointed four white dwarf stars surrounded by dust from shattered planetary bodies which once bore striking similarities to the composition of the Earth. The scientists publish their results in a paper in the journal Monthly Notices of the Royal Astronomical Society.

White dwarfs are the final stage of life of stars like our Sun, the residual cores of material left behind after their available fuel for nuclear reactions has been exhausted. Using the Hubble Space Telescope to carry out the biggest survey to date of the chemical composition of the atmospheres of white dwarf stars, the researchers found that the most frequently occurring elements in the dust around these four white dwarfs were oxygen, magnesium, iron and silicon – the four elements that make up roughly 93 per cent of the Earth.

However an even more significant observation was that this material also contained an extremely low proportion of carbon, which matched very closely that of the Earth and the other rocky planets orbiting closest to our own Sun.

This is the first time that such low proportions of carbon have been measured in the atmospheres of white dwarf stars polluted by debris. Not only is this clear evidence that these stars once had at least one rocky exoplanet which they have now destroyed, the observations must also pinpoint the last phase of the death of these worlds.

The atmosphere of a white dwarf is made up of hydrogen and/or helium, so any heavy elements that come into their atmosphere are dragged downwards to their core and out of sight within a matter of days by the dwarf’s high gravity. Given this, the astronomers must literally be observing the final phase of the death of these worlds as the material rains down on the stars at rates of up to 1 million kilograms every second.

Not only is this clear evidence that these stars once had rocky exoplanetary bodies which have now been destroyed, the observations of one particular white dwarf, PG0843+516, may also tell the story of the destruction of these worlds.

This star stood out from the rest owing to the relative overabundance of the elements iron, nickel and sulphur in the dust found in its atmosphere. Iron and nickel are found in the cores of terrestrial planets, as they sink to the centre owing to the pull of gravity during planetary formation, and so does sulphur thanks to its chemical affinity to iron.

Therefore, researchers believe they are observing White Dwarf PG0843+516 in the very act of swallowing up material from the core of a rocky planet that was large enough to undergo differentiation, similar to the process that separated the core and the mantle of the Earth.

Professor Boris Gänsicke of the Department of Physics at the University of Warwick, who led the study, said the destructive process which caused the discs of dust around these distant white dwarfs is likely to one day play out in our own solar system.

“What we are seeing today in these white dwarfs several hundred light years away could well be a snapshot of the very distant future of the Earth. As stars like our Sun reach the end of their life, they expand to become red giants when the nuclear fuel in their cores is depleted.

‘When this happens in our own solar system, billions of years from now, the Sun will engulf the inner planets Mercury and Venus. It’s unclear whether the Earth will also be swallowed up by the Sun in its red giant phase - but even if it survives, its surface will be roasted.

‘During the transformation of the Sun into a white dwarf, it will lose a large amount of mass, and all the planets will move further out. This may destabilise the orbits and lead to collisions between planetary bodies as happened in the unstable early days of our solar systems.

‘This may even shatter entire terrestrial planets, forming large amounts of asteroids, some of which will have chemical compositions similar to those of the planetary core. In our solar system, Jupiter will survive the late evolution of the Sun unscathed, and scatter asteroids, new or old, towards the white dwarf.

‘It is entirely feasible that in PG0843+516 we see the accretion of such fragments made from the core material of what was once a terrestrial exoplanet.”

The University of Warwick led team surveyed more than 80 white dwarfs within a few hundred light years of the Sun, using the Cosmic Origin Spectrograph onboard the Hubble Space Telescope.


Images and captions

The following high resolution artist impressions are available. They were all created for the University of Warwick by the space artist Mark A. Garlick and are free for use by media or the University but are otherwise copyright as follows: “© Mark A. Garlick / space-art.co.uk / University of Warwick”

First Artist’s impression by Mark A. Garlick

http://bit.ly/KornIK The inner region of an exo-planetary system where four terrestrial planets orbit a solar-like star.

Second artist’s impression by Mark A. Garlick

http://bit.ly/ItlgIP The host star is running out of hydrogen in the core, swells up, and its surface becomes cooler. It is also losing mass, which causes the planets to move further out. The perturbation of the orbits may lead to collisions that will generate large amounts of rocky debris.

Third artist’s impression by Mark A. Garlick

http://bit.ly/IEhrxJ This depicts what the researchers are now observing. A white dwarf sits in the centre of the remnant of a planetary system. Asteroid sized debris is scattered inwards by interaction with the remaining planets and is tidally disrupted as it approaches the white dwarf forming a disc of dust some of which is raining down onto the star. The researchers have found that the composition of the debris that has just fallen onto the four white dwarfs matches the composition of Earth-like rocky worlds.

Image that brings together all three artist’s impressions by Mark A. Garlick together in one sequence http://bit.ly/K02jev

Science contact

Professor Boris Gänsicke, Department of Physics University of Warwick
Tel: +44 (0)2476 574741
Email: boris.gaensicke@warwick.ac.uk


Media contacts

Anna Blackaby
University of Warwick Science Press Officer
Tel: +44 (0)2476 575910
Mob: +44 (0) 7785 433155
Email: a.blackaby@warwick.ac.uk

Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
Email: rm@ras.org.uk


Further information

The new work is published in “The chemical diversity of exo-terrestrial planetary debris around white dwarfs”, B. T. Gänsicke, D. Koester, J. Farihi, J. Girven, S.G.Parsons, E. Breedt, Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper is available at http://arxiv.org/abs/1205.0167

Notes for editors


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