Showing posts with label orange dwarfs. Show all posts
Showing posts with label orange dwarfs. Show all posts

Sunday, May 26, 2024

New Catalog Showcases a Diverse Exoplanet Landscape with Strange, Exotic Worlds

Artist’s rendition of the variety of exoplanets featured in the new NASA TESS-Keck Survey Mass Catalog, the largest homogenous analysis of TESS planets released by any survey thus far. Credit: W. M. Keck Observatory/Adam Makarenko

NASA TESS-Keck Survey is the single largest uniform analysis of TESS planets to date

A new, robust catalog is out featuring 126 confirmed and candidate exoplanets discovered with the National Aeronautics and Space Administration (NASA) Transiting Exoplanet Survey Satellite (TESS) in collaboration with W. M. Keck Observatory on Maunakea, Hawaiʻi.

In this latest installment of the TESS-Keck Survey, the catalog consists of thousands of radial velocity (RV) observations that reveal a fascinating mix of planet types beyond our solar system, from rare worlds with extreme environments to ones that could possibly support life.

The study is published in today’s edition of The Astrophysical Journal Supplement.

“The results that have come from the TESS-Keck Survey represents the single largest contribution to understanding the physical nature and system architectures of new planets TESS has discovered,” says University of Kansas Physics and Astronomy graduate student Alex Polanski, the lead author of the paper. “Catalogs like this help astronomers place individual worlds in context with the rest of the exoplanet population.”

Polanski and a global team of astronomers from multiple institutions spent three years developing the catalog; they took TESS planetary data and analyzed 9,204 RV measurements, 4,943 of which were taken over the course of 301 observing nights using Keck Observatory’s planet-hunting instrument called the High-Resolution Echelle Spectrometer (HIRES).

“The TESS-Keck Survey results fundamentally depend on Doppler spectroscopy from Keck Observatory’s HIRES. The U.S. science community has relied on this workhorse instrument for exoplanet studies for nearly three decades,” says University of Kansas Associate Professor of Physics and Astronomy Ian Crossfield, a co-author of the paper.

The team also obtained an additional 4,261 RV with The University of California Observatories’ Automated Planet Finder at Lick Observatory in California. With the combined total of RV measurements, they were able to calculate the masses of 120 confirmed planets plus six candidate planets.

“RV measurements let astronomers detect, and learn the properties of, these exoplanetary systems. When we see a star wobbling regularly back and forth, we can infer the presence of an orbiting planet and measure the planet’s mass,” says Crossfield.

The wobble produces a regular change in wavelengths due to the Doppler effect, which is detected through the RV method — one of the techniques used to find exoplanets. The phenomenon refers to the gravitational effect an exoplanet has on its host star, where it tugs the star as the planet orbits around it. When the host star moves toward a telescope, its visible light turns slightly bluer; when it moves away from us, the light shifts slightly redder. This is much like how sound behaves; a fire truck’s siren gets higher-pitched as it travels closer to you, and sounds lower-pitched as it drives farther away.

Of the planets profiled in the TESS-Keck Survey, two planets — TOI-1824 b and TOI-1798 c — stand out as examples of worlds that have such peculiar characteristics they give new insight into exoplanet classification and serve as potential touchstones for deepening astronomers’ understanding of the diverse ways planets form and evolve.

TOI-1824 b: A Superdense Sub-Neptune

One of the densest sub-Neptunes in the TESS-Keck Survey catalog, and the subject of another TESS-Keck Survey paper by University of California (UC), Santa Cruz undergraduate Sarah Lange, TOI-1824 b is unusually dense for a planet its size.

“At nearly 19 times the mass of Earth, but only 2.6 times the size of our home planet, TOI-1824 b is an exoplanet oddity,” says co-author Joseph Murphy, a graduate student at the UC Santa Cruz. “Planets similar in size typically have a mass between roughly 6 and 12 times the mass of Earth.”

One explanation for why TOI-1824 b is so massive yet appears much smaller than usual is it could have an Earth-like core surrounded by an unusually thin, hydrogen-dominated atmosphere. Another possibility is the planet has a water-rich core beneath a steam atmosphere.

“This superdense sub-Neptune may be the massive cousin of water worlds, which are small planets with high H2O content purported to exist around red dwarf stars,” says Murphy.

Red dwarfs, or M dwarf stars, are the most common star type in the Milky Way galaxy. They make ideal targets in the search for habitable worlds because M dwarfs are cooler than the Sun; this allows for liquid water to exist on planets orbiting closer to them, therefore making these systems easier to study.


TOI-1798 c: A rare, extreme Super-Earth

TOI-1798 is an orange dwarf, or K-type star, with two planets: TOI-1798 b, a sub-Neptune that has an orbit of about eight days, and TOI-1798 c, a super-Earth that is so close to its host star, it completes one orbit in less than 12 hours. This rare planetary system is one of only a few star systems known to have an inner planet with an ultra-short period (USP) orbit.

“While the majority of planets we know about today orbit their star faster than Mercury orbits the Sun, USPs take this to the extreme. TOI-1798 c orbits its star so quickly that one year on this planet lasts less than half a day on Earth. Because of their proximity to their host star, USPs are also ultra hot — receiving more than 3,000 times the radiation that Earth receives from the Sun. Existing in this extreme environment means that this planet has likely lost any atmosphere that it initially formed,” says Polanski.

With the TESS-Keck Survey’s Mass Catalog, astronomers now have a new database to explore the latest research on worlds that TESS has detected; this paves the way for studying the variables and conditions of their environments in finer detail, particularly ones that could harbor life as we know it.

“There are still thousands of unconfirmed planets from the TESS mission alone, so large releases of new planets like this will become more common as astronomers work to get a handle on the diversity of worlds we see today,” says Crossfield.



Companion Papers

Subgiants Catalog: “The TESS-Keck Survey XXI: 13 New Planets and Homogeneous Properties for 21 Subgiant Systems” (Ashley Chontos et al.)

Individual Systems:


About HIRES
The High-Resolution Echelle Spectrometer (HIRES) produces spectra of single objects at very high spectral resolution, yet covering a wide wavelength range. It does this by separating the light into many “stripes” of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding exoplanets. Astronomers also use HIRES to study important astrophysical phenomena like distant galaxies and quasars, and find cosmological clues about the structure of the early universe, just after the Big Bang.

 About W. M. Keck Observatory
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, The University of California Observatories, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.


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Thursday, January 09, 2020

Goldilocks Stars Are Best Places to Look for Life

Comparison of G, K, and M Stars for Habitability
Credits: NASA, ESA, and Z. Levy (STScI) 
Science: NASA , ESA , and E. Guinan (Villanova University)


Orange Dwarf Stars Most Likely to Host Planets

To date astronomers have discovered over 4,000 planets orbiting other stars. Statistically, there should be over 100 billion planets in our Milky Way galaxy. They come in a wide range of sizes and characteristics, largely unimagined before exoplanets were first discovered in the mid-1990s. The biggest motivation for perusing these worlds is to find "Genesis II," a planet where life has arisen and evolved beyond microbes. The ultimate payoff would be finding intelligent life off the Earth.

A major step in searching for habitable planets is finding suitable stars that could foster the emergence of complex organisms. Because our Sun has nurtured life on Earth for nearly 4 billion years, conventional wisdom would suggest that stars like it would be prime candidates. But stars like our Sun represent only about 10% of the Milky Way population. What's more, they are comparatively short-lived. Our Sun is halfway through its estimated 10 billion-year lifetime.

Complex organisms arose on Earth only 500 million years ago. And, the modern form of humans has been here only for the blink of an eye on cosmological timescales: 200,000 years. The future of humanity is unknown. But what is for certain is that Earth will become uninhabitable for higher forms of life in a little over 1 billion years, as the Sun grows warmer and desiccates our planet.

Therefore, stars slightly cooler than our Sun — called orange dwarfs — are considered better hang-outs for advanced life. They can burn steadily for tens of billions of years. This opens up a vast timescape for biological evolution to pursue an infinity of experiments for yielding robust life forms. And, for every star like our Sun there are three times as many orange dwarfs in the Milky Way.

The only type of star that is more abundant are red dwarfs. But these are feisty little stars. They are so magnetically active they pump out 500 times as much radiation in the form of X-rays and ultraviolet light as our Sun does. Planets around these stars take a beating. They would be no place to call home for organisms like us.

An emerging idea, bolstered by stellar surveys performed by Hubble and other telescopes, is that the orange dwarfs are "Goldilocks stars" — not too hot, not too cool, and above all, not too violent to host life-friendly planets over a vast horizon of cosmic time.

In the search for life beyond Earth, astronomers look for planets in a star's "habitable zone" — sometimes nicknamed the "Goldilocks zone" — where temperatures are just right for liquid water to exist on a planet's surface to nurture life as we know it.

An emerging idea, bolstered by a three-decade-long set of stellar surveys, is that there are "Goldilocks stars" — not too hot, not too cool, and above all, not too violent to host life-friendly planets.

Because our Sun has nurtured life on Earth for nearly 4 billion years, conventional wisdom would suggest that stars like it would be prime candidates in the search for other potentially habitable worlds. In reality, stars slightly cooler and less luminous than our Sun, classified as K dwarfs, are the true "Goldilocks stars," said Edward Guinan of Villanova University, Villanova, Pennsylvania. "K-dwarf stars are in the 'sweet spot,' with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars). The K stars, especially the warmer ones, have the best of all worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life."

For starters, there are three times as many K dwarfs in our galaxy as stars like our Sun. Roughly 1,000 K stars lie within 100 light-years of our Sun as prime candidates for exploration. These so-called orange dwarfs live from 15 billion to 45 billion years. By contrast, our Sun, now already halfway through its lifetime, lasts for only 10 billion years. Its comparatively rapid rate of stellar evolution will leave the Earth largely uninhabitable in just another 1 or 2 billion years. "Solar-type stars limit how long a planet's atmosphere can remain stable," Guinan said. That's because a billion or so years from now, Earth will orbit inside the hotter (inner) edge of the Sun's habitable zone, which moves outward as the Sun grows warmer and brighter. As a result, the Earth will be desiccated as it loses its present atmosphere and oceans. By an age of 9 billion years the Sun will have swelled up to become a red giant that could engulf the Earth.

Despite their small size, the even more abundant red dwarf stars, also known as M dwarf stars, have even longer lifetimes and appear to be hostile to life as we know it. Planets that are located in a red dwarf's comparatively narrow habitable zone, which is very close to the star, are exposed to extreme levels of X-ray and ultraviolet (UV) radiation, which can be up to hundreds of thousands of times more intense than what Earth receives from the Sun. A relentless fireworks show of flares and coronal mass ejections bombard planets with a dragon's breath of seething plasma and showers of penetrating high-energy particles. Red dwarf habitable-zone planets can be baked bone dry and have their atmospheres stripped away very early in their lives. This could likely prohibit the planets from evolving to be more hospitable a few billion years after red dwarf outbursts have subsided. "We're not so optimistic anymore about the chances of finding advanced life around many M stars," Guinan said.

The K dwarfs do not have intensely active magnetic fields that power strong X-ray and UV emissions and energetic outbursts, and therefore they shoot off flares much less frequently, based on Guinan's research. Accompanying planets would get about 1/100th as much deadly X-ray radiation as those orbiting the close-in habitable zones of magnetically-active M stars.

In a program called the "GoldiloKs" Project, Guinan and his Villanova colleague Scott Engle, are working with undergraduate students to measure the age, rotation rate, and X-ray and far-ultraviolet radiation in a sampling of mostly cool G and K stars.They are using NASA's Hubble Space Telescope, Chandra X-ray Observatory, and the European Space Agency's XMM-Newton satellite for their observations. Hubble's sensitive ultraviolet-light observations of radiation from hydrogen were used to assess the radiation from a sample of about 20 orange dwarfs. "Hubble is the only telescope that can do this kind of observation," Guinan said.

Guinan and Engle found that the levels of radiation were much more benign to any accompanying planets than those found around red dwarfs. K stars also have longer lifetimes and therefore slower migration of the habitable zone. Therefore, K dwarfs seem like the ideal place to go looking for life, and these stars would allow time for highly evolved life to develop on planets. Over the Sun's entire lifetime — 10 billion years — K stars only increase their brightness by about 10-15%, giving biological evolution a much longer timespan to evolve advanced life forms than on Earth.

Guinan and Engle looked at some of the more interesting K stars hosting planets, including Kepler-442, Tau Ceti, and Epsilon Eridani. (The latter two were early targets of the late 1950s Project Ozma — the first attempt to detect radio transmissions from extraterrestrial civilizations.)

"Kepler-442 is noteworthy in that this star (spectral classification, K5) hosts what is considered one of the best Goldilocks planets, Kepler-442b, a rocky planet that is a little more than twice Earth's mass. So the Kepler-442 system is a Goldilocks planet hosted by a Goldilocks star!" said Guinan.

Over the last 30 years Guinan and Engle and their students have observed a variety of stellar types. Based on their studies, the researchers have determined relationships among stellar age, rotation rate, X-ray-UV emissions and flare activity. These data have been utilized to investigate the effects of high-energy radiation on planet atmospheres and possible life.

The results are being presented at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.



Contact:

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu

Edward Guinan
Villanova University, Villanova, Pennsylvania
edward.guinan@villanova.edu

Related Links: NASA's Hubble Portal

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