Color catalog aids hunt for life on frozen worlds + 3 exoplanets are actually stars

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Tint of life: Color catalog built to find frozen worlds

RELEASE: Aided by microbes found in the subarctic conditions of Canada’s Hudson Bay, an international team – including researchers from Portugal’s Instituto Superior de Agronomia and Técnico, Canada’s Université Laval in Quebec, and Cornell – has created the first color catalog of icy planet surface signatures to uncover the existence of life in the cosmos.

As ground-based and space telescopes get larger and can probe the atmosphere of rocky exoplanets, astronomers need a color-coded guide to compare them and their moons to vibrant, tinted biological microbes on Earth, which may dominate frozen worlds that circle different stars.

But researchers need to know what microbes that live in frigid places on Earth look like before they can spot them elsewhere. The study, “Color Catalogue of Life in Ice: Surface Biosignatures on Icy Worlds,” published March 15 in the journal Astrobiology, provides this toolkit.

“On Earth, vibrant, biological colors in the Arctic represent signatures of life in small, frozen niches,” said lead author Lígia F. Coelho, an astrobiologist and a doctoral student at the faculty laboratories of Zita Martins, João Canário and Rodrigo Costa at Técnico, who grew and measured this frigid, colorful biota at the Carl Sagan Institute at Cornell (CSI).

This summer, Coelho will become a postdoctoral researcher in the lab of Lisa Kaltenegger, professor in the Department of Astronomy and director of CSI in the College of Arts and Sciences. “The colors from organisms could dominate the whole surface of icy worlds,” Coelho said. “Frozen exoplanets are not lost causes. With upcoming telescopes you could find the telltale signs of microbes – if you know what to look for. That’s why we’ve created this catalog.”

Coelho collected 80 microorganisms from ice and water at Kuujjuarapik, Quebec, working across the frozen Hudson Bay, obtaining ice cores and drilling holes in the ice to take water samples. She acquired samples at the mouth of the Great Whale River in February 2019, during an excursion in collaboration with a team from Université Laval, Canada, and logistic support of the Centre d´étude Nordiques (CEN).

“When searching for life in the cosmos, microbes in these frozen plains of the Arctic give us crucial insight of what to look for on cold new worlds,” Kaltenegger said, explaining that some of this icy microbial life is well-adapted to the harsh radiation bombardment of space – which can be the norm on distant exoplanets under a red sun.

“Having the right tools to detect life forms on icy worlds is fundamental,” said Martins, director of the astrobiology laboratory and professor at Técnico, who came up with the idea for this research with Kaltenegger. “Our study shows that biosignatures are more intense in drier environments, suggesting that locations that are drier than Earth and contain microbial life forms could represent good targets for future space missions.”

After Coelho isolated the microorganisms in Rodrigo Costa’s lab (Técnico), she grew them in upstate New York’s relatively balmy climate in the laboratory of Stephen Zinder, professor emeritus of microbiology in the College of Agriculture and Life Sciences. The measurements to find out how these microbes would look to our telescopes were made in the laboratory of William Philpot, professor in the School of Civil and Environmental Engineering.

”Icy environments on Earth show a surprisingly wide diversity of life and might have even provided the environment for life to originate,” Coelho said. “The color catalog of life on Earth’s subarctic will serve as the guide to search for surface life on icy worlds throughout all the habitable zone, not just in the warm parts.”

Said Kaltenegger: “We are assembling the tools to search for life in the universe, so as not to miss it, taking all of Earth’s vibrant biosphere into account – even those in the breathtaking chilled places of our Pale Blue Dot.”

In addition to Coelho, Kaltenegger, Zinder, Philpot, Jack Madden, Ph.D. ’20, Martins, Canário and Costa (Técnico), M. Glória Esquível (ISA) and Warwick Vincent (U. Laval/CEN) were co-authors on the research.

Look! Up in the sky! Is it a planet? Nope, just a star

RELEASE: The first worlds beyond our solar system were discovered three decades ago. Since then, close to 5,000 exoplanets have been confirmed in our galaxy. Astronomers have detected another 5,000 planetary candidates — objects that might be planets but have yet to be confirmed. Now, the list of planets has shrunk by at least three.

In a study appearing today in the Astronomical Journal, MIT astronomers report that three, and potentially four, planets that were originally discovered by NASA’s Kepler Space Telescope are in fact misclassified. Instead, these suspected planets are likely small stars.

The team used updated measurements of planet-hosting stars to double-check the size of the planets, and identified three that are simply too big to be planets. With new and better estimates of stellar properties, the researchers found that the three objects, which are known as Kepler-854b, Kepler-840b, and Kepler-699b, are now estimated to be between two and four times the size of Jupiter.

“Most exoplanets are Jupiter-sized or much smaller. Twice [the size of] Jupiter is already suspicious. Larger than that cannot be a planet, which is what we found,” says the study’s first author Prajwal Niraula, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences.

A fourth planet, Kepler-747b, is about 1.8 times Jupiter’s size, which is comparable to the very largest confirmed planets. But Kepler-747b is relatively far from its star, and the amount of light it receives is too small to sustain a planet of its size. Kepler-747b’s planetary status, the team concludes, is suspect but not entirely implausible. 

“Overall, this study makes the current list of planets more complete,” says study author Avi Shporer, a research scientist at MIT’s Kavli Institute for Astrophysics and Space Research. “People rely on this list to study the population of planets as a whole. If you use a sample with a few interlopers, your results may be inaccurate. So, it’s important that the list of planets is not contaminated.”

The study’s co-authors also include Ian Wong, NASA Postdoctoral Program Fellow at NASA Goddard Space Flight Center, and MIT Assistant Professor Julien de Wit.

Stellar updates. Rooting out planetary imposters was not the team’s initial goal. Niraula originally intended to look for systems with signs of tidal distortion. “If you have two objects close to each other, the gravitational pull of one will cause the other to be egg-shaped, or ellipsoidal, which gives you an idea of how massive the companion is,” Niraula explains. “So you could determine whether it’s a star-star or star-planet system, just based on that tidal pull.”

When combing through the Kepler catalog, he came upon a signal from Kepler-854b that appeared too large to be true. “Suddenly we had a system where we saw this ellipsoidal signal which was huge, and pretty immediately we knew this could not be from a planet,” Shporer says. “Then we thought, something doesn’t add up.”

The team then took a second look at both the star and the planetary candidate. As with all Kepler-detected planets, Kepler-854b was spotted through a transit detection — a periodic dip in starlight that signals a possible planet passing in front of its star. The depth of that dip represents the ratio between the size of the planet and that of its star. Astronomers can calculate the planet’s size based on what they know of the star’s size. But as Kepler-854b was discovered in 2016, its size was based on stellar estimates that were less precise than they are today.

Currently, the most accurate measurements of stars comes from the European Space Agency’s Gaia mission, a space-based observatory that is designed to precisely measure and map the properties and paths of stars in the Milky Way. In 2016, Gaia’s measurements of Kepler-854 were not yet available. Given the stellar information that was available, the object seemed to be a plausible-sized planet. But Niraula found that with Gaia’s improved estimates, Kepler-854b turned out to be much larger, at three times the size of Jupiter.

“There’s no way the universe can make a planet of that size,” Shporer says. “It just doesn’t exist.”

Tiny corrections. The team confirmed that Kepler-854b was a planetary “false positive” — not a planet at all, but instead, a small star orbiting a larger host star. Then they wondered: Could there be more?

Niraula searched through the Kepler catalog’s more than 2,000 planets, this time for significant updates to the size of stars provided by Gaia. He ultimately discovered three stars whose sizes significantly changed based on Gaia’s improved measurements. From these estimates, the team recalculated the size of the planets orbiting each star, and found them to be about two to four times Jupiter’s size.

“That was a very big flag,” Niraula says. “We now have three objects that are now not planets, and the fourth is likely not a planet.”

Going forward, the team anticipates that there won’t be many more such corrections to existing exoplanet catalogs. “This is a tiny correction,” Shporer says. “It comes from the better understanding of stars, which is only improving all the time. So, the chances of a star’s radius being so incorrect are much smaller. These misclassifications are not going to happen many times more.”

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