Exoplanet may reveal secrets about the edge of habitability

How close can a rocky planet be to a star, and still sustain water and life? (by James Dean, Cornell Chronicle)

A recently discovered exoplanet may be key to solving that mystery, providing important insights about conditions at the inner edge of a star’s habitable zone and why Earth and Venus developed so differently, according to new research led by Lisa Kaltenegger, director of the Carl Sagan Institute and associate professor of astronomy in the College of Arts and Sciences (A&S).

Kaltenegger’s team found that the “super-Earth” LP 890-9c (also named SPECULOOS-2c), which orbits close to the inner edge of its solar system’s habitable zone, would look very different depending on whether it still had warm oceans, a steam atmosphere or if it had lost its water, assuming it once had oceans like Earth’s.

“Looking at this planet will tell us what’s happening on this inner edge of the habitable zone – how long a rocky planet can maintain habitability when it starts to get hot,” Kaltenegger said. “It will teach us something fundamental about how rocky planets evolve with increasing starlight, and about what will one day happen to us and Earth.”

Kaltenegger is the lead author of “Hot Earth or Young Venus? A Nearby Transiting Rocky Planet Mystery,” published June 21 in Monthly Notices of the Royal Astronomical Society: Letters. Co-authors are Rebecca Payne, research associate in the Department of Astronomy (A&S); Zifan Lin ’20, doctoral student at the Massachusetts Institute of Technology; James Kasting, professor emeritus at Pennsylvania State University; and Laetitia Delrez, postdoctoral researcher at the University of Liège in Belgium, who led an international team that reported the discovery of LP 890-9c in September 2022.

A companion paper led by Jonathan Gomez Barrientos ’22, a graduate student at the California Institute of Technology, demonstrates that NASA’s James Webb Space Telescope (JWST) could distinguish between the exoplanet’s potential different atmospheres, making it a prime target for the flagship observatory. Kaltenegger is a co-author with Ryan J. MacDonald, a former research associate at Cornell and now a NASA Sagan Fellow at the University of Michigan, of “A Venus in the Making? Predictions for JWST Observations of the Ultracool M-Dwarf Planet LP 890-9c.”

LP 890-9c is one of two super-Earths orbiting a red dwarf star located 100 light years from Earth, the Delrez team – which included Kaltenegger – announced last year. (NASA’s Transiting Exoplanet Survey Satellite had previously identified LP 890-9b.) They said liquid water or an atmosphere rich in water vapor was possible on LP 890-9c, which is about 40% larger than Earth and circles the small, cool star in 8.5 days.

Those criteria suggested it to be one of the best targets for JWST to study among the known, potentially habitable terrestrial planets, in addition to the TRAPPIST-1 system.

“Professor Kaltenegger and I were thinking this exoplanet might be an excellent target for JWST,” Barrientos said, “but now we’ve proven this hypothesis, and that LP 890-9c may potentially reveal if life is possible on the edge – the inner edge of the habitable zone.”

Her team’s models are the first to detail differences in the chemical signatures generated by rocky planets near the habitable zone’s interior boundary, based on variables including the planet’s size, mass, chemical makeup, surface temperature and pressure, atmospheric height and cloud cover. The calculations were key to estimating how much time JWST would need to confirm the basic composition of an atmosphere – if there is one.

The models span several scenarios thought to reflect stages of rocky planets’ evolution, ranging from a “hot Earth” where life might still be possible, to a desolate Venus featuring a carbon dioxide atmosphere. In between are phases Earth is expected to experience as the sun grows brighter and hotter with age, causing the oceans to gradually evaporate and fill the atmosphere with steam before boiling off entirely.

How long those processes might take is unknown, and the astronomers say LP 890-9c provides a rare opportunity to explore that evolution.

“This planet is the first target where we can test these different scenarios,” Kaltenegger said. “If it’s still a hotter Earth – hot, but with liquid water and conditions for life – then the inner edge of the habitable zone could be teeming with life. If we see that it’s already a full-blown Venus, then water can get lost faster than we anticipate.”

In the companion paper, Kaltenegger and colleagues propose that JWST could confirm the presence of an atmosphere – and whether it is primarily water vapor – in as few as three transits, or passages of the planet across its host star. If further observation is warranted, they estimate a total of eight transits could detect a Venus-like atmosphere, while 20 transits could find evidence of a potentially habitable “hot Earth” scenario.

It’s possible that LP 890-9c has no atmosphere and hosts no life, or that it resembles a Venus with thick clouds that would block light from reflecting and thus yield little information. Deeper investigation promises to provide valuable clues, Kaltenegger said.

“We don’t know what this planet on the edge of habitability could be like, so we have to look,” she said. “This is what real exploration is about.”

Jurassic worlds might be easier to spot than modern Earth

Might a tyrannosaur roam on Trappist-1e, a protoceratops on Proxima Centauri b, or a quetzalcoatlus on Kepler 1047c? (by James Dean, Cornell Chronicle)

Things may not have ended well for dinosaurs on Earth, but Cornell astronomers say the “light fingerprint” of the conditions that enabled them to emerge here – including abundant atmospheric oxygen – provides a crucial missing piece in our search for signs of life on planets orbiting other stars. Click to view

Modeling by Cornell astronomers finds that telescopes could more easily detect an exoplanet with higher levels of atmospheric oxygen than modern Earth, as existed during the dinosaur age.

Their analysis of the most recent 540 million years of Earth’s evolution, known as the Phanerozoic Eon, finds that telescopes could better detect potential chemical signatures of life in the atmosphere of an Earth-like exoplanet more closely resembling the age the dinosaurs inhabited than the one we know today.

Two key biosignature pairs – oxygen and methane, and ozone and methane – appeared stronger in models of Earth roughly 100 million to 300 million years ago, when oxygen levels were significantly higher. The models simulated the transmission spectra, or light fingerprint, generated by an atmosphere that absorbs some colors of starlight and lets others filter through, information scientists use to determine the atmosphere’s composition.

“Modern Earth’s light fingerprint has been our template for identifying potentially habitable planets, but there was a time when this fingerprint was even more pronounced – better at showing signs of life,” said Lisa Kaltenegger, director of the Carl Sagan Institute (CSI) and professor of astronomy in the College of Arts and Sciences (A&S). “This gives us hope that it might be just a little bit easier to find signs of life – even large, complex life – elsewhere in the cosmos.”

Kaltenegger is co-author of “Oxygen Bounty for Earth-like Exoplanets: Spectra of Earth Through the Phanerozoic,” published Nov. 2 in Monthly Notices of the Royal Astronomical Society: Letters. The first author, Rebecca Payne, research associate at CSI and in the Department of Astronomy (A&S), led the new models that detail a critical epoch including the origins of land plants, animals and dinosaurs. Over that period, atmospheric oxygen ranged from below 10% to as high as 35% before stabilizing at the contemporary level of 21%.

Using estimates from two established climate models (called GEOCARB and COPSE), the researchers simulated Earth’s atmospheric composition and resulting transmission spectra over five 100-million-year increments of the Phanerozoic. Each features significant changes as a complex biosphere diversified, forests proliferated and terrestrial biospheres flourished, influencing the mix of oxygen and other gasses in the atmosphere.

“The Phanerozoic is just the most recent 12% or so of Earth’s history, but it encompasses nearly all of the time in which life was more complex than microbes and sponges,” said Payne, an astrobiologist and geologist. “These light fingerprints are what you’d search for elsewhere, if you were looking for something more advanced than a single-celled organism.”

For most of the past 400 million years, oxygen is believed to have ranged within the charcoal “fire window” of 16% to 35%: Any less and fires couldn’t ignite, any more and they couldn’t be extinguished. Its estimated peak around 30%, some 300 million years ago, is thought to have made possible the emergence of large, complex creatures like dinosaurs, which lived from roughly 245 million to 66 million years ago.

While similar evolutionary processes may or may not unfold on exoplanets, Payne and Kaltenegger said their models fill in the missing puzzle piece of what a Phanerozoic Earth would look like to a telescope, creating new templates for habitable planets with varying atmospheric oxygen levels.

Kaltenegger pioneered modeling of what Earth would look like to faraway observers based on changes over time in its geology, climate and atmosphere – our “ground truth,” she said, for identifying potential evidence of life on other worlds.

To date, about 40 rocky exoplanets have already been discovered in habitable zones where oceans could exist, Kaltenegger said. Analyzing an exoplanet’s atmosphere – if it has one – is at the edge of technical capability for NASA’s James Webb Space Telescope, but is now a possibility. However, the researchers said, scientists need to know what to look for. Their models identify planets like Phanerozoic Earth as extremely promising targets for finding life in the cosmos.

They also allow scientists to entertain the possibility – purely theoretical – that if a habitable exoplanet is discovered to have an atmosphere with 30% oxygen, life there might not be limited to microbes, but could include creatures as large and varied as the megalosauruses or microraptors that once roamed Earth.

“If they’re out there,” Payne said, “this analysis lets us figure out where they could be living.”

Dinosaurs or not, the models confirm that from a great distance, such a planet’s light fingerprint would stand out more than a modern Earth’s.

“Hopefully we’ll find some planets that happen to have more oxygen than Earth right now, because that will make the search for life just a little bit easier,” Kaltenegger said. “And who knows, maybe there are other dinosaurs waiting to be found.”

The authors thanked the Carl Sagan Institute and the Brinson Foundation for supporting the research.

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