Exoplanets can reveal secrets about the limits of habitability


June 21, 2023

(Nanowerk News) How close is a rocky planet to a star, and still able to support water and life?

A newly discovered exoplanet may be the key to solving the mystery, providing important insight into 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 professor of astronomy in the College of Arts and Sciences (A&S).

Kaltenegger’s team found that LP 890-9c “super-Earth” (also called SPECULOS-2c), orbiting near the inner edge of the solar system’s habitable zone, would look very different depending on whether it still had warm oceans, a vapor atmosphere or if it has lost its water, assuming it once had oceans like Earth.

“Looking at the planet will tell us what’s happening at the inner edge of the habitable zone — how long rocky planets can maintain habitability when they start to get hot,” Kaltenegger said. “This will teach us something fundamental about how rocky planets evolve with increasing starlight, and about what this will one day happen to us and Earth.” Artist's impressions show the potential evolution of exoplanet LP 890-9c from hot Earth to dry Venus Artist’s impressions show the potential evolution of exoplanet LP 890-9c from hot Earth to dry Venus. (Image: Carl Sagan Institute/R. Payne)

Kaltenegger is the lead author of the paper published in Monthly Notices of the Royal Astronomical Society: Letters (“Hot Earth or Young Venus? The Mystery of a Stone Planet Transiting Nearby”). 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, a postdoctoral researcher at the University of Liège in Belgium, who led the international team reporting the discovery of LP 890-9c in September 2022.

Companion paper (Monthly Notices of the Royal Astronomical Society: Letters, “Venus in the Making? Predictions for JWST Observations of Ultracool M-Dwarf Planet LP 890-9c”) led by Jonathan Gomez Barrientos ’22, a graduate student at the California Institute of Technology, demonstrated that NASA’s James Webb Space Telescope (JWST) can distinguish between the potential atmospheres of different exoplanets, making it a prime target for reliable observatories. Kaltenegger is co-author with Ryan J. MacDonald, a former research associate at Cornell and now a NASA Sagan Fellow at the University of Michigan.

LP 890-9c is one of two super-Earths orbiting a red dwarf located 100 light years from Earth, Delrez’s team – including Kaltenegger – announced last year. (NASA’s Transiting Exoplanet Survey satellite previously identified LP 890-9b.) They say liquid water or a water vapor-rich atmosphere is possible at LP 890-9c, which is about 40% larger than Earth and surrounds a small, cool star at 8 ,5 days.

These criteria suggest it to be one of JWST’s best targets for study among terrestrial planets known to be potentially habitable, other than the TRAPPIST-1 system.

“Professor Kaltenegger and I thought this exoplanet might be an excellent target for JWST,” said Barrientos, “but we have now proven this hypothesis, and that LP 890-9c has the potential to reveal if life may exist at the very edges of the zone. habitable.”

Her team’s model is the first to detail differences in the chemical signatures produced by rocky planets near the interior limits of the habitable zone, based on variables including planetary size, mass, chemical makeup, surface temperature and pressure, atmospheric height, and cloud cover. Calculations are key to estimating how much time it will take for JWST to confirm the basic composition of the atmosphere – if at all.

The model includes several scenarios thought to reflect the stages of rocky planet evolution, from a “hot Earth” where life was possible, to desolate Venus with an atmosphere of carbon dioxide. Among the phases Earth is expected to experience when the sun gets brighter and hotter with age, causing the oceans to gradually evaporate and fill the atmosphere with steam before boiling it all over.

How long this process took is unknown, and astronomers say LP 890-9c provides a rare opportunity to explore this evolution.

“This planet is the first target where we can test these scenarios,” Kaltenegger said. “If Earth were still warmer – hot, but with liquid water and conditions for life – then the inner edge of the habitable zone might be teeming with life. If we see that it’s already a full Venus, then the water could be gone much quicker than we think.

In a companion paper, Kaltenegger and colleagues propose that JWST can confirm the presence of an atmosphere – and whether it is primarily water vapor – in just three transits, or planetary passages across their parent star. If further observations are needed, 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 is possible that LP 890-9c has no atmosphere and no life, or is similar to Venus with thick clouds that block reflected light and thus yield little information. A deeper investigation promises to provide valuable clues, says Kaltenegger.

“We don’t know what a planet that is on the verge of habitability will look like, so we have to look for it,” he said. “This is what true exploration is all about.”


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