【Dear Utol: Catfish Episode 46】

Astronomers have Dear Utol: Catfish Episode 46added two more exoplanetsto a growing study aimed at finding out whether rocky worlds near small, faint stars might have air.

Scientists leading a high-priority observing program with NASA’s James Webb Space Telescopehave added two more rocky planets outside the solar system to their shortlist. These distant worlds — LHS 1140 b and LTT 1445 Ab — both orbit red dwarfs. These stars, also called M-dwarfs, are smaller and cooler than the sun, but far more common in space, making them intriguing places to look for potentially habitable conditions.

The new pair brings the number of targets in the project, first reported by Mashable, to four, though the researchers expect to eventually look at about a dozen exoplanets. 

The campaign will use Webb to look for signs of carbon dioxide, a heat-trapping gas, using a novel method for studying atmospheres, called the secondary eclipse technique. Meanwhile, the Hubble Space Telescopewill focus on the stars themselves, studying their ultraviolet radiation output. Red dwarfs are ubiquitous in the Milky Way, but they’re also known for flaring and blasting out harmful ultraviolet light. That could strip a planet bare, leaving behind nothing but a dead rock.

"The answer for 'Are we special ... or not?' can start to be answered if we start uncovering atmospheres of these rocky exoplanets around M-dwarfs," said Néstor Espinoza, an astronomer heading the implementation team. 

Exoplanets targeted for atmosphere study

LTT 1445 Abis roughly the size of Earth and about 22 light-yearsaway in the constellation Eridanus. The exoplanet orbits the star LTT 1445 A, which, in turn, orbits two other stars in a triple-star system. LHS 1140 bis a bit larger and heavier than Earth, located about 49 light-years away in the constellation Cetus. 

Both were discovered within the past eight years, but LHS 1140 b was the focus of a recent Webb study that suggested the planet could have an ocean half the size of the Atlantic and maybe even a thick, nitrogen-rich atmosphere like Earth. 

"We'll need more JWST observations to verify an atmosphere," said Ryan MacDonald, one of the study's coauthors, on X last year, "but LHS 1140b is looking like one of the most promising potentially habitable super-Earths." 

These two selected exoplanets join GJ 3929 b and LTT 1445 Ac, a sibling to LTT 1445 Ab within the same star system, in the Rocky Worlds program. The first observations could begin as early as the end of this week.

There's a key difference between the first four targets and Earth: These alien worlds are so close to their stars, their years are only a few Earth-days long. That may not be a problem, given that their host stars aren't as hot as the sun, but could atmospheres even survive the constant barrage of harsh radiation? Getting to the bottom of that fundamental question is why this program exists, Espinoza told Mashable. 

"If we end up finding that these M-dwarfs have atmospheres, then immediately the likelihood of maybe life can emerge in M-dwarfs is more likely there than here," he said, comparing them to our own solar system, "perhaps because you will have many more possibilities of generating different types of atmospheres."

Scientists have found signs of atmospheres surrounding many of the 5,900exoplanets discovered so far, but all of them have surrounded gas giant planets, like Jupiter, with air mostly made of hydrogen. The hunt for a more terrestrial world shielded by a protective atmosphere has so far eluded astronomers.

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Jennifer Lotz, who directs Webb and Hubble's operations at the Space Telescope Science Institute in Baltimore, decided last year to initiatethe rocky exoplanet survey using the director's discretionary time, the same way revolutionary science campaigns like the Hubble deep field imagescame to fruition. 

Secondary eclipse technique explained

Webb usually observes exoplanets through transmission spectroscopy, a technique for studying what molecules are present in an atmosphere by analyzing how starlight filters through it. But that method has had its drawbacks. If the starlight were completely uniform, that would be one thing, but red dwarf stars can get sunspots, causing variability in the signals. This problem, called stellar contamination, has recently led Webb scientists to embrace another technique, known as secondary eclipse observations. 

With secondary eclipses, it's a game of hide-and-seek. Scientists measure the signals of the red dwarf and planet when they're next to each other in space from the telescope's vantage point. Then, when the planet travels behind the star, scientists collect the star's signal alone while the planet is blocked. By subtracting the star from the total, the researchers can then isolate the light coming from the planet. Teams will use a particular wavelength filter that can detect carbon dioxide, thought to be a likely atmospheric gas.

Scientists will also take thermal measurements to get an early sense of whether an atmosphere could be present. If the temperature is lower than expected, it's a strong indication that a thick atmosphere is distributing energy from the planet's dayside — the hemisphere facing the star — to the nightside. 

More targets are expected to be announced before October, when the next round of telescope proposals is due.