The evaporating atmosphere of sub-neptune exoplanet TOI-1759b could tell us how habitable planets form


Astronomers sifting through data from a NASA planet-hunting satellite have hit the jackpot: a common type of planet 131 light-years away called TOI-1759b, with an extremely rare story to tell. Thanks to its proximity to its host star, the planet’s atmosphere appears to be evaporating into a dead expanse of space, extremely fast by astronomical standards.

Looking at this radiation-induced “photo-evaporation,” reported in a new study led by Eder Martioli at the Laboratório Nacional de Astrofísica in Brazil, could help us understand one of the weirdest mysteries in modern astronomy. As we continue to discover more planets beyond our solar system – what we call exoplanets – there is an inexplicable absence of Neptune-sized or slightly smaller (“sub-Neptunes”) planets on orbits close to their host stars.

Just out of sheer probability, we should discover more of these medium-sized planets that manage to complete full orbits around their host stars in just a few days. Instead, we end up with a planetary dead zone for the middle planets that astronomers call the “sub-Neptune wasteland.” All of this begs the question: how can there be so many medium-sized planets out there, but if little medium-sized planets orbiting close to their stars?

It’s not just a compelling question for astronomers interested in planetary formation. Measuring atmospheric evaporation could also help us narrow our search for extraterrestrial life in the universe – something many more people can stay behind.

An artist’s conception of HD 209458b, another exoplanet known to lose its atmosphere due to its close orbit to its sun.

NASA, European Space Agency, Alfred Vidal-Madjar (Paris Institute of Astrophysics, CNRS)

One theory is that the sub-Neptunes are just the right size, density, and composition so that when exposed to the hot light of a very nearby star, they simply cannot retain their atmosphere. All that energized hydrogen goes up and out.

On more gaseous, less dense planets with lower gravities, stellar radiation can trigger a process astronomers call “hydrogen escape” that eventually removes all of the gas that makes up most of the atmosphere, leaving behind inert rocky cores that are only a fraction of the planet’s original size.

Larger planets, on the other hand, tend to cling more tightly to their atmospheres, preventing hydrogen from escaping. Smaller planets with higher gravity, including Earth, also benefit from this protection. A small amount of hydrogen is escaping from our own planet’s upper atmosphere, but the escape isn’t fast enough to really matter, so we’re all still warm and fuzzy here.

It seems to be the unique fate of near-orbiting sub-Neptunes to lose their atmosphere and turn into smaller, bare rocks. The whole process can take hundreds of millions or even billions of years.

Everywhere we look in space, we see these tiny, rocky planets in close orbit – the possible remnants of sub-Neptunes that experienced possible stellar rubbing potentially millions or billions of years ago. “Fossils,” Martioli called them.

The hydrogen leak could turn TOI-1759b into one of those fossils, robbing our galaxy of another of its many middle planets and further expanding the sub-Neptunian wasteland.

“If we catch this process in action by detecting the hydrogen leak in TOI-1759b, we should be able to quantify the contribution of this process” to the creation of the desert, Martioli told The Daily Beast.

Martioli and his team found TOI-1759b in tons of data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS), launched in 2018. They announced their discovery in their new paper, a preprint of which has appeared online this month (although it has not yet been peer-reviewed).

TESS’ high-tech camera clutch can inspect distant stars and search for the shadows of passing planets. Over the past two decades, this “transit” survey method has found nearly 5,000 distant exoplanets in star systems other than our own. Their light signatures, recorded with ultraviolet filters, can indicate the movement of gases in their atmospheres.

At least half of all known exoplanets fall between the size of Earth and Neptune, the sub-Neptune range. But only a handful of sub-Neptunes orbit close to their stars. A lucky sub-Neptune with a narrow orbit, NGTS-4b, appears stable, possibly because it has an unusually massive core that clings more tightly to its atmosphere. There is no observable hydrogen escape.

TOI-1759b is farther from its star than NGTS-4b — in fact, TOI-1759b is just outside the normal sub-Neptunian desert boundaries, according to George McDonald, an astronomer at Rutgers. Importantly, the ultraviolet snapshots indicate that it is losing its atmosphere. It is perhaps the best place to observe an atmosphere evaporating in real time since it is so close to us (131 light years, it’s just a stone’s throw into space).

Artist illustration of TESS.


Ideally, someone could point the new James Webb Space Telescope at TOI-1759b, Martioli said. But he’s not counting on that, believing the new space telescope’s 21-foot-wide mirror is overkill for this kind of study. “This planet’s evaporative atmosphere should be easily detectable even with ground-based facilities,” he said.

But spotting the process of hydrogen escape on a distant planet is one thing. Calculate how much many the hydrogen escapes, and how fast, is another matter. “Escaped hydrogen is usually difficult to measure,” Étienne Artigau, a University of Montreal astrophysicist and member of Martioli’s team, told The Daily Beast.

Mathematical hydrogen escape models, based in part on data from Earth’s own atmosphere and the atmospheres of planets and moons in our solar system, still include many assumptions and may not always apply. to distant exoplanets.

Confirming and measuring photoevaporation on TOI-1759b could help us refine our hydrogen escape models and, by extension, our planetary evolution models as well. Not only could it help us spot planets that are lose their atmospheres and make the sub-Neptune a bit less mysterious, it could also tell us where to look for planets that are not lose their atmospheres.

This has obvious implications for our ever-growing search for extraterrestrial life. As far as we know, a stable atmosphere is a prerequisite for biological evolution.

TOI-1759b’s star is a red dwarf, making it smaller and cooler than our own sun, a yellow dwarf. There are many red dwarf systems around us in the Milky Way galaxy. Many of these systems have planets slightly smaller than the sub-Neptunes but slightly bigger than the Earth, called “super-Earths”.

“The very different properties of red dwarfs compared to sun-like stars mean that there are a lot of unknowns about how these planets are truly habitable,” McDonald said.

If astronomers can calculate the hydrogen escape rate on TOI-1759b, they can begin to guess which red dwarf orbiting super-Earths might have very moo escape rates – and possibly stable atmospheres where life could thrive.

These may be the first clues to help finally confirm whether or not we are alone in the universe. And it could all start with an unfortunate sub-Neptune that is pumping a lot of gas into space.


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