They discover water in an unknown and enigmatic exoplanet: could it be habitable?

The most common stars in the universe are red dwarf stars, which means rocky exoplanets are more likely to be found orbiting such a star. Red dwarf stars are cold, so a planet must “squeeze” them into a tight orbit to stay hot enough to potentially host liquid water.

These stars are also active, especially when young, emitting ultraviolet and X-ray radiation that could destroy planetary atmospheres. As a result, An important question in astronomy is whether a rocky planet could maintain or restore an atmosphere in such a hostile environment.

To help answer this question, astronomers used the James Webb Space Telescope, led by the JAR with its partners, European Space Agency (ESA) and the Canadian Space Agency (CSA) , study a rocky exoplanet known as GJ 486 b . It is too close to its star to be in the habitable zone, with a surface temperature of around 430°C. And yet, his observations using the near infrared spectrograph Webb (NIRSpec) shows evidence of water vapor.

If water vapor is associated with the planet, that would indicate that it has an atmosphere despite its scorching temperature and proximity to its star. Water vapor has already been seen on gaseous exoplanets, but to date no atmosphere has been definitively detected around a rocky exoplanet. However, the team warns that the water vapor could be on the star itself, especially on cool stars, and not on the planet at all.

“We’re seeing a signal, and it’s almost certainly due to the water. But we can’t say yet if this water is part of the planet’s atmosphere, which means the planet To an atmosphere, or if we just see a water signature coming from the star,” he said. Sarah Moran of the University of Arizona in Tucson lead author of the study, which will be published in Letters from the Astrophysical Journal .

“Water vapor in the atmosphere of a hot rocky planet would represent a breakthrough for exoplanet science. But we have to be careful and make sure the star is not to blame.” Kevin Stevenson of the Applied Physics Laboratory at Johns Hopkins University in Laurel, Maryland, who is also principal investigator for the program .

GJ 486 b is about 30% larger than Earth and three times as massive, which means it is a rocky world with a stronger gravity than Earth. It orbits a red dwarf star in just under 1.5 Earth days. It should be closed at tide, with a permanent day side and a permanent night side.

Said exoplanet transits its star, crossing in front of the star from our point of view. SIf it has an atmosphere, then when it transits, starlight would filter through those gases, imprint fingerprints in light that allow astronomers to decode its composition using a technique called spectroscopy transmission .

The team observed two transits, each lasting about an hour. They then used three different methods to analyze the data obtained. The results for all three are consistent in that they show an essentially flat spectrum with an intriguing increase at shorter infrared wavelengths. The team ran computer models looking at several different molecules and concluded that the most likely source of the signal was water vapour.

While water vapor could potentially indicate the presence of an atmosphere in GJ 486 b, an equally plausible explanation is water vapor from the star. Surprisingly, even on our own Sun, water vapor can sometimes exist in sunspots because these spots are very cold compared to the star’s surrounding surface. GJ 486 b’s host star is much cooler than the Sun, so even more water vapor would be concentrated in its star spots. As a result, it could create a signal that mimics a planetary atmosphere.

“We observed no evidence that the planet crossed any star points during the transits. But that doesn’t mean there aren’t spots on other parts of the star. And that’s exactly the physical scenario that would imprint this water signal in the data and it could end up looking like a planetary atmosphere.” Ryan MacDonald of the University of Michigan at Ann Arbor, one of the study’s co-authors, explained in a statement.

A water vapor atmosphere would be expected to gradually erode due to stellar heating and irradiation. Therefore, if an atmosphere is present, it should probably be constantly renewed. by volcanoes that expel steam from the interior of the planet. If water is indeed found in the planet’s atmosphere, additional observations are needed to reduce the amount of water present.

Leidy Peña, astronomer at Center for Astrophysics and Related Technologies (CATA) and some Design University explains that for a planet to be habitable it must be considered that it has certain primordial conditions. “ Of course, it’s relevant that it has an atmosphere that shields it from the host star’s radiation to help it maintain a temperature balance. But in addition, the planet must be in a stellar zone around that same star in which the incident radiation flux from that star allows liquid water to exist on the surface of that rocky planet. However, the planet must also have a considerable mass, no less than 0.5 times the mass of Earth. Nor should this planet have a slow rotation that causes very hot days and cold nights or that it should have a very extreme tilt that lengthens the seasons considerably and also that it should have a magnetic field that protects it and therefore other characteristics”, explains the researcher. .

For his part, he astronomer of Faculty of Physical and Mathematical Sciences of the University of Chile, César Fuentes , comments that the habitability condition of a planet, in the context of planets around stars other than the Sun, it has to do with the equilibrium temperature around that star and one thinks or determines what allows the existence of liquid water, i.e. between 0 and 100 degrees Celsius .

“In this case it does not correspond to a habitable planet, however, finding water on its surface, even in a gaseous state, is significant and requires further observation. of James Webb at different wavelengths to be sure that it corresponds to an atmosphere, that it would be the first to be around a rocky exoplanet, and that indeed the molecule on its surface is water. It’s important because it’s the first step to finding water around what an Earth analogue can do around another star.” expert chart.

Moreover, Fuentes details that the discovery of water and the atmosphere in a rocky planet could be the first of its kind , because exoplanets with these characteristics are usually gigantic and hot, like Jupiter. One of the plausible explanations for the existence of water, “It is very likely that this planet has volcanoes and that these volcanoes expel steam inside the planet. But, of course, it would also take a lot more observations to be able to say something like that. Pena’s comments.

Future Webb observations may shed more light on this system. A Webb’s Next Show will use the mid infrared instrument (MIRI) to observe the day side of the planet. If the planet has no atmosphere, or only a thin atmosphere, the hottest part of the day side should be directly under the star. . However, if the hottest spot is moved, it would indicate an atmosphere that can circulate heat.

Ultimately, observations at shorter infrared wavelengths will be needed by another Webb instrument, the near-infrared imager and slitless spectrograph (NIRISS), to differentiate between planetary atmosphere and star spot scenarios. “It’s about putting together several instruments that will really determine whether or not this planet has an atmosphere,” Stevenson said.