For the first time, in the atmosphere of an exoplanet just 111 light-years distant from Earth , astronomers have made a very exciting discovery: they have detected water . 50% of K2-18b’s atmosphere could be water vapor; But unlike other giant exoplanets in which atmospheric water has been detected, K2-18b is a super Earth. It could be rocky, like Earth. The finding has been published in the journal Nature Astronomy .
This discovery could help us understand not only the atmospheres of habitable zone exoplanets in general, but also those of rocky habitable zone exoplanets in close orbit around red dwarf stars.
“Finding water in a potentially habitable world other than Earth is incredibly exciting,” says astronomer Angelos Tsiaras of University College London (UK) and co-author of the work. “K2-18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is Earth unique?”
K2-18b
The planet K2-18b was discovered in 2015, and it has been a somewhat complicated exoplanet to pin down. We know that it orbits very close to a red dwarf star called K2-18, completing its orbit once every 33 days. Furthermore, the planet’s stellar radiation levels are similar to those of Earth (except for the high eruption activity typical of red dwarfs). This is because this 33-day orbit is smack in the middle of the star’s habitable zone, not too hot for liquid water to evaporate from the surface, and not so cold that it freezes completely.
We also know that the planet is about twice the size of Earth and about eight times its mass. Astronomers even narrowed the planet down to two possible types. In 2017, a team determined that it could be a rocky planet with an atmosphere, like Earth, but larger, or a planet with a mostly watery interior, covered by a thick layer of ice, like Enceladus or Europa.
The new research suggests that K2-18b does, after all, have an atmosphere.
The planet was detected by the Kepler space telescope, which detected planets through the transit method (when the star system is perfectly aligned so that the planet passes between us and its star, causing a detectable attenuation of the light from the star).
This transit can also help us study the atmosphere of a planet. For example, some wavelengths can be absorbed by specific gases, producing lines in a spectrum. These can be selected when comparing a spectral profile of the star with a spectral profile of the transit.
But it is not easy. Even detecting the planet in the first place requires highly sensitive instruments capable of detecting faint dips in starlight. As if this were not enough, the spectral absorption lines are also incredibly weak.
The scientists did this using the WFC3 instrument on the Hubble Space Telescope. They took images of eight transits of the planet in front of the star, and combined them to produce a weighted average, creating a spectral profile for the planet.
Computer simulations
Then it was time to find out what that spectral profile was telling them, using modeling. Initially, they ran K2-18b atmosphere models with a range of atmospheric molecules that could produce absorption lines, includingwater (H2O), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and ammonia (NH3).
On the planet’s spectrum, only water could be detected with a high degree of confidence, so the team revised their analysis, using only water as a tracer. Later, they modeled the atmosphere using three different approaches: cloudless, with water vapor in a hydrogen-helium atmosphere; cloudless, with water vapor, hydrogen-helium and molecular nitrogen; and cloudy, with water vapor and hydrogen-helium.
All three simulations produced a statistically significant atmosphere, with values so similar that they could not distinguish the three potential types.
It was not possible, based on current information, to limit the abundance of water; But the models suggested that between 20 and 50% of the planet’s atmosphere could be water vapor. So things,K2-18b seems to be a fairly humid place.
“Although the issue of the habitability of temperate planets around late-type stars is a subject of active discussion, and actual progress requires substantially improved observation constraints, the analysis presented here provides the first direct observation of a molecular signature of a habitable zone exoplanet, connecting these theoretical studies with observations, “the authors conclude.
Referencia: Angelos Tsiaras, Ingo Waldmann, Giovanna Tinetti. Jonathan Tennyson y Sergey N. Yurchenko. “Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b”. Nature Astronomy (septiembre, 2019).
