Twins or cousins? Sub-Neptune exoplanets could provide an answer

Astronomers have discovered a pair of exoplanets orbiting an M dwarf star, one of which is the coldest M dwarf planet found by the TESS satellite observatory, making it accessible for tracking studies of its atmosphere . The work is published in Letters from the astrophysical journal.

The planets orbit the star TOI 904, located 150 light years from Earth, and are both sub-Neptunes, with radii more than twice those of Earth, but smaller than Neptune. Discovered with the Transiting Exoplanet Survey Satellite (TESS), the outer planet, TOI 904c, takes 84 days to circle its star and has an estimated equilibrium temperature of 217 K (-56°C), the dwarf planet M the coldest discovered by TESS. date.

Its companion, TOI 904b, orbits the star every 11 days. The host star is bright enough in the infrared to allow future determinations of the masses of exoplanets and the characteristics of their atmospheres, but cold and dark enough that the star’s brightness does not overwhelm observing telescopes.

TESS is a satellite designed to study exoplanets around the brightest dwarf stars. Its initial two-year mission ended in July 2020, but it has been extended twice and continues to collect data on the brightness of thousands of stars. As exoplanets orbit a star, astronomers look for small, periodic dips in its intensity as the planets pass or transit between TESS and the star.

The data collected by TESS is the subject of very active investigation and reveals that about 75% of the stars found were M dwarfs, both smaller and dimmer than the sun, and to date, more than 7 000 candidate exoplanets have been found, of which more than 400 are now confirmed.

NASA’s previous Kepler mission, which lasted four years and ended in 2013, discovered three exoplanets with comparable orbital periods but orbiting fainter host stars. Since an exoplanet’s atmosphere is studied by observing changes in the host star’s spectrum as its light passes through the exoplanet’s thin outer shell, a brighter star, like TOI 904 here, provides better opportunity on the ground to identify the changes that characterize the atmosphere. chemical composition, such as water vapor, oxygen, ozone and carbon dioxide.

In this study, the exoplanets are of similar size (2.5 and 2.3 Earth radii, respectively) and orbit their star at distances of 0.06 and 0.31 AU (astronomical units, the average distance from the sun to Earth), with a relatively wide separation. between them.

Kepler only discovered around thirty planets at distances greater than 0.15 AU, corresponding to periods greater than 25 days. Thus, the current study will provide a window into an unexplored region of these exoplanets and provide targets for future studies.

Unlike the three exoplanets previously discovered by Kepler, this star is bright enough to provide a unique (so far) opportunity to also determine planetary masses, by determining their average densities. Such knowledge would begin to constrain the composition of the two planets in this system; Similar densities would suggest that they have similar formation histories, or they evolved by different means while orbiting the same star.

The group looked for variations in the transit times of planets b and c, which could indicate the presence of non-transiting exoplanets around the same star, but found no evidence of additional planets in the system.

Exoplanets around M dwarf stars are of particular interest due to the long lifetime of these stellar objects, especially those located in the star’s habitable zone. M-type dwarf stars, also called red dwarfs, are the coolest of the different types of dwarf stars and are by far the most common type of star in the Sun’s neighborhood, and probably in the Milky Way where they represent up to 75% of all stars. . The closest star to Earth, Proxima Centauri, is an M dwarf.

These stars have a radius less than half that of the Sun, with a surface temperature between 2700 K and 4000 K (compared to 5700 K for the Sun). This means that their habitable zones, where liquid surface waters could support life as we know it, are close to the star, usually about 0.2 AU (astronomical units, where the Earth-Sun distance is 1 au).

None of the exoplanets orbiting TOI 904 are in the star’s habitable zone, with “b” being too close (so any water would have evaporated) and “c” being too far away, so water and others compounds present, if they exist, would be frozen.

“The differences and similarities in the composition of these two planets can give us an idea of ​​when and where they formed around their host star,” said Mallory Harris, a doctoral student. candidate at the University of New Mexico who is the lead author of the study. “This could tell us more about how planets form around M dwarf stars in general.”

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