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

Astronomers have found a pair of exoplanets orbiting an M dwarf star, one among which is the coldest M dwarf planet discovered by the TESS satellite observatory, making it accessible for follow-up research of its ambiance. The work is published in The Astrophysical Journal Letters.

The planets orbit the star TOI 904, which is 150 light years from Earth, and are each sub-Neptunes, with radii over two occasions that of Earth, however smaller than Neptune. Found with the Transiting Exoplanet Survey Satellite tv for pc (TESS), the outer planet, TOI 904c, takes 84 days to circle its star and has an estimated equilibrium temperature of 217 Ok (-56°C), the coldest M dwarf planet found by TESS so far.

Its companion, TOI 904b, orbits the star each 11 days. The host star is vibrant sufficient within the infrared to permit future determinations of the exoplanet lots and traits of their atmospheres, however cool and dim sufficient that the star’s luminosity doesn’t overwhelm observing telescopes.

TESS is a satellite designed to check exoplanets across the brightest dwarf stars. Its authentic two-year mission led to July 2020, nevertheless it has been prolonged twice and continues to be gathering information on the brightness of hundreds of stars. As exoplanets orbit a star, astronomers search for small, periodic declines in its depth because the planets go, or transit, between TESS and the star.

The info collected by TESS is below very energetic investigation and divulges that about 75% of stars discovered have been M dwarfs, each smaller and dimmer than the sun, and so far it has discovered over 7,000 candidate exoplanets, with over 400 now confirmed.

NASA’s earlier Kepler mission, which lasted 4 years and led to 2013, discovered three exoplanets with comparable orbital intervals however orbiting dimmer host stars. As a result of an exoplanet’s ambiance is studied by observing adjustments within the host star’s spectrum as its mild passes by way of the skinny outer shell of the exoplanet, a brighter star, like TOI 904 right here, gives a greater ground-based alternative to determine adjustments that characterize the ambiance’s chemical composition, corresponding to water vapor, oxygen, ozone and carbon dioxide.

On this research, the exoplanets are of comparable measurement (2.5 and a pair of.3 Earth radii, respectively) and orbit their star at distances of 0.06 and 0.31 au (astronomical items, the common distance from the sun to the Earth), with a comparatively extensive separation between them.

Kepler found solely about 30 planets at distances higher than 0.15 au, comparable to intervals higher than 25 days. So, the present research will provide a window into an unexplored area of such exoplanets and supply targets for future research.

Not like the three earlier exoplanets found by Kepler, this star is vibrant sufficient to supply a singular alternative (to this point) to find out the planetary lots as properly, figuring out their common densities. Such information would start to constrain the composition of each planets on this system; comparable densities would counsel they’ve comparable formation histories, or else they advanced by way of totally different means whereas orbiting the identical star.

The group regarded for variations within the transit occasions of planets b and c, which might point out the presence of non-transiting exoplanets across the similar star, however discovered no proof of extra planets within the system.

Exoplanets round M dwarf stars are of specific curiosity because of the lengthy lifetimes of such stellar objects, particularly these within the star’s liveable zone. Kind M dwarf stars, additionally referred to as crimson dwarfs, are the good of the assorted forms of dwarf stars and are simply the most typical kind of star within the sun’s neighborhood, and possibly the Milky Way galaxy the place they comprise as much as 75% of all stars. The closest star to Earth, Proxima Centauri, is an M dwarf.

These stars have radii lower than half of the sun’s, with a floor temperature between 2700 Ok and 4000 Ok (in comparison with the sun’s 5700 Ok). This implies their liveable zones, the place liquid floor water might enable life as we all know it to exist, are near the star, usually about 0.2 au (astronomical items, the place the Earth-sun distance is 1 au).

Neither of the exoplanets orbiting TOI 904 are within the star’s liveable zone, with “b” being too shut (so any water would have evaporated away) and “c” being too far, so any water and different compounds there, in the event that they exist, can be frozen.

“The variations and similarities within the compositions of those two planets can provide us an concept of when and the place they fashioned round their host star,” stated Mallory Harris, a Ph.D. candidate on the College of New Mexico who’s the research’s lead writer. “This might inform us extra about how planets kind round M dwarf stars typically.”

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