Astronomers Find Sub-Neptune Planet That May Have Liquid Water, Vital Ingredient for Life

A team of astronomers has discovered a new sub-Neptune planet orbiting a nearby dwarf star. The icy exoplanet, twice the size of the Earth with over five times its mass, orbits its star at a distance that could allow it to harbor liquid water.

That isn't all that is extraordinary about this world's orbit, however. It passes around its parent star, TOI-2257, in an extremely flattened elliptical orbit.

Yet because the exoplanet, designated TOI-2257b, is so close to its stars at just around 135 million miles away, it completes an orbit once roughly every 35 earth days.

The team's investigation of this star and its planet, which are around 188 light-years from Earth, has been pre-published on the arXiv pre-print server.

Astronomers from institutions across the globe discovered the planet using NASA's Transiting Exoplanet Survey Satellite (TESS). The space-based telescope detects tiny dips in light that planets cause when they pass in front of their parent stars, a process known as the transit method of exoplanet detection.

TESS has been employing this technique since its launch in 2018 to search for exoplanets around 200,000 of the brightest stars in close proximity to the sun.

In its three years of operation, it has detected 4,600 possible exoplanets, of which 167 have been confirmed.

After spotting a possible transiting exoplanet around the M-dwarf star TOI-2257, the team performed a follow-up investigation with telescopes here on Earth. This confirmed the presence of an exoplanet with 2.19 times the radius of earth and 5.7 times its mass.

Just because the planet is larger than Earth, that doesn't mean that its star is greater in size than our own, however.

The eight billion year-old-star TOI-2257 has just one-third the mass of the sun, and around a third of our star's mass also. It is also cooler than our star at around 3,200 degrees Celsius compared to our star's average temperature of 5,500 degrees Celsius.

Sub-Neptunes are considered to be planets that have a smaller radius than Neptune, but could still have more mass. Based on our current understanding, they are the most common planets in the galaxy, which makes it strange that the solar system doesn't have one.

The authors of the paper say that TOI-2257b is currently one of only around 20 known sub-Neptunes that exist in a zone around their star that allows water to exist in a liquid state. TOI-2257.

Despite having this key element for life, the authors add that life at the planet's surface is improbable due to its temperature of around -16 degrees Celsius, and its high pressure. Yet, there remains a chance that living organisms could dwell in its clouds. This is something that has been proposed before for sub-Neptunes such as TOI-2257 before.

The possibility marks this planet out as a potential target for the James Webb Space Telescope (JWST) following its launch in December this year.

One of the key missions of the JWST will be the investigation of the atmospheres of exoplanets in the search for the tell-tale signatures of living organisms.

The fact that the planet has an orbit with the highest eccentricity ever found around an M-star, and has the third greatest eccentricity of any discovered sub-Neptune makes it an ideal target for the study of planetary systems other than our own.

Sub-Neptune
An illustration of a Sub-Neptune planet. Astronomers have discovered a new Sub-Neptune that is twice as large as Earth and could harbour liquid water. Hubble/M.Kornmesser/ESA