A red dwarf star just 105 light-years away could host at least one habitable world.
In close orbit around the cool, dim star, scientists have found and confirmed two rocky exoplanets – one comfortably within a distance of the star known as the habitable zone.
Follow-up observations will need to be made to further determine its nature, but the discovery is exciting, suggesting that other such worlds may be waiting to be discovered in the solar neighborhood.
Even though the exoplanet is not habitable, habitable zone worlds are relatively rare, especially rocky ones.
So in any case, the discovery is an important data point for characterizing the population of these exoplanets.
Both exoplanets were discovered following observations by NASA’s TESS exoplanet-hunting telescope.
As it stared at a small red dwarf star named LP 890-9 (AKA TOI-4306), it picked up the faint, regular dips in starlight characteristic of an exoplanet orbiting between us and the star, passageways called transits, over a 2.7 -day period.
Transit data can tell us a lot about an exoplanet. There is the fact of its existence, to begin with. Then there is the orbital period. And, based on the decrease in starlight, scientists can also infer the diameter of the exoplanet.
But, in order to confirm the detection and get more data on exoplanets, more observations are needed using different instruments.
“This tracking,” explains astronomer Laetitia Delrez of the University of Liège in Belgium, “is particularly important in the case of relatively cool stars, such as TOI-4306, which emit most of their light in the near infrared. and for which TESS has a rather limited sensitivity.
She and her team used telescopes from the SPECULOOS consortium (Search for habitable Planets EClipsing ULtra-cOOl Stars), sensitive to the near-infrared wavelengths emitted by TOI-4306. Transit data obtained in these observations confirmed the existence of the exoplanet, named LP 890-9b.
Next, the team searched for exoplanets that might have been missed by TESS – and found a second world, slightly further from the star than LP 890-9b. This exoplanet, named LP 890-9c, has an orbital period of 8.4 days.
Other data, including radial velocity measurements that determine the gravitational pull an exoplanet exerts on a star and thus derive their mass, allowed the team to characterize the two exoplanets in detail.
Although the two masses were not measured, radial velocity data gave scientists an upper mass limit for the two exoplanets.
LP 890-9b is about 1.32 times the diameter of Earth and up to 13 times its mass.
LP 890-9c is about 1.37 times the diameter of Earth and up to 25 times its mass.
These measurements are consistent with the density of rocky worlds, like Earth, Mars, and Venus, rather than gaseous or icy worlds like Jupiter or Neptune.
This means that exoplanets can be classified as super-Earths – rocky worlds larger than Earth and smaller than Neptune.
Where they orbit the star — specifically, the outer exoplanet, LP 890-9c — is where it gets interesting.
It’s one of the first things scientists look for when assessing the potential habitability of an alien world. An exoplanet too close or too far from its star will respectively be too hot or too cold for life as we know it.
But there is a temperate or habitable zone in the orbital neighborhood of every star in which liquid water could comfortably rest on a planetary surface.
LP 890-9c may be in a close orbit to its star relative to Earth, but that star is much, much cooler and dimmer than the Sun.
At its orbital position, the exoplanet is in its star’s conservative habitable zone, receiving levels of stellar radiation similar to Earth’s.
After the TRAPPIST system, it is the second most promising potentially habitable world discovered to date, according to the researchers.
“But we shouldn’t get ahead of ourselves. Being in the right place doesn’t guarantee a palm beach,” says astronomer Robert Wells of the University of Bern in Germany.
“Our neighbor planet Venus, which is, so to speak, a carbon dioxide-rich pressure cooker, at nearly 500 degrees Celsius, is also close to this so-called habitable zone around the Sun.”
LP 890-9c is very close to the point at which a young planet could be caught in a runaway greenhouse effect, like Venus, but we don’t know for sure what’s going on there.
A second team of scientists, including some of the authors of LP 890-9, have already submitted a preprint questioning this mystery, but we probably won’t know until we get observations of the atmosphere of the exoplanet, if it has one.
The James Webb Space Telescope is already proving its worth. The telescope, as you can imagine, is in high demand, so we may have to wait a while for sightings.
On the other hand, a target as promising as the LP 890-9c may be worth a scramble, because whatever the outcome, we have something to gain from it.
“It is important,” says astronomer Amaury Triaud from the University of Birmingham in the UK, “to detect as many temperate terrestrial worlds as possible to study the diversity of exoplanet climates, and eventually to be able to measure how often biology has emerged in the Cosmos.”
The research has been published in Astronomy & Astrophysics.
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