Astronomers discover an Earth-sized exoplanet orbiting a nearby ultra-cool dwarf star

An international team of astronomers reports the discovery of a new Earth-sized exoplanet orbiting an ultra-cool dwarf star just 54.6 light-years away. The newly discovered alien world, called SPECULOOS-3 b, is slightly smaller but much hotter than our planet. The finding was reported in an article published May 15 in the journal Nature Astronomy.

The Search for Planets EClipsing ULTra-cOOl Stars (SPECULOOS) project aims to find potentially habitable exoplanets around some of the solar neighborhood’s smallest and coldest stars. It uses a network of six robotic telescopes with an aperture of 1 meter: the four telescopes of the SPECULOOS-South Observatory (SSO) in Chile, Artemis, the first telescope of the SPECULOOS-North Observatory (SNO) in Tenerife, and the SAINT-EX telescope at the San Pedro Martir Observatory in Mexico.

One of the stars observed as part of the SPECULOOS program is SPECULOOS-3 (also known as LSPM J2049+3336) – an ultracool dwarf of spectral type M6.5, about eight times smaller and ten times less massive than the Sun. The star is estimated to be 6.6 billion years old and has an effective temperature of 2,800 K.

A group of astronomers led by Michaël Gillon of the University of Liège in Belgium recently detected a transit-like signal in the light curves of SPECULOOS-3. Follow-up observations of this star have shown that this signal is caused by an exoplanet the size of Earth.

“We present the SPECULOOS project’s detection of an Earth-sized planet in a 17-hour orbit around an ultracool dwarf of spectral type M6.5 located 16.8 parsecs away,” the researchers wrote in the paper.

According to the article, SPECULOOS-3 b has a radius of about 0.977 Earth radii and orbits its parent star every 17.28 hours. The planet’s equilibrium temperature was estimated at about 553 K.

The mass and therefore the composition of SPECULOOS-3 b remains unknown. However, the astronomers assume that this planet has a rocky composition, which would have allowed such a small planet to retain a significant amount of hydrogen in such a short orbit. Furthermore, they add that all currently known Earth-sized planets in NASA’s exoplanet archive have masses that imply rocky compositions.

The researchers note that if SPECULOOS-3 b indeed has a rocky composition, its expected mass would be about 0.93 Earth masses. Follow-up spectroscopic observations could result in the detection of radial velocity signals from SPECULOOS-3 b, which could lead to the first measurement of its mass.

In their closing remarks, the paper’s authors emphasize that the high irradiance of SPECULOOS-3 b, combined with the infrared brightness and Jupiter-like size of its host, makes it one of the most promising rocky exoplanets to explore with the James Webb Space Telescope. JWST) to perform a detailed characterization of the emission spectroscopy.