Astronomers have discovered 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, designated 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 Astronomy of nature.

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

One of the stars observed by 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 10 times less massive than the Sun. The age of the star is estimated to be 6.6 billion years and its effective temperature is 2800 K.

A team 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. Subsequent observations of this star determined that this signal was caused by an extrasolar planet the size of Earth.

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

According to the paper, SPECULOOS-3 b has a radius of approximately 0.977 that of Earth and orbits its host star every 17.28 hours. The equilibrium temperature of the planet was estimated to be about 553 K.

The mass and thus the composition of SPECULOOS-3 b remains unknown. However, astronomers believe that this planet has a rocky composition for such a small planet in such a short orbit that it has retained a significant envelope of hydrogen. Additionally, they add, all currently known Earth-sized planets in NASA’s exoplanet archive have masses indicative of rocky compositions.

The researchers note that if SPECULOOS-3 b is indeed rocky in composition, then its expected mass would be around 0.93 Earth masses. Subsequent spectroscopic observations could lead to the detection of SPECULOOS-3 b’s radial velocity signals, which could lead to the first measurement of its mass.

In the concluding remarks, the authors of the paper emphasize that the high irradiance of SPECULOOS-3 bv combination with its infrared luminosity and Jupiter-like size make it one of the most promising rocky exoplanets to be investigated by the James Webb Space Telescope (JWST) for detailed emission spectroscopic characterization.

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