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An international team of astronomers reports the detection of two new brown dwarfs orbiting distant stars using NASA's Transiting Exoplanet Survey Satellite (TESS). The newfound objects are about 30 times more massive than Jupiter. The finding was detailed in a paper on March 7 to the arXiv preprint server.

Brown dwarfs (BDs) are intermediate objects between planets and stars, occupying the mass range between 13 and 80 Jupiter masses (0.012 and 0.076 solar masses). Although many brown dwarfs have been detected to date, these objects orbiting other stars are a rare find.

TESS is conducting a survey of about 200,000 of the brightest stars near the sun with the aim of searching for transiting exoplanets. So far, it has identified more than 7,500 candidate exoplanets (TESS Objects of Interest, or TOI), of which 612 have been confirmed so far.

Now, a team of astronomers led by Elina Y. Zhang of the University of Hawaii has identified a transit signal in the light curve of two distant stars, namely: TOI-4776 (TIC 196286578) located some 1,206 light years away and TOI-5422 (TIC 80611440) at a distance of 1,134 light years. Follow-up observations of these stars found that the transit signals are caused not by planets but more massive objects.

"TOI-4776 b and TOI-5422 b are two newly discovered low-mass BDs by TESS that transit their host stars in nearly circular orbits. We used TESS and ground-based light curves, ground-based RV [radial velocity] follow-up, and parallax measurements from Gaia DR3 to characterize the BDs," the researchers explained.

According to the paper, TOI-4776 b is the size of Jupiter but about 32 times more massive than the solar system's biggest planet. The brown dwarf orbits its star every 10.41 days, at a distance of approximately 0.1 AU from it. The host is an F-type star slightly larger and more massive than the sun, with an effective temperature of 6,011 K and is estimated to be 5.4 billion years old. The equilibrium temperature of the brown dwarf was calculated to be 1,032 K.

The brown dwarf TOI-5422 b turned out to be smaller and less massive than TOI-4776 b鈥攊ts radius was found to be 0.81 Jupiter radii, while its mass was measured to be 27.7 Jupiter masses. TOI-5422 b has an orbital period of 5.37 days, an equilibrium temperature of 1,355 K, and is separated from the parent star by about 0.06 AU. When it comes to the host TOI-5422, it is a subgiant star with a radius of 1.48 solar radii and a mass of 1.05 solar masses. The star has an age of about 8.2 billion years and its effective temperature is 5,744 K.

Summing up the results, the authors of the paper underlined that TOI-5422 b is one of the oldest transiting brown dwarfs so far discovered. They added that this brown dwarf is likely spinning up its host star and its orbit is aligned with the stellar spin axis.

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