Two Mini-Neptunes Found Orbiting Sun-Like Star

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Using NASA’s Transiting Exoplanet Survey Satellite (TESS) and the HARPS fiber-fed Echelle spectrograph mounted at the 3.6-m telescope of ESO’s La Silla observatory, astronomers have discovered and confirmed two massive transiting planets around the G-type dwarf star TOI-763.

An artist’s impression of the TOI-763 planetary system. Image credit:

An artist’s impression of the TOI-763 planetary system. Image credit:

“There are relatively few planets smaller than Neptune for which both the size and mass has been measured,” Professor Malcolm Fridlund, an astronomer at Leiden Observatory and Onsala Space Observatory at the Chalmers University of Technology, and his colleagues wrote in their paper.

“Only 70 such planets are reported orbiting G-stars in the NASA archive as of June 2020.”

“Most of these have large uncertainties leading to errors in density of a factor of two or more,” they noted.

“This is due to the fact that hitherto the exoplanetary space missions have been searching relatively faint stars where although the diameters are known with high precision, the follow-up observations to acquire the planetary masses have usually had large errors.”

“It is therefore important that thanks to the launch of the TESS mission relatively bright stars are now being searched for exoplanets.”

TOI-763 is a relatively bright G-type star located 311 light-years away in the constellation of Centaurus.

Also known as TIC 178819686, CD-39 7945 and 2MASS J1575245-3945275, the star is around 10% smaller than the Sun.

The inner planet in the system, named TOI-763b, has an orbital period of 5.6 days, a mass of 9.8 Earth masses and a radius of 2.4 Earth radii.

The second planet, TOI-763c, has an orbital period of 12.3 days, a mass of 9.3 Earth masses and a radius of 2.9 Earth radii.

“We find that both TOI-763b and c should contain large amounts of water, but demonstrate significant differences between them,” the astronomers wrote.

The transit signals of TOI-763b and c were detected in the photometric data from the TESS satellite.

The planetary nature of these signals was then confirmed by the HARPS instrument.

“We find that these planets all belong in the density regime of ‘ice-planets’ and that their density versus radii distribution can be described by a first-degree polynomial with a very small scatter,” Professor Fridlund and co-authors wrote.

“All the planets, including TOI-763b and c, that fall along the ‘ice-track’ and orbiting stars similar to our Sun, are found in the compact arrangement with short orbital periods, similar to what has been discovered so far for smaller planets orbiting low-mass stars in general.”

In addition to the two confirmed planets, they also identified a third transit signature that could be interpreted as one additional planet, TOI-763d.

If confirmed by future studies, it should have a similar mass as TOI-763b and c and an orbital period of 47.8 days.

The team’s paper was published online in the Monthly Notices of the Royal Astronomical Society.


M. Fridlund et al. The TOI-763 system: sub-Neptunes orbiting a Sun-like star. MNRAS, published online August 19, 2020; doi: 10.1093/mnras/staa2502

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