Citizen scientists from the Planet Hunters TESS (PHT) project have discovered a Saturn-sized planet orbiting around TOI 813, a bright subgiant star located 865 light-years away. Designated TOI 813b, it is the first planet discovered by the PHT project.
An artist’s impression of the Saturn-sized exoplanet TOI 813b. Image credit: Sci-News.com.
Evolved stars are rare in the solar neighborhood, as the subgiant and giant phases of stellar evolution are short-lived. Consequently, only a few exoplanets are known around such stars.
TOI 813 is a bright subgiant which is in the process of evolving away from the main sequence and onto the red giant phase.
Also known as TIC 55525572, 2MASS J04504658-6054196 and CD-61 970, the star is approximately 3.73 billion years old. It has an estimated mass of 1.3 solar masses and a radius of 1.9 solar radii.
TOI 813b orbits the star once every 84 days at a distance of only 0.4 AU. The planet is about 6.7 times bigger than Earth and 42 times more massive.
“TOI 813b is interesting for many reasons,” said Nora Eisner, a Ph.D. student in the Department of Physics at the University of Oxford.
“First, because it is orbiting around an evolved, subgiant star. The subgiant phase of a star occurs when a star runs out of its nuclear fuel source and, in a desperate attempt to find another source of energy, expands its outer layers and contracts its core.”
“Our Sun has not yet reached this stage of its life — it still has around half of its fuel source left — but it will undoubtedly one day also become a subgiant star.”
“There is a noticeable lack of well-studied planets around these types of stars, however, they may be able to help us predict what will happen to the planets within our own Solar System in the very, very distant future.”
“Do planets survive this stage of a star’s life? And if so, how do their characteristics change?”
“We investigated this further for TOI 813 by modeling the size of the star over time. This analysis showed that TOI 813b will sadly be engulfed by its evolving host in approximately 780 million years!”
“TOI 813b is also interesting as it currently has the longest orbital period of any validated TESS planet,” she added.
TOI 813b was initially identified as a single-transit event by citizen scientists taking part in the PHT project.
“The primary goal of the project is to harness the power of citizen science to find transit events in the TESS data that were missed by the main TESS pipeline and by other teams of professional astronomers,” Eisner and colleagues said.
“PHT builds on the success of the original Planet Hunters project (PH), which used Kepler and K2 data.”
Eisner and her team then confirmed the discovery using several ground-based telescopes.
“In order to determine whether TOI 813b really is a planet, we had to find out as much as possible about the host star,” Eisner explained.
“This can be done using spectroscopy, whereby we split light up into its individual wavelengths, much like a prism splits light into a rainbow. From this we can derive properties such as temperature and composition of the host star.”
“We obtained these observations using the Wide Field Spectrograph instrument on the 2.3-m Australian National University telescope and the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the ESO 3.6-m telescope in Chile.”
A paper describing the discovery of TOI 813b will be published in the Monthly Notices of the Royal Astronomical Society.
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N.L. Eisner et al. 2019. Planet Hunters TESS I: TOI 813, a subgiant hosting a transiting Saturn-sized planet on an 84-day orbit. MNRAS, in press; arXiv: 1909.09094
