UNIVERSITY PARK, Pa. – A planet observed crossing in front of, or transiting, a low-mass star has been determined to be about the size of Jupiter. While hundreds of Jupiter-sized planets have been discovered orbiting larger sun-like stars, it is rare to see these planets orbiting low-mass host stars and astronomers think this discovery could help them better understand how these giant planets form.
“This is only the fifth Jupiter-sized planet transiting a low-mass star that has been observed and the first with such a long orbital period, which makes this discovery really exciting,” said Caleb Cañas, lead author of the paper and a doctoral student at Penn State and NASA Earth and Space Science Fellow.
Originally detected by NASA’s Transiting Exoplanet Survey Satellite (TESS) spacecraft, astronomers characterized the planet’s mass, radius, and its orbital period using the Habitable-zone Planet Finder (HPF), an astronomical spectrograph built by a Penn State team and installed on the 10 meter Hobby-Eberly Telescope at McDonald Observatory in Texas. A paper describing the research appears in the September 2020 issue of The Astronomical Journal and is publicly accessible on arXiv.
“A transiting Jupiter-sized planet is amenable to further observations to see how well the orbit is aligned with the spin-axis of the host star and to constrain how it could have formed,” said Cañas. “Furthermore, the low mass of the host star and the long orbital period result in a Jupiter with a moderate temperature compared to similar planets detected with NASA’s Kepler space telescope.”
The host star, TOI-1899, is a low-mass — M dwarf — star about 419 light years away from Earth. The planet, TOI-1899 b, is two-thirds the mass of Jupiter, 10% larger in radius than Jupiter, and is 0.16 astronomical units — a measure defined as the distance between the Earth and the sun — from its host star such that a full year on TOI-1899 takes only 29 Earth days. For comparison, the four other transiting Jupiter-size planets around comparable stars complete their orbits in less than 4 days.
The planet was detected by TESS using the transit method, which searches for stars showing periodic dips in their brightness as a telltale sign of an orbiting object crossing in front of the star and blocking a portion of its light. The signal was later confirmed as a planet using precision observations from the HPF spectrograph that measure the planet’s mass by analyzing how it causes its host star to wobble.