Even tiny stars have giant planets
Stars several times less massive than our own Sun host Jupiter-size planets
Even the smallest of stars can have the largest of planets. Stars as small as 20% the mass of our Sun can host planets the size of Jupiter. This finding is unexpected, as such low-mass stars were thought to have insufficient material to form such large planets. The research was recently published in specialised journal Monthly Notices of the Royal Astronomical Society.
Lead author of the study, Dr Ed Bryant, Research Fellow at Mullard Space Science Laboratory at UCL said:
“Low-mass stars are better at forming giant planets than we thought. Our results raise serious questions for planet formation models. In particular, our detection of gas giants orbiting stars as low as 20% of the mass of the Sun poses a conflict with current theory.”
To search for planets orbiting low mass stars, the researchers made use of data observed by the NASA TESS mission. TESS observes the brightness of stars over time to detect changes caused by the passing of a planet in front of the star, known as a planetary transit, an effect similar to that of a solar eclipse. Using TESS data, Bryant and his team looked at 91,306 low-mass stars. They found hundreds of potential planetary signals, most of which did not pass extensive vetting and were caused by other astrophysical or instrumental effects. However, 15 signals passed all checks and most of these are thought to be the result of giant, Jupiter-size planets orbiting these low mass stars.
Such planets are unexpected. The most common theory for how such giant planets form, a set of models known as core accretion models, struggles to produce such planets around low-mass stars. This is because low-mass stars have a limited reservoir of material out of which to form planets. Core accretion models therefore predict that giant planets should be increasingly rare around stars of lower mass, with occurrence of giant planets predicted to approach zero for stars below half the mass of the Sun.
According to the new study by Bryant and his team, such giant planets around low-mass stars do exist, albeit rarely. To measure how frequent such planets occur, they injected artificial planet signals into the TESS observations. They then ran their planet detection algorithms on these artificial planets to see which fraction they were able to recover. Combining the knowledge of how many planets were detected, and the fraction of planets they missed, they could then estimate how common giant planets are around these stars. Overall they found that about 1 in 500 stars with a mass between 0.088 and 0.71 times the mass of the Sun has a planet the size of Jupiter at orbital periods shorter than 10 days.
One possible explanation for the discrepancy between the observed planet occurrence and the theoretical predictions is that such planets do not form through core accretion, but that another set of models known as gravitational instability needs to be invoked. Another option is that the amount of material available to form planets around low-mass stars has been underestimated, speculates coauthor Dr. Dan Bayliss, Associate Professor of Physics at the University of Warwick:
“It’s possible we don’t understand the masses of these protoplanetary disks as well as we thought we did. Powerful new instruments such as the James Webb Space Telescope will be able to study the properties of these disks in more detail.”
Further studies to investigate the properties of the newly discovered planets, and measure their masses, are underway.
The study by Bryant, Bayliss, and Van Eylen was published in Monthly Notices of the Royal Astronomical Society in May 2023. Van Eylen is the editor of the Extrasolar Times and Bryant is a member of his research team. The study was previously featured in the UCL news and a press release by Warwick University and reported on by Sky & Telescope.