Stars less than half the mass of our Sun may host giant worlds like Jupiter, in conflict with the most widely accepted theory of how such planets form, a new study reveals.
Gas giants, like other planets, form from disks of material that surround young stars. According to nuclei accretion theory, they first form a core of rock, ice, and other heavy solids, attracting an outer layer of gas once this core is massive enough (about 15 to 20 times that of Earth). . However, low-mass stars have low-mass disks that models predict would not provide enough material to form a gas giant in this way, or at least not fast enough before the disk ruptures. In the study, accepted for publication in the Monthly Notices of the Royal Astronomical Society (MNRAS), researchers from University College London (UCL) and the University of Warwick observed 91,306 low-mass stars, using observations from the Exoplanet Survey Satellite in NASA Transit (TESS), and in 15 cases found dips in light brightness corresponding to a gas giant passing in front of the star.
Since then, five of the 15 potential giant planets have been confirmed as planets using independent methods. One of these confirmed planets orbits a star that is one fifth the mass of the Sun, which would not be possible according to planet formation models. Lead author Dr Ed Bryant, from the Mullard Space Science Laboratory at UCL and formerly at the University of Warwick, who started the work as part of his PhD, said: “Low-mass stars are better for forming giant planets of “What 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 Sun’s mass raises a conflict with current theory.”
Co-author Dr Vincent Van Eylen, from the Mullard Space Science Laboratory at UCL, added: “The fact that, although rare, gas giants exist around low-mass stars is an unexpected finding and means that models of planet formation will need to be reviewed.” One possible interpretation is that gas giants do not form through core accretion, but through gravitational instability, where the disk surrounding a star fragments into planet-sized clumps of dust and gas. If this is the case, low-mass stars could harbor very large gas giants, two to three times the mass of Jupiter. However, this is considered unlikely, since the disks around low-mass stars do not appear to be massive enough to fragment in this way.
Another explanation, the researchers say, is that astronomers have underestimated how massive a star’s disk can be, meaning that small stars could form giant planets through core accretion after all. This could be because we have miscalculated the mass of the disks that we can see through telescopes, or because the disks are more massive early in a star’s life, when they are very hard to see (because they are embedded in dust clouds), compared to later in a star’s life when we can observe them.
In their paper, the researchers sought to identify how often giant planets occurred around low-mass stars, testing whether this rate of occurrence matched what core accretion models would predict. They used an algorithm to identify signals from transiting gas giants in the light emitted by low-mass stars. They then examined these signals, ruling out a number of false positives. To determine the probability that their method would detect actual gas giants orbiting these stars, they inserted simulations of thousands of signals from transiting planets into the actual TESS starlight data and then ran their algorithm to see how many of these planets would be detected.
Now the researchers are working to confirm as planets (or rule out) nine of the 15 candidate planets they identified (five so far have been confirmed as planets, with one false positive). These candidates could potentially be companion stars or there could be another reason for the dips in brightness.