Three planets circle stars that should have swallowed them, a study finds

When a star like the Sun runs low on hydrogen in its core, it swells into a red giant. Planets that orbit within about 1 au of such a star are expected to be dragged in and engulfed as it grows; 1 au is the distance from Earth to the Sun. Afterward the star shrinks back and settles down to burn helium in its core, becoming what astronomers call a red clump giant.

A team at Yunnan University and Yunnan Observatories of the Chinese Academy of Sciences has now shown that three known planets orbit red clump giants from inside that danger zone. The study, led by Lin and Qian, is published in the Proceedings of the National Academy of Sciences. Each planet orbits inside the largest size its star likely reached as a red giant, so by the usual rules it should not have survived, the authors say.

Starquakes tell a star's stage of life

The planets themselves are not new. Other teams found all three planets years ago by measuring the small wobble each planet raises in its star. What is new is what the team learned about the stars. A red giant that is still swelling and a red clump giant that has already swelled and shrunk can look almost the same in a telescope. Their insides are different, and that difference shows up in the way the stars vibrate.

Those vibrations, called starquakes, make a star's brightness flicker by tiny amounts. The team read them in brightness data from NASA's TESS telescope, a method called asteroseismology, and found that all three hosts are red clump giants. That means each star has already been through its largest size, with its planet still in orbit.

The result rests on one team's analysis. For 24 Boo, an independent measurement with the CHARA telescope array agrees with the starquake values for the star's size but not for its mass, and the authors note that their error ranges leave out some possible systematic errors. They still place all three stars securely among red clump giants.

Three survivors

The closest of the three, 24 Boo b, circles its star every 30 days at 0.183 au, less than half of Mercury's distance from the Sun. No planet is known closer to a red clump giant. HD 12648 b orbits at 0.516 au with a year of 133 days, and TYC 4282-605-1 b at 0.489 au with a year of 102 days.

For 24 Boo b and TYC 4282-605-1 b, the orbit sits inside the largest size the star reached under every star model the team tested. For HD 12648 b, the answer depends on how the models treat convection, the churning that carries heat through a star's outer layers. Under one setting, the chance that it should have been engulfed falls from near certainty to about 43 percent.

The wobble method gives only a minimum mass for each planet, because the tilt of its orbit is unknown. Those minimum masses are 0.76 times Jupiter's for 24 Boo b, 2.5 for HD 12648 b and 14.3 for TYC 4282-605-1 b. That puts all three among giant planets or substellar companions, objects heavier than planets but lighter than stars. Astronomers usually draw the line between giant planets and brown dwarfs at about 13 Jupiter masses, where deuterium, a heavy form of hydrogen, can begin to fuse. TYC 4282-605-1 b is already above that line at its minimum mass, so it is likely a brown dwarf.

A new class of planetary system

Only one planet like these was known before: 8 Ursae Minoris b, reported in 2023. Three more, the authors say, mark the start of a new class of planetary systems. They suggest ways the planets could have survived, such as a past merger with a companion star, the swallowing of an inner planet that reshaped the orbit of an outer one, or a planet slipping free as the star shrank. The authors call these possibilities that their study does not test.

The question matters close to home. The Sun will become a red giant too, billions of years from now, and one widely cited model of its future finds that even Earth would not escape being engulfed. Every planet found orbiting a star that has already been through that phase is a test of such models.