Astronomers are scratching their heads over a recently discovered planet. The planet, dubbed TOI-6894b, orbits a star 238 light-years from Earth. The planet is huge; the star is tiny, a red dwarf only 20 percent the mass of our own sun. And that’s where the head scratching comes in.
Low-mass stars do not typically spawn large planets, or at least that’s what astronomers used to think. Though the process of planet formation is not well understood, the basic idea is that the disc of gas, dust, and debris that surrounds young stars begins to rotate, and eventually, due to gravity and other factors, coalesces into planets.
“The thing we observe for young stars that are in the process of formation is that the size and the mass of the disc is scaled in proportion to the size of the star. So the bigger the star, the more massive the disc, and the smaller the star, the less massive the disc, roughly speaking,” says Eric Agol, an astrophysicist at the University of Washington. And that’s why you don’t expect a small star to give rise to a giant planet. There’s just not enough material to work with.
Large Planets, Like TOI-6894 b
But clearly, it can happen. In 2023, for example, astronomers found a planet with the mass of Neptune orbiting a star with a mass nine times less than that of our sun. And while other large planets orbiting small stars have been found, those planets are much more distant from their stars, says Agol. This makes sense, he says, because the farther from the star you go, the more material there is to form planets.
Such systems seem to be very rare, though scientists are actively looking for them.
“I would be very excited if we were to find more large planets with low-mass stars, and I think we can't rule out the possibility that there are more large planets like TOI-6894 b out there,” says Edward M. Bryant, astrophysicist at the University of Warwick and leader of the team that discovered TOI-6894 b.
Read More: Image Shows a Humongous Amount of Dust, Revealing How Planets Form
Planet Formation Process
Though no one yet knows how this small star spawned such a large planet, there are several possibilities. One is that the planet formed by a process called gravitational disc instability, explains Bryant. This is when the disc of gas and dust surrounding the star breaks into fragments and collapses into a planet, though Bryant says that the exact conditions needed for this to happen are still unknown.
Another possibility, says Agol, is that the planet actually formed further away from the star, then migrated inward. It might have done that because it lost the angular momentum that prevents it from falling into its star.
That could happen if it interacted with a companion star. It could also happen if, in the process of disc formation, the disc of asteroids and gas carried away some of the angular momentum of the planet, allowing it to move inward, he explains. Yet a third possibility, Agol says, is that multiple planets formed, and when they interacted, one of them was pushed closer to the star.
Exploring Future Planet Formation
This is an exciting discovery for understanding planet formation. However, TOI-6894 b also offers a good opportunity for the James Webb Space Telescope (JWST), says Agol. When a planet transits its star, it obscures the disk of the star, causing a dip in the star’s brightness, he explains.
“For most giant planets, you get a dip that’s about one percent in depth. In this case, the net value is more like 17 percent, which is really dramatic,” says Agol.
This means the planet probably has a very extended atmosphere, which will make it easier to study.
“We could possibly learn something about the composition of the planet’s atmosphere, and that will help inform our models of planet formation as well,” says Agol. “This is going to be a very exciting target for JWST to look at.”
The discovery doesn’t completely upend the thinking on planet formation, says Bryant, but it does highlight the need to better understand the environments in which these planets form, he says.
“The TOI-6894 system will be an important benchmark system as we explore these environments and scenarios in the future.”
Read More: James Webb Space Telescope Captures Smallest Exoplanet Ever Seen in Historic First
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Nature Astronomy. A transiting giant planet in orbit around a 0.2-solar-mass host star
NASA Exoplanet Archive. TOI-6894 Overview
Science. A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models
Avery Hurt is a freelance science journalist. In addition to writing for Discover, she writes regularly for a variety of outlets, both print and online, including National Geographic, Science News Explores, Medscape, and WebMD. She’s the author of Bullet With Your Name on It: What You Will Probably Die From and What You Can Do About It, Clerisy Press 2007, as well as several books for young readers. Avery got her start in journalism while attending university, writing for the school newspaper and editing the student non-fiction magazine. Though she writes about all areas of science, she is particularly interested in neuroscience, the science of consciousness, and AI–interests she developed while earning a degree in philosophy.
