Astronomers have discovered a world outside the solar system about 240 light-years away in space that is a freak of nature.
Somehow, a little red dwarf star only one-fifth the size of the sun gave birth to an enormous baby — an exoplanet that is a little larger than Saturn, although it weighs about half as much as our ringed gas giant.
Discovered in a sweeping investigation of NASA's Transiting Exoplanet Survey Satellite data, this world, TOI-6894 b, and its host star have set a new record for their incongruous sizes. Together they are the smallest known star to have an orbiting giant planet. If there were a Guinness Book of Galactic Records, this one would have a landslide victory for the titleholder. The star, TOI-6894, is just 60 percent the size of the next smallest star with such a planet.
The pair's existence breaks all the rules of what scientists know about planet formation.
"We don’t really understand how a star with so little mass can form such a massive planet!" said Vincent Van Eylen, a researcher at the University College London, in a statement. "By finding planetary systems different from our solar system, we can test our models and better understand how our own solar system formed."
Edward Bryant, who led the research team, found the behemoth first by poring over TESS space telescope data of over 91,000 small red dwarfs, aka M-type stars. Then he used the European Southern Observatory's Very Large Telescope in Chile to reveal TOI-6894b.
According to How to Make a Really Humongous Planet 101, it should be difficult — nearly impossible — for stars this tiny to do this. That’s because the disks of gas and dust swirling around young stars are the construction materials for planets. Small stars tend to have smaller and lighter disks.
Gas giants like our own Jupiter or Saturn need a lot of stuff to form their cores. They then are able to attract a lot more gas quickly from their surroundings to collect an atmosphere. The mechanics are called "core accretion," and it seems to work best when the building materials are plentiful.
But TOI-6894b seems to be playing by a different rulebook. It's about 53 times the weight of Earth and made partly of heavy elements, according to a paper on its discovery published in Nature Astronomy. In fact, the exoplanet is thought to have about 12 Earths'-worth of those chemicals. That’s way beyond what most small young stars are thought to have in their midst.
Some scientists don't want to throw the baby out with the bathwater: Though the exoplanet doesn't fit neatly into the core-accretion model, it could have formed in a similar way but with a tweak. Perhaps this world started collecting ingredients to form its core very early in its star's life, when the disk was still chock-full of raw material. Or maybe instead of growing a large core quickly to pull in more gas in a runaway process à la Jupiter-like planets, TOI-6894b could have just kept hoarding gas and heavy elements gradually over time.
But even that would require a bigger original supply of dust. In a survey sample of 70 disks around small stars, only five had enough material to build a planet on the scale of TOI-6894b, according to the new paper.
Another idea, called gravitational instability, suggests the disk could collapse under its own weight to create a planet directly. But the discovery team for TOI-6894 b points out that the process doesn't quite work for something the size of this exoplanet — at least according to computer simulations.
Whatever the origin story, TOI-6894b is leading the ranks of other known gas giants orbiting small and faint stars that astronomers want to study. Scientists also have their eyes on LHS 3154 b, GJ 3512 b, and TZ Ari b.
"This discovery will be a cornerstone for understanding the extremes of giant planet formation," Bryant said.
The next step for the research team is to use the James Webb Space Telescope to study the exoplanet's atmosphere, which will occur within the next year. By measuring the various materials in the planet, the researchers may be able to determine the size and structure of its core. That could answer the question of whether TOI-6894 b formed through one of the known models.
They also have a hunch the exoplanet's atmosphere is rich in methane, something Webb could help confirm. TOI-6894 b is unusually cool for a gas giant, about 300 degrees Fahrenheit. Most of the gas giants known are "hot Jupiters," with temperatures between 1,340 and 3,140 degrees Fahrenheit. Such a discovery of a relatively chilly gas giant would be very rare, the researchers said.
"Most stars in our galaxy are actually small stars exactly like this," said Daniel Bayliss, a coauthor from the University of Warwick in the United Kingdom. "The fact that this star hosts a giant planet has big implications for the total number of giant planets we estimate exist in our galaxy."
