If exoplanet research is to be believed, the Milky Way galaxy could be like some kind of fantastic candy land.
First, there was the discovery of exoplanets with the density of cotton candy. Now, astronomers say they’ve discovered a world that’s comparable in marshmallow density. It is, they say, the fluffiest exoplanet yet discovered orbiting a red dwarf star.
This is important. It means worlds with significant gas envelopes can be found closely orbiting small, stormy dwarf stars, which astronomers had previously suspected could strip any closely orbiting planet of a large proportion of its atmospheres.
Since atmospheres are believed to be one of the key planetary features that allow life to form and thrive, this would have implications for our understanding of the habitability of planets orbiting red dwarf stars.
“Giant planets around red dwarf stars have traditionally been thought to be difficult to form,” says planetary astronomer Shubham Kanodia of the Carnegie Institution for Science’s Earth and Planets Laboratory.
“So far this has only been looked at with small samples from Doppler surveys, which have typically found giant planets further away from these red dwarf stars. Until now we haven’t had a large enough sample of planets to find gas planets close to in a robust manner.”
Red dwarf stars are by far the most numerous stars in the Milky Way. They are very small, cool, and faint—so faint, in fact, that none of them can be seen with the naked eye, even though they make up about 73 percent of all the stars in the Milky Way.
Because they are small, they burn more slowly and less hotly than stars like our Sun, which means they have significantly longer lifespans. It is estimated that the useful life of our Sun is about 10 billion years. Red dwarf stars are expected to live for billions of years. This longevity, along with the abundance of red dwarf stars, means that life, if it is going to arise anywhere, could arise on a planet orbiting a red dwarf star.
But red dwarfs can also be very, very angry, whipping the space around them with powerful flares that could irradiate and sterilize any nearby exoplanets and strip them of their atmospheres. And because these stars are so cool, for an exoplanet to have a temperature conducive to life as we know it, that planet would have to be within the flare range. So, you know, that’s a problem.
But it may not be, as this new world suggests. It’s called TOI-3757b, and it’s a gas giant orbiting a red dwarf star in the constellation Auriga, about 580 light-years away.
TOI-3757b was detected by the TESS space telescope, which finds exoplanets by detecting the regular dips in light caused by the planet’s passage in front of the star. If you know the brightness of the star, how much light is blocked tells you the size of the exoplanet. From this, we know that TOI-3757b is slightly larger than Jupiter.
Then, to get the exoplanet’s mass, the researchers looked for changes in the star’s light that showed the exoplanet’s gravitational pull. Since gravity is related to mass, this gave us a mass of about 85 Earths.
Jupiter, for context, has a mass of about 318 Earths, with an average density of 1.33 grams per cubic centimeter. The average density of TOI-3757b is 0.27 grams per cubic centimeter. This is an extremely fluffy exoplanet, so fluffy that it’s not clear how it could have formed so close to its star: it completes one orbit every 3.43 days.
Kanodia and his colleagues think there could be two factors at play. Gas giants first form with a rocky core, around which gas accumulates to form a thick, extended atmosphere. Because the red dwarf star has few heavy elements compared to other gas giant red dwarf stars, the rocky core may have formed a bit more slowly, which would have slowed the accretion of gas and affected the density of the world.
Second, the orbit appears to be slightly oval, which means that its distance from the star varies. Perhaps as it approaches, the atmosphere heats up and expands.
Other astronomers have suggested that inflated exoplanets could have extended ring systems, like Saturn; but Kanodia and the other researchers found that TOI-3757b is simply too close to its star to support a stable ring system. So it’s probably just a very inflated atmosphere.
The team hopes to find and study other marshmallow worlds out there, to help figure out how they form and survive in a place where it should be difficult for them to do so.
“Finding more such systems with giant planets, which were previously theorized to be extremely rare around red dwarfs, is part of our goal of understanding how planets form,” says Kanodia.
Hopefully they are sufficiently stocked with sugary snacks.
The team’s research has been published in The Astronomical Journal.