A Fluffy "Marshmallow" World Has Been Discovered With Incredibly Low Density

A Fluffy “Marshmallow” World Has Been Discovered With Incredibly Low Density

If exoplanet research is to be believed, the Milky Way galaxy could look like some kind of fantasy candy land.

First, there was the discovery of exoplanets with the density of cotton candy. Now astronomers say they have discovered a world comparable to the density of marshmallows. It is, they say, the fluffiest exoplanet discovered to date orbiting a red dwarf star.

It is important. This means that worlds with significant gas envelopes can be found in close orbit around small stormy dwarf stars, which astronomers previously suspected could rob nearby planets of much of their atmospheres.

Since atmospheres are considered one of the main 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 considered 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 examined with small samples of Doppler surveys, which have generally found giant planets further away from these red dwarf stars. So far we haven’t had a large enough sample of planets to find nearby gaseous planets in a robust way.”

Red dwarf stars are by far the most numerous stars in the Milky Way. They are very small, cold and dim – so dim, in fact, that none can be seen with the naked eye, despite making up about 73% of all stars in the Milky Way.

Because they’re small, they burn slower and much cooler than stars like our Sun, which means they have a much longer lifespan. The lifespan of our Sun is estimated at about 10 billion years. Red dwarf stars are expected to live for billions of years. This longevity, coupled with the abundance of red dwarf stars, means that life, if it were to emerge somewhere, could emerge on a planet orbiting a red dwarf star.

But red dwarfs can also be really, really grumpy, blasting the space around them with powerful flares that could irradiate and sterilize any exoplanets in close orbit, and strip them of their atmospheres. And because these stars are so cold, for an exoplanet to have a temperature conducive to life as we know it, that planet would have to be within range of fire. So, you know, it’s a problem.

But maybe not, 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 using the TESS space telescope, which finds exoplanets by detecting regular dips in light caused by the planet passing in front of the star. If you know how bright the star is, the amount of light blocked tells you how big the exoplanet is. From this we know that TOI-3757b is a bit bigger than Jupiter.

Then, to get the mass of the exoplanet, the researchers looked for changes in the light of the star that show the gravitational pull exerted on it by the exoplanet. Since gravity is related to mass, this gave us a mass of about 85 Earths.

Jupiter, by 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. It’s an extremely fluffy exoplanet – so fluffy that it’s unclear how it could have formed so close to its star: it completes an orbit every 3.43 days.

Kanodia and his colleagues believe there could be two factors at play. First, gas giants form with a rocky core, around which gas collects to form a thick, expansive atmosphere. Because the red dwarf star is poor in heavy elements compared to other red dwarfs with gas giants, perhaps the rocky core formed a little slower, which would have delayed gas buildup and affected the density of the world.

Second, the orbit appears to be slightly oval, which means that its distance from the star varies. Maybe as he gets closer, the atmosphere heats up and expands.

Other astronomers have suggested that puffy exoplanets might have extended ring systems, like Saturn; but Kanodia and the other researchers found that TOI-3757b is simply too close to its star to maintain a stable ring system. So it’s probably just a really bloated atmosphere.

The team hopes to find and study other such marshmallow worlds, to help understand 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 once thought to be extremely rare around red dwarfs — is part of our goal to understand how planets form,” says Kanodia.

We hope they are sufficiently equipped with sweet snacks.

The team’s research has been published in The Astronomical Journal.

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