Puffy, Marshmallow-Like Planet Could Float in a Bathtub

Astronomers encounter a lot of intriguing celestial bodies while studying space: a nebula that looks like a tarantula, massive black holes, ice volcanoes and a moon that sounds like dial-up internet, to name a few.

Now, they’ve found a unique exoplanet that’s strikingly similar to a popular sweet treat. The planet, called TOI-3757 b, has the same average density as a marshmallow.

Situated some 580 light-years away from Earth, the exoplanet is orbiting a red dwarf star in the Auriga constellation. Scientists located the distant gas giant using the 3.5-meter telescope at Kitt Peak National Observatory outside of Tucson, Arizona, and recently shared more details about their discovery in The Astronomical Journal.

Though discovering a new exoplanet—a planet outside of our solar system—isn’t uncommon, this one stands out.

For one, with its marshmallow-like fluffiness, TOI-3757 b is the lowest-density planet scientists believe they’ve ever found orbiting a red dwarf star. They calculated its average density to be around 17 grams per cubic feet, which is about one-quarter the density of water. This means it would float if placed in a “planet-sized, cosmic bathtub,” as Chris Young writes for Interesting Engineering.

It’s also unusual for a gas giant to be orbiting a red dwarf star, which can emit flares that are strong enough to strip a planet’s atmosphere and, as Kevin Hurler writes for Gizmodo, “roast it like a s’more.” The planet’s host is a “cool” red dwarf star, also known as an M dwarf. These types of stars, which are very common in the Milky Way and throughout the universe, are the dimmest and smallest of stars that fuse hydrogen into helium in their cores, known as main sequence stars. They are cool compared to other stars, such as our Sun (a yellow dwarf star), but they are still extremely active.

“Finding more such systems with giant planets—which were once theorized to be extremely rare around red dwarfs—is part of our goal to understand how planets form,” says study co-author Shubham Kanodia, an astrophysicist at Carnegie Institution for Science’s Earth and Planets Laboratory, in a statement.

Discovery Alert!
A Jupiter-like planet orbits a cool red dwarf star ~580 light-years from Earth in the constellation of Auriga the Charioteer. TOI-3757b is the lowest-density planet ever detected around a red dwarf. It has the density of a MARSHMALLOW. https://t.co/tYpWteTmWt pic.twitter.com/mK92TY7CU9

— NASA Hexoplanets (@NASAExoplanets) October 20, 2022

Scientists began taking a closer look at TOI-3757 b after NASA’s Transiting Exoplanet Survey Satellite (TESS) noticed the host star dimmed slightly when the exoplanet passed in front of it. In addition to the 3.5-meter telescope at Kitt Peak, they also observed the planet with ground-based instruments on the Hobby-Eberly Telescope (HET) at the McDonald Observatory in Texas and at the Red Buttes Observatory (RBO) in Wyoming.

From their analyses, they determined TOI-3757 b orbits its red dwarf star every 3.5 days and is slightly larger than Jupiter at roughly 100,000 miles wide. Though they don’t know for certain, scientists suspect the exoplanet has such a low density because its host star has lower-than-normal amounts of heavy elements, which in turn causes the planet’s rocky core to form more slowly. That means the exoplanet isn’t pulling in as much gas as other gas giants do, keeping its density down.

Another possible explanation for the low density is the slightly elliptical shape of the planet’s orbit. When it gets closer to the red dwarf, the star's excess heat may be causing the planet’s atmosphere to “bloat,” per the statement. Scientists now hope the new James Webb Space Telescope will be able to point its sophisticated instruments toward the exoplanet and shed further light on its “puffy nature,” says study co-author Jessica Libby-Roberts, a researcher at Pennsylvania State University, in the statement.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.