Cotton-Candy Planets: The Puffiest Worlds Found So Far

The word "planet" tends to bring solid ground to mind, or at least a heavy ball of gas like Jupiter. So it is worth pausing over the news out of NASA's Transiting Exoplanet Survey Satellite: two newly measured worlds are as wide as Jupiter but weigh so little that, if you could find a bathtub large enough, they would float. Their bulk density sits near that of cotton candy, roughly 0.05 grams per cubic centimeter. Astronomers call this rare class a super-puff, and these two are the puffiest yet confirmed.

The planets, catalogued TOI-791 b and TOI-791 c, orbit a Sun-like star about 1,100 light-years away. The findings were published in the Monthly Notices of the Royal Astronomical Society. For an amateur observer, the star itself is far too faint to see without a telescope and a chart, and the planets are invisible to any backyard instrument. But the story of how they were weighed is exactly the kind of measuring that patient observers can appreciate, and it points to a few things you can actually do under a dark Maine sky.

How you weigh a planet you cannot see

No telescope resolves these worlds as disks. Their existence is inferred from two complementary tricks, and understanding both is half the fun.

The first is the transit method, which is what TESS is built for. When a planet crosses in front of its star from our line of sight, it blocks a sliver of light, and the star dims by a tiny, repeating amount. The depth of that dip tells you how wide the planet is compared to the star. A Jupiter-sized planet crossing a Sun-sized star drops the brightness by about one percent, a change no eye can detect but a sensitive detector can graph as a clean light curve.

Size alone does not give you density. For that you need mass, and mass comes from the radial velocity method: as the planet tugs its star, the star wobbles slightly toward and away from us, shifting its light bluer and redder in a rhythm. The size of that wobble reveals the planet's weight. Put the two together, a width from the transit and a mass from the wobble, and you get density. In the case of TOI-791 b and c, the answer came out startlingly low.

Why a planet ends up this fluffy

A density near cotton candy means these are gas giants with wildly inflated atmospheres, puffed up far beyond what their modest mass would seem to allow. Astronomers do not fully agree on how that happens, and the leading ideas are worth knowing.

• Youth and heat. A young planet is still warm from formation, and a hot atmosphere swells. Some super-puffs may simply be caught early, destined to shrink as they cool over billions of years.
• Tidal heating. A close, slightly stretched orbit can flex a planet and pump energy into it, keeping the atmosphere inflated longer than it otherwise would be.
• An observing illusion. A few candidates may look puffy because faint planetary rings or high-altitude hazes make the planet block more starlight than its true surface would, inflating the apparent size.

Which explanation fits TOI-791's pair is a question for follow-up work, likely with instruments that can probe the atmospheres for their makeup. That is the same detective spirit that keeps our own hobby interesting: a measurement raises a better question.

Bringing it back to the backyard

There is a real, hands-on version of this science that amateurs contribute to every clear night. Recording the tiny dip of a planet crossing a bright, nearby star is within reach of a small telescope, a stable mount, and a camera doing careful differential photometry, comparing the target star against steady neighbors. Networks of amateur observers help confirm transit timings and catch changes that space telescopes cannot watch continuously.

You do not need to chase a transit to take something from this news, though. The next time you are set up under a dark sky, look toward the Summer Triangle and remember that the same simple ideas, a star that dims when something passes in front of it and wobbles when something pulls on it, are how we know that two planets the size of Jupiter and nearly as light as air float somewhere out there. New to the sky? Our beginner's guide to stargazing is the place to start, and for a bright summer signpost see how to find Boötes and Arcturus.

Clear skies, and keep looking up. If you would like to observe with us, learn more about our editorial team and club or return to the latest guides on the home page.