Two planets the size of Jupiter, lighter than cotton candy

Space 4 min 80 sources

Astronomers found a pair of 'super-puff' worlds 1,110 light-years away — Jupiter-sized but so faint in mass that Jupiter itself is up to 35 times denser. Plus a planet that brushes magnetic fields with its star, and a new tool to triage the search for life.

Key takeaways

Astronomers found two Jupiter-sized planets so light they'd float on water — Jupiter is up to 35 times denser — 1,110 light-years away.
We never saw the planets: their size came from the starlight they block, their mass from how their gravity tugs each planet's timing, and density from combining the two.
A separate planet appears to brush magnetic fields with its star, and a new Stanford tool helps astronomers skip rocky worlds that can't hold an atmosphere.

The week’s strongest space story isn’t a launch or a rover. It’s two planets that shouldn’t hold together — and what they reveal about how we measure worlds we will never visit.

A pair of cosmic puffballs

Astronomers announced two exoplanets — planets orbiting a star other than the Sun — that are each about the size of Jupiter, yet so light they are the least dense planets ever found [43]. Both circle a small star called TOI-791, about 1,110 light-years away [43]. To put that distance in scale: light, the fastest thing there is, takes 1,110 years to cross it. We see this star as it was around the year 916.

The numbers are the story. Jupiter is the biggest planet in our solar system, and it’s already a lightweight — mostly gas. But Jupiter is 28 times denser than one of these new planets, TOI-791 c, and 35 times denser than the other, TOI-791 b [43]. Their density is lower than cotton candy. Find an ocean big enough, and they’d float.

Astronomers call these “super-puffs,” and only a handful are known. “It is even rarer to find two in the same system,” said George Dransfield of Oxford University, who led the team [43].

How you weigh a planet you can’t see

Here’s the part worth slowing down for: no telescope took a picture of these planets. We learned their size one way and their mass a completely different way.

The size came from NASA’s TESS spacecraft — the Transiting Exoplanet Survey Satellite — which watches stars for a tiny dip in brightness when a planet crosses in front [43]. A bigger planet blocks more light, so the size of the dip tells you the size of the planet. These two block the star for an unusually long time — transits of 11 hours each, some of the longest ever recorded [43].

The mass came from the planets tugging on each other. The pair are locked in a rhythm where the inner planet makes five laps for every three of the outer one [43]. As they swing past, their gravity nudges each other, shifting the exact timing of those star-crossings. Measure the nudge, and you can work out how heavy each planet is — without ever weighing it directly [43]. Combine the size (big) with the mass (tiny), and you get the density (almost nothing).

First spotted by volunteers in the Planet Hunters citizen-science group sifting TESS data, the planets were confirmed with telescopes around the world, including one in Antarctica [43]. The team now wants the James Webb Space Telescope, NASA’s big infrared observatory, to read the planets’ atmospheres for carbon, nitrogen and oxygen — clues to how such featherweight worlds form [43].

A planet that touches its star

Elsewhere, another oddity: a planet so close to its star that their magnetic fields appear to connect [48]. The star, GJ 436, is a red dwarf — a small, cool star half the Sun’s mass — about 30 light-years away [48]. Its one known planet, four times Earth’s mass, whips around it every 2.6 days [48].

Researchers combing years of archived observations found the star’s outer layer brightens on a rhythm that tracks the planet’s orbit [48]. Their reading: the planet’s magnetic field reaches the star’s, and the meeting lights up a thin shell of the star called the chromosphere [48]. It’s a reminder that “close to a star” out there means things our own solar system never prepared us for — metal vapour in the air, atmospheres puffed thin, magnetism that bridges the gap [48].

A filter for the search for life

With thousands of known exoplanets and billions thought to exist — roughly one per star in the galaxy — the real bottleneck isn’t finding worlds; it’s deciding which few deserve precious telescope time [42]. Stanford researchers released a tool, called STEHM, that screens small rocky planets for one prerequisite of life: whether a world can build and hold onto an atmosphere over billions of years [42]. It can’t tell you a planet is alive. It can tell you which ones aren’t worth the look — and that is its own kind of progress [42].

02 · Lesson · why it matters

The big thing in front of you is usually the thing telling you the least

Size is loud and easy to measure. What a thing is actually made of is quiet, invisible, and only shows up when you put two separate measurements together.

A planet as big as Jupiter that floats

Two planets just turned up that break the picture in your head. They’re each about as big as Jupiter — the giant of our solar system. And they are so light that Jupiter is up to 35 times denser. Their stuff is thinner than cotton candy. Drop one in a big enough ocean and it would bob on the surface.

Big and heavy usually travel together in our minds. A boulder is heavy; a beach ball is light. These planets snap that link. Same size as the heavyweight, almost none of the weight. The size told the astronomers almost nothing about what they were dealing with.

The loud measurement and the quiet one

So how did anyone know? Not from a photo — there is no photo. The planets are 1,110 light-years away, far too distant and too dim to see directly.

The size was the easy part. A spacecraft watched the star and caught a tiny dip in its light each time a planet crossed in front. Bigger planet, bigger dip. That measurement is loud — it practically announces itself. And it’s the one that fooled you, because it said “Jupiter-sized” and your mind filled in “Jupiter-heavy.”

The mass was the hard part, and it came from somewhere else entirely. The two planets pull on each other as they orbit, nudging the timing of those star-crossings by a few minutes. Measure the nudge and you can work out how heavy each one is — even though nothing was ever placed on a scale. That measurement is quiet. It hides inside the timing. You have to know to look for it.

Density — what the thing is really made of — is neither measurement alone. It’s size and mass set against each other. You only get the truth by holding two unrelated readings up together. Either one by itself lies.

We do this with everything

This isn’t only about planets. It’s how we misread almost everything.

The loud feature — the size of a company, the volume of a confident voice, the price of a thing, the follower count, the muscle — is the one we can see without effort. So it’s the one we judge by. And it’s usually the feature that tells us the least about what’s actually inside. The quiet feature — the debt under the company, the substance under the confidence, the durability under the price — is the one that decides how the thing behaves. But it’s invisible, and you only find it by pairing it with something else.

A big, sturdy-looking institution can be hollow. A small, quiet one can be dense with the thing that matters. The cotton-candy planet is a perfect, literal version of a mistake we make daily: reading the size and assuming we’ve read the substance.

It’s tempting to take this as a clever filter to use on others — to spot the puffed-up thing across the room. But the harder half is that we are also the planet being measured. Other people read us by our loud feature — our title, our certainty, the size of our claim — and fill in the rest, the same way you filled in “heavy” from “big.” We do it to ourselves, too. We mistake the visible part of our own life for the whole of it.

Nobody gets density for free. The astronomers needed two telescopes’ worth of patience and a trick of orbital timing to weigh a world they couldn’t see. Knowing what something — or someone — is really made of always costs a second, quieter measurement. The size is just the thing that gets your attention first.

On the whole

The most striking fact about these planets is also the most ordinary lesson hiding in them: the feature that grabs you is rarely the feature that matters, and the one that matters is the one you can’t see until you look twice. We weigh the world by what’s loud, then act surprised when it doesn’t behave the way its size promised. Seeing the whole here means holding the easy reading a little more loosely — yours of others, theirs of you, your own of yourself — and remembering that the truth of a thing is a property you have to earn, not one you can glance at. Educate yourself for humble decisions.

03 · Lab · your turn

Weigh a World

Rehearse judging a planet by its size, then taking the second measurement that reveals size told you almost nothing.

04 · Hope · carry this

A handful of volunteers and a telescope in Antarctica just weighed a world floating 1,110 light-years away, simply by watching it tug on its neighbour. We keep finding ways to know the unreachable — patient, clever, two careful measurements at a time.