How can a giant planet be lighter than cotton candy? Scientists just found two

Super-puff planets have stunned astronomers after an international research team discovered two giant worlds with densities so low they are lighter than cotton candy. The newly identified exoplanets, TOI-791 b and TOI-791 c, are roughly the size of Jupiter but contain surprisingly little material packed into enormous atmospheres. Their discovery is reshaping what scientists know about planet formation and the strange diversity of worlds beyond our Solar System.

Located about 1,110 light-years away in the constellation Volans, these unusual planets orbit an F7-type dwarf star. Their densities are among the lowest ever measured for giant planets, challenging the traditional idea that large planets must always be massive and tightly packed.

The finding, led by researchers from the University of Oxford with collaborators from Université Côte d’Azur, Observatoire de la Côte d’Azur, and the University of Birmingham, reveals a rare planetary system where two “fluffy” worlds exist together. Published in the Monthly Notices of the Royal Astronomical Society, the study offers a new window into how planets grow, hold their atmospheres, and evolve over billions of years.

How can planets become lighter than cotton candy?

The discovery of TOI-791 b and TOI-791 c seems almost impossible at first glance. Both planets are similar in size to Jupiter, the largest planet in our Solar System, yet they are dramatically less dense.

TOI-791 b has a density of only 0.038 grams per cubic centimeter, while TOI-791 c measures around 0.047 grams per cubic centimeter. Jupiter, by comparison, has an average density of 1.33 grams per cubic centimeter.

That means these distant planets are around 28 to 35 times less dense than Jupiter. Their densities are even below cotton candy, which typically sits near 0.05 grams per cubic centimeter.

But the comparison does not mean these planets are made of sugar-like material. Instead, scientists believe they are likely huge balls of gas with extremely expanded atmospheres surrounding smaller solid cores. These worlds belong to a rare category known as “super-puff” planets. The name comes from their enormous size compared with their surprisingly low mass.

Why are super-puff planets important for astronomy?

For years, astronomers have searched for planets that break the usual rules of planetary science. Super-puff planets are among the strangest examples because they force researchers to rethink how planets form.

Most giant planets are expected to gather large amounts of gas and become dense, like Jupiter and Saturn. Super-puffs appear different. Their atmospheres may contain large amounts of hydrogen and helium, creating a huge outer layer around a relatively small core.

One leading theory suggests these planets formed far away from their parent star, in colder regions of a young planetary system. In those distant zones, gases could cool quickly and collect around developing planetary cores.

What makes the TOI-791 system so unusual?

The two planets are not only rare because of their low densities. They also exist as a gravitationally connected pair. TOI-791 b and TOI-791 c are locked in a 5:3 mean-motion resonance. This means that when the inner planet completes five orbits around its star, the outer planet completes almost exactly three.

This relationship creates a repeating gravitational dance. The planets constantly pull on each other, causing tiny changes in the timing of their movements.

Those small shifts helped researchers estimate their masses. By combining transit observations, where planets pass in front of their star, with gravitational measurements, scientists calculated their extraordinary densities.

The discovery involved years of observations and international teamwork. The planets were first identified as candidates through the Planet Hunters TESS citizen-science project, where volunteers examine data collected by NASA’s Transiting Exoplanet Survey Satellite.

TOI-791 b was spotted in 2019, while TOI-791 c was identified later in 2023. Citizen scientists helped flag the signals that professional astronomers could investigate further.

Each transit lasted more than 11 hours, making them some of the longest complete planetary transits ever captured from the ground. The discovery highlights how modern astronomy increasingly depends on global collaboration, combining space missions, ground-based observatories, and public participation.

The next major step may come from the James Webb Space Telescope. Researchers plan to study the chemical makeup of these enormous atmospheres.

Scientists hope Webb observations can reveal whether the planets contain carbon-, nitrogen-, and oxygen-based molecules. Those details could help explain where these planets formed and how they became so inflated.

A planet can be enormous yet incredibly light. It can look like a gas giant but behave unlike anything nearby. These super-puff planets remind scientists that the universe still has many surprises hidden among the stars.

FAQs:

Could super-puff planets lose their giant atmospheres over time?
Yes, it is possible. A planet’s outer gases can escape into space if radiation from its star heats the atmosphere strongly enough. Studying these planets helps scientists understand how planetary atmospheres survive or disappear over billions of years.

What can these unusual planets teach us about Earth?
Although Earth is completely different from super-puff worlds, studying distant planets helps scientists understand the variety of possible planetary environments. It also provides clues about how common or unusual Earth’s own formation history might be.

Why are multiple-planet systems more valuable for research?
When several planets orbit the same star, their interactions provide extra information. Their gravitational relationship can reveal hidden details about their masses, movements, and past evolution that would be difficult to learn from a single planet.

Could there be many more super-puff planets waiting to be discovered?
Very likely. Current discoveries depend on available telescope technology and observation time. As missions improve, astronomers expect to find more unusual planets that challenge existing ideas about how worlds form.