Super-puff exoplanets discovered by researchers are lighter than cotton candy

Two newly identified worlds challenge expectations about how massive planets should feel. Orbiting a faint dwarf star roughly 1,110 light-years away in the southern skies, these objects are about as wide as Jupiter yet astonishingly airy — so much so that scientists liken them to being lighter than cotton candy.

Designated TOI-791b and TOI-791c, the pair stand out for having some of the lowest measured bulk densities among known giant exoplanets. Their discovery and follow-up observations are reshaping how astronomers think about planetary formation and atmospheric evolution in closely packed systems.

Jupiter-sized but unexpectedly feather-light: the numbers that stunned astronomers

Both planets are comparable in radius to Jupiter, but their masses are tiny in comparison, producing densities that are extreme outliers when stacked against familiar Solar System bodies.

• TOI-791b has an estimated density near 0.038 grams per cubic centimeter.
• TOI-791c measures only slightly denser at about 0.047 grams per cubic centimeter.

To give this context: Jupiter’s average density is about 1.33 g/cm³, Earth’s is roughly 5.5 g/cm³, and spun sugar—cotton candy—comes in around 0.05 g/cm³. That places these newly found planets among the most diffuse giant planets ever recorded, even lighter than candy floss.

How these “super-puff” planets were found: a mix of crowdsourcing and extreme observing conditions

The initial signals that led to the planets’ identification came not from a single observatory but from volunteers scanning data. Citizen scientists working with the Planet Hunters TESS project flagged irregular transit signatures in NASA’s TESS photometry, prompting professional follow-up.

Those follow-up measurements combined resources across the globe. Notably, one of the key ground-based contributions came from the Antarctic Search for Transiting ExoPlanets (ASTEP). The unique conditions of the Antarctic winter — prolonged darkness and stable observing windows — allowed astronomers to record uninterrupted transits that lasted more than 11 hours, the longest continuous ground-based transit observations ever completed for a planet.

Instruments and teams that made the case

• Space-based survey: TESS (Transiting Exoplanet Survey Satellite)
• Citizen science initiative: Planet Hunters TESS
• Ground-based monitoring: ASTEP in Antarctica plus multiple telescopes across several continents

Combining transit depths (which set the radii) with timing variations and other mass-sensitive measurements enabled the team to infer the tiny masses and therefore the extraordinary low densities of TOI-791b and TOI-791c.

Locked in an orbital dance: resonance, timing shifts, and what they reveal

The pair aren’t solitary wanderers; they share a strong gravitational relationship. The planets are in a near 5:3 mean-motion resonance, meaning the inner world completes five orbits in almost the same time the outer world completes three. That configuration causes repeated mutual tugs that slightly alter the schedule of each transit.

Those timing deviations — called transit-timing variations (TTVs) — provided a way to estimate the planets’ masses without relying solely on radial-velocity measurements, which can be hard to obtain for faint host stars. By modeling the TTVs, researchers constrained the planets’ masses and confirmed how unusually inflated their volumes are for such low mass.

Why are these planets so puffy? Theories and puzzles about formation

Super-puff planets are rare, and finding two in the same system deepens the mystery. Astronomers are considering several possibilities to explain their extreme puffiness:

• Inflated, hydrogen-rich envelopes that have escaped significant contraction.
• High-altitude aerosols or hazes that affect transit measurements and mimic a larger apparent radius.
• Young age or residual heat from formation that keeps atmospheres extended.
• Atmospheric escape processes coupled with low core mass, producing ephemeral bloated envelopes.

None of these explanations is yet definitive for TOI-791b and c. The fact that they likely formed from the same protoplanetary disk — termed “siblings” by the research team — adds a constraint: whatever process created their puffiness must be compatible with the system’s shared origin and dynamical history.

Next steps: what astronomers plan to learn with powerful telescopes

To distinguish among formation scenarios and to probe atmospheric composition, the researchers have proposed further observations using space-based facilities. The James Webb Space Telescope (JWST) is a prime candidate to search for molecular signatures — carbon-, nitrogen-, and oxygen-bearing gases — that would illuminate the chemical makeup of the extended envelopes.

Planned investigations will target:

1. Transmission spectroscopy to detect molecules and aerosols in the atmospheres.
2. Additional transit monitoring to refine mass and orbital interaction models.
3. Comparative studies between the two planets to understand why both became super-puffy in the same system.

Coordinated observing campaigns that combine space telescopes with long-baseline ground assets — especially sites providing long continuous watches like Antarctica — are expected to be essential for teasing apart these subtle signals.

Why this discovery matters for exoplanet science and searches

Finding two extreme low-density giants orbiting the same star gives astronomers a rare laboratory for testing models of planetary structure and evolution. These systems challenge assumptions about how gas giants cool and contract, how atmospheres survive stellar irradiation, and how multi-planet dynamics influence observable properties.

Only a handful of super-puff planets are known, and identifying a sibling pair offers an especially powerful datapoint: it pushes investigators to reconcile atmospheric physics, system architecture, and formation history within a consistent framework.

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Michael Thompson

Michael Thompson is an experienced journalist covering U.S. and global news. With ten years on the front lines, he breaks down political and economic stories that matter. His precise writing and keen attention to detail help you grasp the real‑world impact of every event.