Planets more than 1,100 light years from Earth are as big as Jupiter, yet less dense than candy floss.
Astronomers from the University of Birmingham have helped to identify a pair of the lowest-density giant planets ever detected - so-called ‘super‑puff’ worlds with densities lighter than candy floss.
The newly characterised planets, TOI‑791 b and TOI‑791 c, orbit a distant dwarf star around 1,110 light years from Earth in the southern constellation Volans.
The international research team, led by the University of Oxford, published its findings in Monthly Notices of the Royal Astronomical Society. The study reveals that both planets may have formed from the disc of gas and dust surrounding their young star, making them ‘siblings’.
Being able to use a telescope in Antarctica, leveraging its incredibly long nights and optimal astronomical conditions, enables to collect data like no other telescope on Earth.
Even though the planets are about as big as Jupiter, they are made of material which is very spread out – making them extremely light for their size. TOI‑791 b has a density of just 0.038 grams per cubic centimetre, while TOI‑791 c has a density of 0.047 grams per cubic centimetre - around 28 to 35 times less dense than Jupiter.
Their densities are even lower than candy floss, typically about 0.05 grams per cubic centimetre. By comparison, Earth’s density is 5.5 grams per cubic centimetre—more than 100 times greater.
When a planet passes in front of its host star - an event known as a ‘transit’ - it slightly dims the star’s light, and the research team were able to calculate the planet’s size from the amount of observed dimming.
In this system, the researchers also detected subtle variations in the timing of the transits, caused by the two planets gravitationally tugging on one another as they orbit the star. Analysing these timing shifts allowed the team to estimate the planets’ masses and calculate their remarkably low densities.
The discovery draws on eight years of observations, including data from the Antarctic Search for Transiting ExoPlanets (ASTEP) telescope at Concordia Station in Antarctica. The site’s prolonged winter darkness enabled astronomers to repeatedly capture uninterrupted planetary transits lasting more than 11 hours, among the longest ever recorded from the ground, without which the mass of the two planets would not have been estimated.
Professor Amaury Triaud, from the University of Birmingham and the UK’s Principal Investigator of ASTEP, said: ”Being able to use a telescope in Antarctica, leveraging its incredibly long nights and optimal astronomical conditions, enables to collect data like no other telescope on Earth.”
Only four other planetary systems are known to host multiple super‑puff planets, making TOI‑791 an exceptionally rare laboratory for studying how such worlds form and evolve.
Lead author Dr George Dransfield, from the University of Oxford - formerly at the University of Birmingham - said: “Only a handful of these super‑puffy planets are known, and it is even rarer to find two in the same system. Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve.”
Dr Dransfield began collecting and analysing the observations for TOI-791 whilst doing her PhD research at the University of Birmingham, work for which she later received the Royal Astronomical Society’s Michael Penston prize, for best PhD thesis in Astronomy and Astrophysics.
The planets were initially flagged as candidates by volunteers taking part in the Planet Hunters TESS citizen science project, which analyses data from NASA’s Transiting Exoplanet Survey Satellite. Identified in 2019 and 2023, the planet candidates were later confirmed and characterised using observations from telescopes around the world, work orchestrated by Dr Dransfield.
Professor Tristan Guillot, Principal Investigator of ASTEP at Université Côte d’Azur (France), added: “These multi-planetary systems are complex, with gravitational interactions evolving over very long timescales. This discovery highlights the importance of continued international collaboration in astronomy, bringing together observations from Antarctica, space telescopes and observatories across several continents to reveal the true nature of these extraordinary planets.”
Despite growing interest in these unusual worlds, scientists are still debating how super‑puff planets form. One leading theory suggests they possess vast hydrogen- and helium-rich atmospheres, built up when the planets formed in colder, outer regions of their protoplanetary disc, where gas could accumulate rapidly around a solid core.
Professor Amaury Triaud concludes: “This system offers a unique laboratory for understanding how super‑puff planets form and evolve. We propose to carry out space-based observations using the James Webb Space Telescope to assess whether these puffy atmospheres contain carbon-, nitrogen-, and oxygen-bearing species, which could reveal new insight into how these unusual planets formed.”
Professor of Exoplanetology
Staff profile for Professor Amaury Triaud
