International researchers are engaged in a complex study of two planets in deep space that appear to have an average density lower than that of candy floss.
The planets orbiting star TOI-791, located 1,100 light-years away - 10.4 quadrillion kilometres - from Earth, are as large as Jupiter but possess only about one-thirtieth of its mass, according to the team led by Georgina Dransfield of the University of Oxford.
The scientists, who shared their findings this week in the journal Monthly Notices of the Royal Astronomical Society, now hope more precise observations will shed light on the formation and evolution of these enigmatic objects.
"Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system," said Dransfield. "Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve."
Amateur researchers discovered the two planets of TOI-791 in archival data from the TESS (Transiting Exoplanet Survey Satellite) space telescope in 2019 and 2023.
Since 2018, TESS has been monitoring the brightness of millions of stars to search for planets. When a planet passes in front of its star during its orbit, as seen from Earth, it regularly dims the star's brightness slightly, thus revealing its existence.
Average density of 0.04 grams per cubic centimetre
From the degree of dimming of the starlight, the researchers could deduce the size of the planets, prompting their comparison with Jupiter.
For several years after the discovery, Dransfield and her colleagues tracked the planets' transits and noticed slight irregularities. Their gravitational pull causes them to interfere with each other, and from these disturbances, the team could calculate the planets' mass.
The result was surprising: Both are much less dense than Jupiter, the fifth planet in our Solar System. This results in an average density of about 0.04 grams per cubic centimetre - even slightly lower than the typical density of 0.05 grams per cubic centimetre for candy floss.
For comparison, Earth's average density is 5.5 grams per cubic centimetre while Jupiter's is 1.3 grams. However, Dransfield and her colleagues rule out any actual fluffy consistency of the planets, as a celestial body made of such material would quickly disintegrate under the influence of gravity.
Rather, the researchers suspect that the planets' enormous size - and thus their low density - is merely an illusion created by an extensive shell of gas or dust.
The planets could have a relatively small core of rock and metals with a correspondingly high density, but this core is enveloped in a vast atmosphere of hydrogen and helium that absorbs starlight during transits.
Computer simulations show that such planets can form in cool, dust-poor regions around a young star. An alternative explanation would be enormous dust clouds, which, for example, could envelop a planet after asteroid impacts, making it appear larger.
The team now plans to examine the two planets more closely using the James Webb Space Telescope, which is orbiting the Sun about 1.5 million kilometres from the Earth.
With its high resolution, the telescope should be able to peer into the atmospheres of the planets and thus solve the mystery of their low density.
