They are very unusual. An international collaboration exposed them. They are two of the lowest density giant planets ever detected. The strange super-fluffy planets have densities lower than that of cotton candy. The study is led by the University of Oxford (United Kingdom).
The two planets, named TOI-791 b and TOI-791 c, orbit a type F7 dwarf star. It is about 1,110 light years from Earth in the southern constellation of Volans. Both planets are similar in size to Jupiter. But they are extraordinarily diffuse: TOI-791 b has a density of just 0.038 grams per cubic centimeter. The other has a density of 0.047 grams per cubic centimeter. By comparison, Jupiter's average density is 28 to 35 times greater.
Minimum density
Its density is even lower than that of cotton candy. This usually has a density of about 0.05 grams per cubic centimeter. In contrast, the density of the Earth is 5.5 grams per cubic centimeter.
These planets are believed to be "siblings" and formed together from the same disk of gas and dust that surrounded their young star. They are linked by a unique gravitational relationship known as the 5:3 mean motion resonance. It means that for every five orbits the inner planet completes, the outer planet completes almost exactly three. This gravitational interaction causes the planets to attract each other on a recurring basis.
Strange superfluffy planets are very rare. And it's 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.
Combined telescopes
The densities of the planets were measured by combining observations of their sizes and masses using telescopes around the world. When a planet passes in front of its host star (an event known as a transit), it slightly dims the star's light. The magnitude of this dimming reveals the size of the planet. In this system, the researchers also detected subtle variations in the timing of transits. They were caused by the gravitational attraction between the two planets as they orbit the star. By analyzing these timing changes, the team was able to estimate the masses of the planets and calculate their densities. They turned out to be extraordinarily low.
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