To date, astronomers have confirmed the existence of 5,638 extrasolar planets in 4,199 star systems. In the process, scientists have discovered many worlds that have exceeded all expectations. This is certainly true of the “hot Neptunes,” planets that resemble the “ice giants” of the outer solar system but orbit much closer to their stars. However, when a team of astronomers led by Johns Hopkins University discovered TIC365102760 b (also known as Pheonix), they observed something completely unexpected: a planet the size of Neptune that maintained its atmosphere by puffing up.
Sam Grunblatt, an astrophysicist in JHU's William H. Miller III Department of Physics and Astronomy, led the research. He was joined by an international team that included Nicholas Saunders, NSF Graduate Research Fellow, Shreyas Vissapragada, Steven Giacalone, Ashley Chontos and Joseph M. Akana Murphy, 51 Pegasi b Fellows, and researchers from many prestigious institutes and universities. The paper describing their findings (which recently appeared in the Astrophysical Journal) is part of a series called “TESS Giants Transiting Giants.”
Artist's impression of JG436b, a hot Neptune located about 33 light-years from Earth. Image credit: STScI
Puff planets are a new class of incredibly rare exoplanets, estimated to make up 1% of all planets in our galaxy. The team discovered Pheonix by combining data from the Transiting Exoplanet Survey Satellite (TESS) with radial velocity measurements obtained by the High Resolution Echelle Spectrometer (HIRES) at the Keck Observatory. Their data showed that Pheonix is 0.55 times the size of Jupiter but only 0.06 times as massive, and orbits a red giant star with a period of 4.21285 days (about six times closer to its star than the distance between Mercury and the Sun).
Based on the age and temperature of its star, as well as the planet's remarkably low density, the team hypothesized that Phoenix's gaseous envelopes must have been separated billions of years ago. Based on its density, the team also estimates that the planet is the most bloated “puff planet” yet (about 60 times less dense than the densest “hot Neptune”) and that it will begin rotating into its star in about 100 million years. As Grunblatt explained in a JHU HUB press release:
“This planet is not evolving as we thought. It appears to have a much larger, less dense atmosphere than we expected for these systems. How it was able to retain this atmosphere despite being so close to such a large parent star is the big question.”
“It is the smallest planet we have ever found around one of these red giants, and probably the lowest-mass planet that has a [red] “The biggest star we've ever seen. That's why it looks really strange. We don't know why it still has an atmosphere, while other 'hot Neptunes', which are much smaller and denser, seem to lose their atmosphere in much less extreme environments.”
Artist's impression of Phoenix, the “hot Neptune” orbiting a red giant star 8 billion light-years from Earth. Image credit: Roberto Molar Candanosa/JHU
These findings could provide new insights into the late evolution of planetary systems and help scientists predict what will happen to the solar system in a few billion years. According to standard models of stellar evolution, our sun will exit its main sequence phase, expand into a red giant, and eventually engulf the inner planets. Based on these findings, they predict that Earth's atmosphere may not evolve as astronomers previously expected. Instead of our sun blasting it away, our atmosphere could expand and become incredibly “puffy.”
Pheonix is the latest bloated planet that the international team has studied using TESS data. While bloated planets are considered rare, exoplanets like Pheonix are particularly difficult to find due to their small size and low density. In the future, Grunblatt and his colleagues plan to search for more of these smaller worlds and have already identified a dozen potential candidates by combining transit and radial velocity data.
Further reading: John Hopkins University, The Astrophysical Journal
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