Astronomers have spied three more exoplanets. But the discovery may not last long. Each planet is in a separate solar system, and each planet orbits dangerously close to its star. Even worse, all the stars are dying.
Three doomed planets.
TESS (Transiting Exoplanet Survey Satellite) and other planetary hunting efforts have found thousands of exoplanets in the last few years and decades. Exoplanets vary widely, from Earth-like planets in the quiet habitable zones of their stars to planets so hot that evaporated iron falls like rain.
But these three exoplanets have something in common. They have very short orbits - some of the shortest ever found - around sub-giant or giant stars. For planets like these, the writing is on the wall: they will spiral inward toward their stars, which will eventually engulf them.
A team of scientists presented these dismal results in a new paper entitled "TESS Giants Transiting Giants II: The Hottest Jupiters orbiting Evolved Stars." The Astronomical Journal accepted the paper and it is up on the pre-press page arxiv.org. Samuel Grunblatt is the lead author. He is a postdoc at the American Museum of Natural History and the Flatiron Institute.
The discovery may sound macabre: three doomed planets that will spiral inward toward their end, with stars also on their way to death as they leave the main sequence. But that would be a misrepresentation of the discovery. Think of it instead as planetary archeology.
"These discoveries are crucial to understanding a new frontier in exoplanet studies: how planetary systems evolve over time," Grunblatt said in a press release. "These observations provide new windows for planets nearing the end of their lives before their host stars swallow them."
The three planets are TOI-2337b, TOI-4329b and TOI-2669b. TOI stands for Tess Object of Interest, and the number is the star. "b" indicates the planet closest to the star in each system.
All the planets are gas giants that look like Jupiter in our solar system. Astronomers found them during a study looking for new planets orbiting evolved host stars. The worlds range from 0.4 to 1.8 Jupiter masses and 0.8 to 1.8 Jupiter radii. The stars range from about 1.2 to 1.5 solar masses and about 2.3 to 4.1 solar radii.
The planets have a wide range of densities, indicating that each of the solar systems underwent a chaotic period of planet-to-planet interactions. Astronomers believe that the history of these interactions contributed to the density variations through unpredictable warming rates and time scales.
The James Webb Space Telescope may be able to tease some of the details of at least one of these systems. When examining the TOI-4329 system, the presence or absence of water vapor and CO2 in the planet's atmosphere may limit the place where the planet was formed. It could also provide insight into the types of planetary interactions that put the planet into its current orbit.
In planetary star configurations like these, astronomers expect the planets to inflate as they move inches closer to the stars and as the stars expand. The stars will eventually engulf the planets as their orbits spiral inward. So far, TESS has not detected any signs of a decaying orbit. But TESS found these planets during its primary mission. It will collect more data during its extended missions, and this data will form a longer baseline of observations. The more extended baseline may indicate that the planets are already in their spirals of death.
Planetary inflation is becoming an important measure of understanding exoplanets. Why do some planets as massive as Jupiter have much larger radii? Gas giants experience radiation cooling from their inner depths, which inhibits inflation over long time scales that begin shortly after formation. Proximity to a star can delay this cooling and promote inflation.
But there is also growing evidence that proximity to a star can actively cause inflation through stellar flux. In an earlier paper by lead author Greenblatt in 2017, the authors wrote: "Stellar currents flowing to the planets 'deep convective interior could therefore explain their current size, an indication that the planet's' inflation 'is directly related to stellar radiation rather than delayed atmospheric cooling after the planets' formation. "
But planetary inflation is a complicated issue. In the same paper from 2017, the authors write that "Planets can be inflated by methods that are more strongly dependent on other factors such as atmospheric metallicity than incident flux."
Although there is no indication of orbital decay in the planets yet, there is evidence of inflation for at least one planet. TOI-2337b has the shortest orbital period of any world ever found around a red giant star, but it shows no signs of inflation. The TOI-2669b system is the most developed system of the three, but the planet shows no signs of inflation either. However, TOI-4329b shows signs of inflation.
TOI-2669b is a lighter target than the other two in this study. That makes it a great target for spectroscopy with the James Webb Telescope. These data, along with tighter constraints on the planet's eccentricity, "... could place new constraints on the planet's inflation and migration mechanisms and time scales."
The three planets in this study are just the beginning. TESS should find many more of them. "We expect to find 10-100 of these evolved transit planet systems with TESS that provide new details on how planets interact with each other, inflate and migrate around stars, including those like our sun," said Nick Saunders, a graduate student at UH IfA and co-author of the study.
TESS had some help in this work. While finding the planets with the transit method, these results reveal only candidate exoplanets. Confirmation as exoplanets requires terrestrial follow-up observations. The Keck Observatory provided them via its High-Resolution Echelle Spectrometer (HIRES) instrument. Data from HIRES concretized the discoveries.
"The Keck observations of these planetary systems are crucial to understanding their origins, and help reveal the fate of solar systems like our own," said UH IfA astronomer Daniel Huber, co-author of the study. Studies like this can also be small steps towards answering humanity's most important question: Are we alone?
All of this may sound academic to a casual reader. But our own sun will one day leave the main sequence and begin to expand into a red subgigant and will likely engulf the Earth. It is doubtful whether humanity will survive until then, as it will not happen for billions of years. Extinction is the norm here on Earth, but perhaps humanity will somehow beat the cosmic odds.
In the meantime, we are at least learning more about nature and the universe we are a part of.