A gas giant almost twice the mass of Jupiter and as dense as aluminum is orbiting its Red Giant host star so tightly that it could plunge to its fiery death within just a million years — a death sentence to be carried out in the morning, in cosmic terms.

This exoplanet, TOI-2337 b, is just one of three such “hot Jupiters” found orbiting perilously close to their stars that were announced Thursday at a press conference for the 239th American Astronomical Society meeting. Together, they could give astronomers new insights into the behavior of gas giants more generally and open a window into the lives of planets condemned by gravity and clinging to the ledge.

“These planets are in such extreme places that actually less than 10 years ago, no one thought that they actually existed,” Samuel Grunblatt, a postdoctoral fellow at the American Museum of Natural History and lead author of a forthcoming paper in Astronomical Journal, said Thursday.

What’s new?— Out of the thousands of exoplanets discovered so far, the three gas giants — TOI-2337 b, TOI-4329 b, and TOI-2669 b — have the shortest periods of exoplanets orbiting sub-giant or giant stars, or what Grunblatt calls “evolved” stars.

“Going back to 2013, there were no planets which were known around these evolved stars,” he says, but TESS began finding “hot Jupiter” gas giants with short orbital periods around these evolved stars. “TESS has now led to the confirmation of the three shortest period planets ever found around evolved stars.”

TOI-2669 b orbits a red giant star roughly every six days, while TOI-4329b orbits a subgiant star in just under three days.

But TOI-2337 b is the strangest of the three new exoplanets. While TOI-4329 b has an average density similar to cork, “TOI-2337 b is actually 10 times as dense, more like a solid block of aluminum,” Grunblatt said.

That density, combined with TOI-2337 b orbiting its red giant star in just under three days, means astronomers expect the planet to complete a death spiral into its star within 1 million years, the shortest time to death for any known planet.

This NASA simulation of a hot Jupiter shows the searing effects from their home stars. NASA/JPL-Caltech/MIT/Principia College

How they did it— NASA’s Transiting Exoplanet Survey Satellite, or TESS, was instrumental in discovering the three gas giants, which first showed up in TESS images taken in 2018 and 2019. TESS monitors stars for changes in brightness associated with the transit — that is, passing of a planet — in front of that star. The changes in brightness and their duration help astronomers detect and characterize the exoplanets transiting the stars.

“TESS is really revolutionizing our ability to find these evolved transiting planets, revealing the hottest planets transiting evolved stars with the shortest period so far,” Grunblatt said.

Grunblatt and his colleagues confirmed the discovery of the three exoplanets using a spectrometer at the Keck Observatory on Maunakea, Hawaii.

Why it matters— Given that barely a decade ago, astronomers didn’t think planets like TOI-2337 b existed at all, the discovery of these exoplanets adds to a new but expanding catalog of worlds that are expanding our understanding of what is possible in the universe.

“We’re starting to build up a population, but there’s still a lot of questions that we need a larger planet population to answer,” Grunblatt said.

When astronomers only had our solar system as a model, which was not that long ago, it would have seemed strange to think gas giants could end up anywhere near their stars. After all, the gas giants of our Solar System are the outer planets.

But hot Jupiters are turning out to be fairly abundant in the universe, and just how they come to orbit so close to their stars could tell us a lot about why our own Jupiter kept its distance — and what that might mean for life here and throughout the cosmos.

The study of these systems should tell us how giant planets move throughout their lives, how that affects their smaller neighbors and then puffs them up during a fiery death dive into their host stars,” Grunblatt said.

What’s next— Scientists would like to understand why TOI-2339 b is so much denser than other exoplanets and get more accurate characterizations of their attributes, something the ongoing TESS mission can help with.

And of course, NASA will soon have a new tool in place for exoplanetary astronomers.

TOI-4329, Grunblatt said, is an attractive follow-up target for the James Webb Space Telescope when it comes online later this summer. Webb can measure the amounts of water and carbon dioxide in TOI-4329 b’s atmosphere, which “can tell us more about the location where this planet formed, and will provide constraints on how far the planet had to migrate and up at its current orbit.”

Abstract: Giant planets on short-period orbits are predicted to be inflated and eventually engulfed by their host stars. However, the detailed timescales and stages of these processes are not well known. Here we present the discovery of three hot Jupiters (P < 10 d) orbiting evolved, intermediate-mass stars (≈ 1.5 M , 2 R < R? < 5 R ). By combining TESS photometry with ground-based photometry and radial velocity measurements, we report masses and radii for these three planets between 0.4 and 1.8 MJ and 0.8 and 1.8 RJ. TOI-2337b has the shortest period (P=2.99432 ± 0.00008 d) of any planet discovered around a red giant star to date. Both TOI-4329b and TOI-2669b appear to be inflated, but TOI-2337b does not show any sign of inflation. The large radii and relatively low masses of TOI-4329b and TOI-2669b place them among the lowest density hot Jupiters currently known, while TOI-2337b is conversely one of the highest. All three planets have orbital eccentricities below 0.2. The large spread in radii for these systems implies that planet inflation has a complex dependence on planet mass, radius, incident flux, and orbital properties. We predict that TOI-2337b has the shortest orbital decay timescale of any planet currently known, but do not detect any orbital decay in this system. Transmission spectroscopy of TOI-4329b would provide a favorable opportunity for the detection of water, carbon dioxide and carbon monoxide features in the atmosphere of a planet orbiting an evolved star, and could yield new information about planet formation and atmospheric evolution.

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