Astronomers Find Scorching Hot Exoplanet Where a Year Lasts 16 Hours

Astronomers have discovered a blisteringly hot exoplanet that orbits its parent star in just 16 hours. At a distance of 1.5 million miles from its star, the planet has a "dayside" surface temperature of around 6000 F. This means the planet is as hot as some small stars.

The exoplanet, located 855 million light-years from Earth, is an example of a class of worlds called "hot Jupiters"—massive balls of gas comparable in size to the solar system's own gas giant, but which race around their parent stars, often completing an orbit in as short a period as under 10 days. Much more rapidly than the 12 Earth years that Jupiter takes to orbit the sun.

But this exoplanet, designated TOI-2109b, is extreme even for a hot Jupiter. The world, which is five times as massive as Jupiter, has a 16-hour orbit around its star. This is the shortest known orbit of any discovered hot Jupiter, with its proximity also making it the second hottest exoplanet in this class ever found.

This short orbital period could eventually spell doom for TOI-2109b, however. Astronomers believe that TOI-2109b is in the process of "orbital decay." This means that it is spiraling into its star. But the extremely short orbit of TOI-2109b means that the planet is spiraling toward its star faster than other hot Jupiters.

"In one or two years, if we are lucky, we may be able to detect how the planet moves closer to its star," NASA Goddard Space Flight Center researcher Ian Wong said. "In our lifetime we will not see the planet fall into its star. But give it another 10 million years, and this planet might not be there."

Wong, who was a post-doctorate at the Massachusetts Institute of Technology (MIT) when part of the team that discovered TOI-2109b, is the lead author of a paper detailing the research in The Astronomical Journal.

The team spotted TOI-2109b using NASA's Transiting Exoplanet Survey Satellite (TESS). TESS spots exoplanets by the tiny dips in light they cause as they cross the face of their parent star, the so-called "transit method" of exoplanet detection.

TESS has thus far proved incredibly successful in spotting exoplanets, as of April this year the craft had detected over 2,200 worlds outside of the solar system.

TESS began observing TOI-2109, a star located in the southern portion of the Hercules constellation, in May 2020. The star had been identified by the mission as a TESS Object of Interest due to the possibility that it might host an orbiting planet.

The TESS team analyzed the light from the star, looking for the telltale periodic dips in output caused by a transiting planet. They discovered dips that corresponded with an exoplanet crossing its face every 16 hours.

The planet is tidally locked, meaning it has a blisteringly hot dayside and a cooler nightside, which astronomers haven't yet been able to properly characterize.

"Meanwhile, the planet's night side brightness is below the sensitivity of the TESS data, which raises questions about what is really happening there," Avi Shporer, research scientist at MIT's Kavli Institute for Astrophysics and Space Research, said. "Is the temperature there very cold, or does the planet somehow take heat on the dayside and transfer it to the night side? We're at the beginning of trying to answer this question for these ultrahot Jupiters."

To answer this and other mysteries surrounding the exoplanet, the team now hopes to follow up on the discovery of TOI-2109b with investigations involving the Hubble Space Telescope or even the James Webb Space Telescope, set to launch in December.

"Ultrahot Jupiters such as TOI-2109b constitute the most extreme subclass of exoplanet," Wong says. "We have only just started to understand some of the unique physical and chemical processes that occur in their atmospheres—processes that have no analogs in our own solar system."

One puzzle surrounding hot Jupiters that the team could solve is how these systems come to be in the first place. "From the beginning of exoplanetary science, hot Jupiters have been seen as oddballs," Shporer says. "How does a planet as massive and large as Jupiter reach an orbit that is only a few days long?

"We don't have anything like this in our solar system, and we see this as an opportunity to study them and help explain their existence."

Hot Jupiter
An illustration of a hot Jupiter exoplanet orbiting its star. astronomers used the TESS telescope to discover such a world that has a surface temperature as hot as a small star. Ames/JPL-Caltech/NASA