TESS discovered an ultra-short-period planet (USP) that is also very light. GJ 367 b is the name of the planet, and it orbits its star in just eight hours. The planet is roughly the size of Mars and half the mass of Earth, making it one of the lightest planets ever discovered.
Ultra-short-period planets are small, compact worlds that race around their stars, completing an orbit and a single, scorching year in less than 24 hours. One of the unsolved mysteries of exoplanetary science is how these planets came to be in such extreme configurations. Astronomers have discovered a super-light, ultra-short-period planet (USP). GJ 367 b is the name of the planet, and it orbits its star in just eight hours. The planet is roughly the size of Mars and half the mass of Earth, making it one of the lightest planets ever discovered.
GJ 367 b orbits a nearby star 31 light years from our sun, which is close enough for researchers to determine properties of the planet that were not possible with previously detected USPs. For example, the team discovered that GJ 376 b is a rocky planet with a solid core of iron and nickel, similar to Mercury’s interior.
The habitable zone would be somewhere between a two- to three-week orbit for this class of star. We have a good chance of seeing other planets in this system because this stars is so close and so bright.
George Ricker
Because of its close proximity to its star, astronomers estimate that GJ 376 b is bombarded with 500 times more radiation than the Earth receives from the sun. As a result, the planet’s dayside can reach temperatures of up to 1,500 degrees Celsius. Any substantial atmosphere would have long vaporized away under such extreme temperatures, along with any signs of life, at least as we know it.
However, there is a possibility that the planet has habitable partners. Its star is a red dwarf, also known as a M dwarf, which is a type of star that typically hosts multiple planets. The discovery of GJ 367 b around such a star suggests that there may be more planets in this system, which could aid scientists in understanding the origins of GJ 376 b and other ultra-short-period planets.
“The habitable zone would be somewhere between a two- to three-week orbit for this class of star,” says team member George Ricker, senior research scientist at MIT’s Kavli Institute for Astrophysics and Space Research. “We have a good chance of seeing other planets in this system because this star is so close and so bright. It’s as if a sign says, ‘Look here for extra planets!'”
The team’s findings were published in the journal Science. The study was led by researchers from the German Aerospace Center’s Institute of Planetary Research, in collaboration with an international group of researchers, including MIT co-authors Ricker, Roland Vanderspek, and Sara Seager.
Transit tests
The new planet was discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), an MIT-led mission led by Ricker. TESS scans the night sky for changes in the brightness of the nearest stars. TESS data is analyzed by scientists to look for transits, or periodic dips in starlight that indicate a planet is crossing and briefly blocking a star’s light.
TESS recorded a patch of the southern sky that included the star GJ 376 for about a month in 2019. Scientists at MIT and elsewhere examined the data and discovered a transiting object with an eight-hour orbit. They performed a number of tests to ensure that the signal did not come from a “false positive” source, such as a foreground or background eclipsing binary star.
After confirming that the object was indeed an ultra-short-period planet, they used the High Accuracy Radial Velocity Planet Searcher (HARPS), an instrument installed on the European Southern Observatory’s telescope in Chile, to examine the planet’s star in greater detail.
Based on these measurements, they determined that the planet is among the lightest discovered to date, with a radius 72 percent that of Earth and a mass 55 percent that of Earth. These dimensions imply that the planet has an iron-rich core.
The researchers then narrowed down various scenarios for the planet’s interior composition to find the best fit. According to the data, an iron core likely makes up 86 percent of the planet’s interior, similar to the composition of Mercury.
“We’re discovering a Mars-sized planet with the composition of Mercury,” says Vanderspek, an MIT principal research scientist. “It’s one of the smallest planets discovered to date, and it’s in a very tight orbit around a M dwarf.”
Scientists hope to detect signals from other planets in the system as they continue to study GJ 367 b and its star. The properties of these planets, such as their spacing and orbital orientation, could reveal information about how GJ 367 b and other ultra-short-period planets formed.
“Understanding how these planets get so close to their host stars is a bit of a detective story,” says Natalia Guerrero of the TESS team. “Why does this planet lack an outer atmosphere? How did it get so close? Was this a peaceful or violent process? Hopefully, this system will provide us with some additional information.”
