A newfound alien world could shed some light on one of the darkest and strangest corners of the exoplanet family tree.
The planet, known as GJ 367b, circles a small, dim red dwarf star just 31 light-years from the sun, its discoverers announced in a new study. (For perspective, our Milky Way galaxy is about 100,000 light-years wide.)
GJ 367b is a rocky world about 70% as large as Earth and 55% as massive, making it one of the lightest known exoplanets, study team members said. It completes one orbit every 7.7 hours and is therefore an"ultra-short period" (USP) planet — a mysterious and little-studied class of world.
"We already know a few of these, but their origins are currently unknown," study co-lead author Kristine W. F. Lam, of the Institute of Planetary Research at the German Aerospace Center, which is known by the German acronym DLR, said in a DLR statement.
"By measuring the precise fundamental properties of the USP planet, we can get a glimpse of the system's formation and evolution history," Lam added.
Finding and characterizing an extreme world
Lam, co-lead author Szilárd Csizmadia, also of the DLR's Institute of Planetary Research, and their colleagues discovered GJ 367b using data gathered by NASA's Transiting Exoplanet Survey Satellite (TESS), which launched to Earth orbit in April 2018.
TESS hunts for planets using the "transit method," noting the tiny brightness dips caused when a world crosses its star's face from the spacecraft's perspective. The discovery team spotted such a dip in TESS observations of the red dwarf GJ 367, which is about half as wide as our sun, then confirmed that the signal was caused by a transiting planet.
The TESS observations revealed GJ 367b's super-short orbital period, as well as its size relative to its host star. The researchers characterized the planet further with the aid of the High Accuracy Radial Velocity Planet Searcher (HARPS), an instrument installed on the European Southern Observatory's 3.6-meter telescope in Chile.
The HARPS measurements showed how much GJ 367b was tugging on its host star, which allowed the study team to calculate the exoplanet's mass. Combining the various observations enabled the astronomers to determine GJ 367b's density, which is higher than that of Earth.
"The high density indicates the planet is dominated by an iron core," Csizmadia said in the DLR statement. "These properties are similar to those of Mercury, with its disproportionately large iron and nickel core that differentiates it from other terrestrial bodies in the solar system."
GJ 367b is not a good candidate to harbor life as we know it. Because of its extreme proximity to its host star, the planet is blasted by stellar radiation, absorbing about 500 times the amount that Earth receives from the sun, study team members said.
If GJ 367b ever had a substantial atmosphere, it was almost certainly lost to space long ago. And the planet is likely tidally locked, always showing the same face to its host star, with temperatures reaching up to 2,700 degrees Fahrenheit (1,500 degrees Celsius) on its scorching day side, the researchers said.
It's unclear how GJ 376b formed, but a few different scenarios could explain its structure and composition, Lam said. For example, it's possible that the planet formed from oddly iron-rich (from our point of view) building blocks. Or GJ 376b may be the remnant of a larger planet, much of whose mass was stripped away by stellar radiation or a giant impact.
"If the planet is a remnant core of a former gaseous planet, then the gaseous planet should be no bigger than a Neptune-sized planet," Lam told Space.com via email.
Related: 7 ways to discover alien planets
A relatively nearby world
GJ 367b's weirdness is intriguing to planetary scientists and astrophysicists.
“Understanding how these planets get so close to their host stars is a bit of a detective story," TESS team member Natalia Guerrero, an astrophysics PhD student at the University of Florida, said in a statement issued by the Massachusetts Institute of Technology (MIT).
"How did it move close in? Was this process peaceful or violent? Hopefully this system will give us a little more insight," added Guerrero. She did not participate in the new study, which was published online Thursday (Dec. 2) in the journal Science.
More insight may indeed come in the not-too-distant future. Given GJ 367's relative proximity to Earth, astronomers should be able to study it using a variety of instruments going forward.
For example, researchers could search the system for other planets, including worlds that might be capable of supporting alien life.
"For this class of star, the habitable zone would be somewhere between a two- to three-week orbit," study co-author George Ricker, a senior research scientist at MIT's Kavli Institute for Astrophysics and Space Research, said in the MIT statement. (The habitable zone is the range of orbital distances in which liquid water could exist on a world's surface.)
"Since this star is so close by, and so bright, we have a good chance of seeing other planets in this system," Ricker said. "It's like there's a sign saying, 'Look here for extra planets!'"
Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or on Facebook.
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Michael Wall is a Senior Space Writer with Space.com and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.