Metal Planet Appears to Be Lightest World Around Another Star Ever Found

The sport of searching for exoplanets, or planets around other star systems, has a new featherweight champion — a small scientist called GJ 367 b with about half the mass of Earth. The lightest exoplanet found so far, GJ 367 b slips around its parent star in a fast 7.7 days and is unusually dense, apparently made of almost pure iron.

Called “Super Mercury” for its location and composition, the existence of the lightweight entity challenges aspects of planetary formation theories. This may be a hint that microcosms come in a much wider variety than previously thought.

“It’s so wonderful to find a planet like this,” said Diana Valencia, an astrophysicist at the University of Toronto Scarborough, who was not involved in the discovery. “It’s amazing.”

GJ 367 b orbits a cool red star located about 31 light-years from Earth. Researchers first discovered it using NASA’s Transiting Exoplanet Survey Satellite (TESS), a space telescope that finds nearby worlds by measuring how dim their stellar hosts are as they move in front of the faces of stars. Because this eclipse produces a small trough of light received from the star, astronomers can use it to estimate the size of the planet.

The newly discovered world is about 5,700 miles wide, about three-quarters of our planet. Follow-up observations using the High Resolution Radial Velocity Planet Searcher (HARPS), an instrument on a telescope at La Silla Observatory in Chile, allowed researchers to discover its mass.

The HARPS detector looks for the tiny oscillations of exoplanets it induces on their parent stars, which get stronger as the planet gets heavier.

With its mass and diameter in hand, scientists can calculate the density of GJ 367 b, which indicates that it is far from most exoplanets. It is closer in size to Earth or Venus but in composition more like Mercury, which is mainly made of iron.

“It’s a strange ball,” said Christine W.F. Lamm, an astronomer at the German Aerospace Center in Berlin and lead author of a research paper published Thursday in the journal Science.

Since it is located near its mother, it is always possible that one side of GJ 367 b will face the glowing star. Dr. Lamm added that temperatures on the day side would rise around 2,700 degrees Fahrenheit, hot enough to melt rocks and minerals, making it a potential lava world.

GJ 367 b offers scientists a way to study how worlds closer to stars form. The leading theory for how a place like Mercury was created is that early in the history of the solar system, a rocky Earth-like planet formed near the sun. Huge space rocks later struck this entity, stripping the world of its shell and mantle. This means that Mercury is essentially a giant planetary core located next to the fiery Sun.

The problem with such a scenario, said Dr. Valencia, is that it doesn’t fully work. The collisions may have smashed the outer layers of primordial Mercury away, but the material wouldn’t go far. Rocks and minerals trapped by the Sun’s gravity will remain nearby in orbit and eventually find their way to the surface of the object.

Dr. Valencia said it’s possible to invoke special circumstances for why this doesn’t happen, but the presence of GJ 367 b and similar things means such planets aren’t exactly rare.

“We have to think about how the Mercury Super craft can be produced reliably, not sporadically,” she added.

One possibility is that there is still something missing from the planetary creation models. Elements as dense as iron may end up somehow closer to the star during its younger days, Dr. Valencia said. For now, she added, this idea is still speculative, although worlds like GJ 367 b may begin to push scientists in such a direction.

The team that has already discovered the small planet is planning further observations of the system. They would also like to use a giant telescope to capture the light from GJ 367 b, which could reveal if it has an atmosphere or if its surface is really molten.

Detecting larger worlds has always been easier than detecting smaller ones, and researchers have found a group of giant Jupiter-like entities with all kinds of configuration and orbital characteristics. As improved telescope techniques have opened up exploration at the other end of the size spectrum, exotic small objects such as GJ 367 b continue to appear.

“It’s hard to remember that these planets have their own stories,” said Jonathan Fortney, a planetary scientist at the University of California, Santa Cruz, who was also not involved in the study. “It’s another way to show that the end planetary outcomes can be enormously diverse.”

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