Metals are everywhere in the universe, from hot gas giant planets that rain molten iron to heavy elements formed when stars go supernova. However, the exoplanet GJ367b is better than them all because this planet is made entirely of metal. GJ367b is an extreme planet. This "super Mercury" orbits its star every 7.7 hours and was first discovered by NASA's TESS planet hunter in 2015.
Now, scientists from the University of Turin in Italy and the State University of Thuringia in Germany have made the latest measurements of the planet using ESO's HARPS spectrograph and initial TESS observations. They found that the object is nearly twice as dense as Earth, suggesting it is likely made of solid iron.
Although GJ367b is now a solid iron planet, it may have once been the core of an ancient rocky planet.
"Thanks to our precise estimates of mass and radius, we explored the potential internal composition and structure of GJ367b and found that it is expected to have an iron core. However, the iron core occupies more than 90% of the planet's surface," the scientists said in a recent study published in The Astrophysical Journal Letters.
When GJ367b was discovered, it was just another exoplanet in a distant star system. Because the size difference between it and the star is not large, it is relatively easy for TESS to identify it. When the star's brightness drops, TESS will capture an exoplanet that is overtaking the star because its light is temporarily blocked by a planet. Several factors make GJ367b more obvious. While it's still small compared to stars, it's not as small as Earth compared to the Sun, so it blocks more light during transit. It also orbits very closely and is incredibly fast.
But what it is made of is not so obvious. Finding the density of an object based on its mass and radius allows scientists to understand its composition. TESS measured GJ367b's radius based on how much it blocked light. To determine the planet's mass, scientists used later radial velocity measurements, which detect the planet's gravitational pull on its host star.
The density of GJ367b is astonishing, 1.85 times the density of the Earth, which is basically the same as the density of iron. It is now the densest planet with the shortest orbital period known, and the densest super Mercury. But how could an entire planet be made only of iron?
"It is currently unclear how low-mass, high-density planets like GJ367b formed. Possible pathways may include formation from material richer in iron than is commonly thought in protoplanetary disks," the scientists said in the same research report.
But there are many other possible avenues. All the more likely formation scenarios are based on the fact that GJ367b was once a rocky planet, not unlike Earth or Mars. Its two companion planets are in more distant orbits and are also rocky, so all three may have formed in the same way. However, GJ367b has had a unique history since then, with its outer rocky layers gradually disappearing until almost only its inner core remains.
GJ367b's outer layers may have been stripped away in a collision or series of collisions, which is what is thought to have happened to Mercury. If an object, or enough objects, hit it with the right mass and impact speed, the rocky layers could be released and disappear.
Another possibility is that GJ367b faced intense radiation while orbiting so close to its star, burning away everything else except its solid iron core. External matter may have sublimated and then disappeared into space. GJ367b may also have experienced some combination of collision and irradiation to become the metallic planet it is today.
How it came to get so close to the star in the first place is also an unsolved mystery, given that it's unlikely to have formed there. Scientists believe that gravitational interactions with other planets may have caused it to migrate inward from where it formed.
Further study of this planet may ultimately tell us more about how rocky planets and planets with short orbital periods form and evolve.