Scientists have discovered a new planet that defies all expectations. It's a lava world, but it's also cold. In fact, it's the coldest lava world we've ever seen with an atmosphere.
Researchers spotted HD 3167 b, a rocky super-Earth covered in lava, orbiting a star 154 light-years away. This planet orbits so closely to its star that a "year" on the world (one revolution around its star) takes only one Earth day. But it's not just a lava-covered rock — it's the smallest lava world researchers have ever found. And yet, it has managed to hang onto its atmosphere.
"What's so surprising is that the closer a rocky planet orbits its star, the harder it should be to have an atmosphere, because it's bombarded by stellar wind and gets more high-energy photons from the star. But it seems that many of these lava worlds do," co-author Edwin Kite, an associate professor of geophysical sciences at the University of Chicago said in a statement.
Since the first exoplanet (or planet outside of our solar system) was confirmed over 30 years ago, scientists have so far discovered over 6,300 different exoplanets. From hot Jupiter-like gas giants to small Neptune-like worlds, an incredible variety of planets has been found. Amongst those are a handful of "lava worlds," or rocky worlds that are hot enough that their surfaces are melted into burning lava.
In this new study, the team was able to make observations with the James Webb Space Telescope's MIRI (mid-infrared instrument), which can see the planet in mid-infrared light, allowing researchers to estimate planet temperatures and see if there are any signs of an atmosphere. With this instrument, they observed the planet during what's called a "secondary eclipse," meaning they looked for differences in mid-infrared light that occurred when the planet went behind its star.
This planet is tidally locked, which is common amongst lava worlds. This means that it has one side that always faces its star and one side that is perpetually shrouded in darkness. But this newly-found world is a little different from the rest. If the planet had no atmosphere, its dayside would be "as hot as theoretically possible," being so close to its star, lead author Brandon Park Coy, a graduate student in Kite's group, said in the same statement. But that's not what they found. The team found an unusually cool planet with an atmosphere, a planet that "is noticeably cooler than its expected maximum," Coy added.
The researchers concluded that the planet appears to have an atmosphere that cools its dayside by reflecting light from its star and essentially redirecting that heat to the planet's nightside. So while it's hot enough to be a lava world, it's the coolest one ever spotted with an atmosphere.
Strangely, while this planet's temperature helped this team to conclude it likely has an atmosphere, scientists have previously spotted atmospheres on ultra-hot lava worlds. This raises a number of questions about how these planets form their atmospheres and at what temperatures.
This discovery is part of a larger program which aims to answer this question. This program, led by astronomer Megan Weiner Mansfield at the University of Maryland, is examining a group of 10 different lava worlds to see at which temperatures these planets tend to form atmospheres.
With these findings, this discovery also supports a growing conclusion amongst scientists that the majority of lava worlds likely have atmospheres. Scientists are expanding what is possible in the field of exoplanet science by exploring strange, far-off worlds that can help us better understand our own planet and how life might form and survive in the universe.
"These planets are too hot for life, but by studying them, we can say something about the processes that matter for other rocky worlds," Kite said.
"Despite how inhospitable they are for life, we're also interested in studying these kinds of planets because we think early Earth might have looked a lot like a lava world," Coy added. "We think that very early in the solar system's history, when the terrestrial planets formed, they were extremely hot due to the energy from all of the planetesimal collisions. Earth had what's known as a magma ocean stage with an entirely liquid surface. This result gives us a window into studying what conditions may have been like in Earth's first couple of million years."
This work is described in a new study published in The Astrophysical Journal Letters.
