A well-known pink object beyond the solar system has thrown astronomers a curveball.
Using the James Webb Space Telescope, researchers found the so-called Pink Planet, GJ504b, has salt in its atmospheric clouds and a chemical mix they say is unlike anything they have analyzed before. The findings were published on Thursday, June 18, in The Astronomical Journal.
GJ504b was first discovered in 2013 and orbits a sun-like star about 57 light-years from Earth. It has a mass about 25 times that of Jupiter. Despite its nickname, it may not be a planet. Astronomers instead describe it as a “planetary-mass companion,” a planet-size object orbiting a parent star, because it could be a brown dwarf, a failed star that did not gain enough mass for hydrogen fusion in its core.
It is one of the coldest planetary-mass companions found with ground-based telescopes, with a temperature of about 290 degrees Celsius.
“The Pink Planet is the coldest companion ever discovered using ground-based instruments,” team leader Aneesh Baburaj of Northwestern University said in a statement.
“Many teams all around the world performed follow-up observations to study its light, but it was too faint for ground-based instruments. That made it a perfect target for JWST.
“When we finally obtained its spectrum, it immediately looked interesting. But once we started digging deeper into the data, we realized it was not like anything we have analyzed before.”
The team studied the object by measuring its faint electromagnetic radiation emissions and filtering out the bright glare of its parent star. They found its relative coolness is tied to age. Gas giant planets and brown dwarfs are born very hot and cool as they age, and the new research estimates GJ504b is between 2.5 billion and 4 billion years old.
By breaking its light into individual wavelengths, the researchers also worked out its chemical composition, using the characteristic wavelengths that elements absorb and emit.
“In the past, other astronomers observed the companion for an entire night with some of the biggest telescopes in the world to obtain a spectrum,” Baburaj said.
“And they could not see the object. With JWST, our entire observation took around two hours, and we were successful.”
The JWST data showed water, carbon dioxide, methane and ammonia in the atmosphere. But the observations did not match models of the atmosphere until the team added something unexpected, salt clouds deep in the atmosphere.
“We ran simulations with clouds, and the results aligned with what we know about cold planets,” Baburaj said.
“We tried three different types of clouds, and salt clouds fit best. When we accounted for salt clouds, it subdued the signature of molecules hidden deeper in the companion’s atmosphere. Then, the results became physically possible.
“This is the first time we’ve found that salt clouds are critical to explaining the spectrum of an object. It’s a good reminder to account for clouds in our models.”
The researchers said GJ504b also appears unusually rich in elements heavier than hydrogen and helium, which astronomers call metals. They said that means they still cannot pin down its origin or determine if it formed like a planet or like a star.
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