The Pink Planet

Artist's conception of GJ 504b
Image Credit: NASA's Goddard Space Flight Center/S. Wiessinger

Planets are hard to "see" because they orbit around stars that are much bigger and brighter. For this reason, most of the planets we know of outside our own solar system have been detected and studied using measurements and mathematics.

In recent years, though, it's become increasingly possible to detect certain kinds of planets from the direct observations of scientific telescopes, which can use filters to capture infrared radiation and turn it into visible images.

The planet GJ 504b, dubbed the "pink planet" was recently imaged in this way using infrared observations from the Subaru Telescope on Mauna Kea in Hawaii. Most of the planets observed so far by direct imaging have been large and relatively far away from the sun they orbit. The pink planet fits that description, but it currently holds the record for being the least massive planet observed in this way. Even so, it's a large planet like Jupiter.  It's that similarity to Jupiter together with its distance from its sun that makes GJ 504b interesting to astronomers.

If planets like Jupiter form within the gas disc of a young star, as many astronomers have believed, then you'd expect to find such planets no farther away from their sun that the planet Neptune is from our sun -- a distance of 30 AU (astronomical units). Contrary to this prediction, GJ 504b is estimated to be 43.5 AU from its sun. That's difficult to explain with any of the current theories about how planets are formed.

What's easier to explain is why GJ 504b is pink. It's part of a relatively new solar system that was formed only ~160 million years ago, making it 30 times younger than our earth, so the planet is still cooling down. It's lost some of its heat, making it cooler than other planets that have been directly imaged, but it's still hot enough to have a magenta glow.