NASA has announced a candidate for something astronomers have considered, but never seen: a planet that has had a makeover, appearing billions of years younger than its true age.
"When planets are young, they still glow with infrared light from their formation," said UCLA's Michael Jura, co-author on the new paper. "But as they get older and cooler, you can't see them anymore.”
A rejuvenated planet would be one that has brightened again so that infrared telescopes can see it late in life. Astronomers have created a theoretical scenario where this might happen, but are keen to learn if it actually occurs.
To understand how such rejuvenation could take place, it is important to understand that the planets we are talking about are gas giants – like Jupiter – rather than rocky worlds like Earth. If enough mass was to settle onto them, the infalling material would create friction that would warm the planet up, potentially making it visible to our telescopes.
As stars move from the red giant stage of their evolution to become white dwarfs, they lose half their mass. In the process, a large planet could, in theory, capture enough of the material blown off the star to give itself a new lease on life.
Such planets would be expected to orbit stars that have only recently become white dwarfs. Using NASA's Wide-field Infrared Survey Explorer, Blake Pantoja, then an undergraduate student, found a surprising amount of infrared light coming from the area around PG 0010+280, a white dwarf whose temperature suggests it has been cooling for 16 million years.
In The Astrophysical Journal Letters, Pantoja and co-authors report that a search for earlier observations of the same star found that the Spitzer Space Telescope detected the same thing in 2006, although no one noticed it at the time.
About 40 white dwarfs have an infrared halo, but these have all been older than PG 0010+280 and have silicon, iron and other heavier elements in their atmospheres. These elements sink to the core of white dwarfs, so their presence in the atmosphere suggests recent renewal.
The same elements are common in asteroids. Consequently, the halos have been attributed to disks formed from the remnants of asteroids ripped apart by the extreme gravitational field around an object as dense as a white dwarf.
However, PG 0010+280's atmosphere looks clean, favoring the alternative theory: The infrared light is coming from an object 1.3 times as wide as Jupiter and with a temperature around 1,000°C (1,832°F).
"I find the most exciting part of this research is that this infrared excess could potentially come from a giant planet, though we need more work to prove it," said lead author Dr. Siyi Xu of UCLA. "If confirmed, it would directly tell us that some planets can survive the red giant stage of stars and be present around white dwarfs."
For the moment, the possibility is difficult to test. However, the James Webb Space Telescope, due to be launched in 2018, may be able to tell if the glow really is coming from a planet whose star has rekindled its warm inner glow.