Mysterious match-up as giant planet spotted orbiting tiny star
NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered a strange star system, where a giant gassy planet is tightly orbiting a tiny white dwarf. Not only is this the first time such an arrangement has been seen, but it raises some very odd questions about how the planet survived the star’s expansive “death” in the first place.
The planet, named WD 1856 b, is roughly the size of Jupiter, making it about seven times bigger than the star it’s orbiting, named WD 1856+534. That’s a huge disparity – stars are normally much bigger than planets. In our own solar system, for example, you could squeeze about 1,000 Jupiters into the Sun.
The difference is that WD 1856+534 is a white dwarf star, a tiny remnant of what was once a Sun-like star. This one is only about 40 percent bigger than Earth and half the Sun's mass. The planet, which is roughly the size of Jupiter, is so close that it orbits the white dwarf every 34 hours, which is 60 times faster than Mercury’s orbit around the Sun.
All of that is bizarre enough by itself. We’ve never seen such a huge planet orbiting such a tiny star, let alone that closely. Previous gas giants orbiting white dwarfs have been smaller and seem to be evaporating. But the really weird part is that it’s hard to explain just how this system came to be in this arrangement.
White dwarfs are born when bigger stars, of the same class as our Sun, reach the end of the main sequence phase of their lives. When they run out of fuel, they swell up into red giant stars, before throwing off their outer layers and leaving just the core behind. That core is the white dwarf, which will go on to shine for many billions of years more.
In the process of becoming a red giant, these stars will gobble up any planets too close to them. When, for example, the Sun does this in about five billion years’ time, it’ll destroy Mercury, Venus and probably Earth. So how did WD 1856 b survive? In its current orbit it would have been incinerated long ago.
“WD 1856 b somehow got very close to its white dwarf and managed to stay in one piece,” says Andrew Vanderburg, lead author of the study. “The white dwarf creation process destroys nearby planets, and anything that later gets too close is usually torn apart by the star’s immense gravity. We still have many questions about how WD 1856 b arrived at its current location without meeting one of those fates.”
The team says that the most likely explanation is that the planet originally formed much further out from the star, beyond the reach of its red giant phase. Later on, the gravitational influence of other objects could have slowly nudged it into a closer orbit. The prime suspects are other gas giant planets born in the same system, but other candidates include two other nearby stars, or even a rogue star swinging through at some point.
We don’t yet have enough data to fill in the rest of the story, but the astronomers say that further observations could help.
The research was published in the journal Nature. The team discusses the study in the video below.