As the 1970s Jerry Reed pop song went: “When you’re hot, you’re hot!” But a planet? Not!
That’s the story from this week’s American Astronomical Society meeting in Washington D.C. where astronomers reported that NASA’s Kepler planet-hunting observatory has found two normal stars orbited by objects that are too blistering hot to be planets but too small to be stars.
So what are they? It’s anybody’s guess. They’re simply called “objects of interest” by the Kepler team.
The Kepler space telescope is monitoring the light from 100,000 stars to search for planets that briefly pass in front of their stars.
The scheme is to get a representative sample that will yield an estimate of the abundance of Earth-like planets in the galaxy. (By definition this means planets the size of Earth orbiting at just the right distance from their parent star to have moderate temperatures for water and life as we know it.)
In sifting through the Kepler data taken so far, postdoctoral student Jason Rowe found a very curious light signature. When an object passed behind its central star, the light from the system dropped significantly. This means the object — called KOI 74b — must be glowing fiercely with its own light that was blocked out when the object was eclipsed.
In fact calculations show that the mystery object is hotter than the parent star. It is seething at 70,000 degrees Fahrenheit while the parent star is 17,000 degrees Fahrenheit. The strange object can’t be a star because the transit data show that it is no bigger than Jupiter. It is so close to the central star it completes an orbit in 23 days.
If that isn’t weird enough, a super-hot Neptune-sized companion called KOI 81b was found orbiting another star with a 5.2-day orbital period.
Both parent stars are hot and short-lived A-class stars that bathe the companions in a torrent of ultraviolet light. What’s more, gravitational tidal effects would heat the interiors of the companion objects. But that still probably doesn’t explain why the mystery objects are so unbelievably hot, even if they were newborn planets. (The hottest confirmed exoplanet to date in 3,700 degrees Fahrenheit.)
One idea is that a white dwarf migrated close to the primary star and was gravitationally stripped of some of its mass. With less mass and therefore lower gravity the dwarf would swell to the size of Jupiter.
The companion stars are each only about 300 million years old. Could a shorter-lived slightly more massive star have accompanied the primary star and burned out quickly to become a white dwarf? If so, how did the white dwarf migrate so closely to its star to be in such short period orbit?
Kepler cannot measure the mass of the hot companions. But if ground-based measurements can be made of the star’s wobble due to the tug of the companion, it might shed more light on the companion’s pedigree.
Perhaps Kepler has stumbled across some entirely new class of celestial object.