A “Planet” Hotter Than the Sun’s Own Surface, If It’s There at All
6,910.74 K. That is the modeled dayside temperature of a candidate planet called KOI-55 c, also cataloged as Kepler-70c, orbiting the exposed core of a dying star. The Sun’s own visible surface, its photosphere, runs at 5,772 K. Do the ratio and KOI-55 c comes out about 1.2 times hotter than the Sun itself, a planet, supposedly, outshining a star in raw temperature. Nobody is sure this planet exists.
The measuring stick
6,910.74 K needs a stick to lean on, and the best one available is the Sun itself. The photosphere, the layer of the Sun that actually reaches your eyes, measured at 5,772 K, is about as hot a “surface” as most people will ever picture. KOI-55 c’s modeled temperature comes in at around 1.2 times that. Not lava hot. Hotter than the thing that makes lava.
How the candidate turned up
On December 21, 2011, a team led by Stéphane Charpinet announced two candidate planets circling the star KOI-55, also known as Kepler-70 or KIC 5807616, in the journal Nature, using data from NASA’s Kepler space telescope. Kepler’s main trick is photometry: watching a star’s brightness over and over, for the tiny dip of a planet crossing in front of it. These two candidates weren’t found that way. There was no dip. Instead, the star’s light brightened and dimmed on a short, regular cycle, consistent with a dayside reflecting and re-radiating light back toward the telescope. The star sits about 3,600 light-years (1,100 parsecs) away in the constellation Cygnus, at an apparent magnitude of 14.87, too faint for any backyard telescope to pick out of the sky around it.
Why the star makes it plausible at all
KOI-55 is a hot subdwarf B star: what is left of a star’s core after most of its outer layers are gone, measured at about 28,000 K, nearly five times the Sun’s own photospheric temperature. Charpinet’s team offered a reason a planet might sit that close and still be there, suggesting the candidates likely sank into the star’s outer layers during its red-giant phase and came through intact. The star itself left its red-giant stage around 18.4 million years ago and has been fusing helium in its core ever since, the aftermath any surviving planet would have had to live through.
Where 6,910.74 K actually comes from
Nothing has read this planet’s temperature off a thermometer or a spectrum; no instrument has ever caught light coming directly from KOI-55 c itself. The figure is a model output: take the radiation pouring off a 28,000 K star at close range, assume how much of it a candidate surface would reflect rather than absorb, an albedo, or reflectivity, of 0.1 in the original paper, and calculate the balance point where absorbed and radiated energy match. That balance point is called an equilibrium temperature, and it is an estimate, not a measurement. Shift the assumed albedo and the figure shifts with it. The same method gives the companion candidate, Kepler-70b, a modeled temperature near 7,288 K, in the range of an F-type star’s surface. Both numbers carry the same asterisk.
What is still open
The asterisk is the real story. NASA’s Exoplanet Archive lists KOI-55 c as a controversial detection, not a confirmed planet. Krzesinski’s 2015 analysis of the same Kepler light curves, the brightness-over-time plots the whole claim rests on, argued the candidates were doubtful. A 2019 study by Blokesz, Krzesinski, and Kedziora-Chudczer went further, proposing that the periodic signal is pulsation: the star itself vibrating at frequencies earlier models hadn’t accounted for, with no orbiting planet needed to explain it. A possible third, smaller body between the two candidates, floated in the original data, was never confirmed either. And a proposed 7:10 orbital resonance between the two candidates, along with a claim that they would have the closest approach of any known planetary pair, only means something if the planets are real to begin with.
The picture that doesn’t exist
No camera has photographed KOI-55 c, and none currently could: everything claimed about it comes from small, repeating shifts of brightness in Kepler’s photometry of its star, not from a resolved point of light. Any glowing, close-orbit image of this “planet” is an illustration built to match a disputed number, not a photograph of anything that ever left the object itself. What is real and photographable is the other half of this story: hot subdwarf stars like KOI-55, and the Kepler light-curve data that started the argument in the first place. That is the picture worth showing. The planet stays a question mark.