KOI-55 b: A Catalog Number Hotter Than the Sun’s Own Face
Picture the Sun’s photosphere, the visible skin of light you were told as a kid never to stare at. Its effective temperature sits at about 5,772 K, the reference value the International Astronomical Union adopted in 2015 for solar comparisons everywhere. Now set a supposed planet beside it: KOI-55 b, listed in the NASA Exoplanet Archive at 7,777.78 K. Do the division and you get roughly 1.35, this “planet” runs about 35 percent hotter than the star it orbits.
A planet outglowing a star’s own face sounds like a typo. It isn’t a typo. But it also isn’t settled science, and how that number ended up in a catalog is stranger than the number itself.
The measuring stick, with its fine print
7,777.78 K against 5,772 K is the comparison that matters here, because it’s the one that turns “hot planet” into a contradiction. A blast furnace doesn’t arrive with a “planet” label already stuck on it. The Sun’s face does, it’s the benchmark most of us already carry for “star-hot,” and KOI-55 b’s catalog figure clears that bar.
Both numbers describe a radiating surface, not a thermometer reading. The solar value is a fixed reference constant built from decades of observation. KOI-55 b’s number is an ESTIMATE, pulled from modeling a distant brightness signal, precise to two decimal places on the page, less precise underneath. The team that first analyzed this object modeled temperatures closer to 7,000–7,400 K for it, one estimate landing near 7,288 K. Those extra decimal places in the catalog number are a rounding artifact, not added accuracy.
How we think we know
Nobody has photographed KOI-55 b. There’s no image to show, not even a fuzzy one, only diagrams, and any illustration of this object is exactly that: a drawing built from numbers, not a camera exposure.
What exists instead is a light curve. In 2011, Stéphane Charpinet and colleagues published “A compact system of small planets around a former red-giant star,” describing tiny, repeating brightness changes in the light from KOI-55, a hot subdwarf B star, essentially a stellar core that swelled into a red giant, shed its outer layers, and shrank back down. Working with data from NASA’s Kepler space telescope, the team read the pattern as starlight reflecting off small bodies in tight orbit, not the shadow of a transit crossing the star’s face. Science coverage that December nicknamed the pair “deep-fried planets,” skimming the star’s exposed core.
A tight little orbit, if it’s really out there
The catalog puts KOI-55 b’s orbit at 20,744.986 seconds, about 5.76 hours, shorter than a single work shift, at roughly 0.01 astronomical units from its star (an astronomical unit being Earth’s average distance from the Sun, about 93 million miles). Its listed mass is 2.628 × 10²⁴ kilograms, about 0.44 times Earth’s mass, mass, not weight, a measure of material, not a measure of pull. Its radius comes in at roughly 3,008 miles (4,841 km), about 0.76 times Earth’s radius. Its estimated density, 5,530 kg/m³, lands close to Earth’s own. Nothing about the bulk recipe is unusual. If this is a real world, proximity did all the heating; size and composition did none of it.
What’s still open
Every one of those numbers carries the same asterisk as the temperature. In 2019, Anna Blokesz and colleagues argued that the brightness wobble Charpinet’s team read as reflected starlight might instead be pulsation from the star itself, KOI-55 ringing at frequencies beyond what earlier models expected, no planet required. The NASA Exoplanet Archive now flags both KOI-55 b and its sibling, KOI-55 c, as controversial. Reflected light and stellar pulsation both fit the same light curve. That’s an OPEN question, not a footnote waiting to be corrected.
So the 7,777.78 K sits in the archive as a catalog value for a body that might not exist at all. If it does, it’s a modest, Earth-mass world with an ordinary density, hotter than the Sun’s face for one plain reason: it sits closer to its star than almost anything we’ve ever tried to measure.