WASP-33 b: A Dayside Edging Toward Tungsten’s Melting Point
Picture the filament inside an old incandescent bulb, the one piece of metal that can glow white-hot for years without melting. Now imagine a whole planet’s daytime side closing in on that same temperature. WASP-33 b’s dayside carries an estimated 4,547 °F (2,781.7 K; 2,508 °C), about three-quarters of the way to tungsten’s melting point of 6,191 °F (3,695 K; 3,422 °C), the highest melting point of any metal in common engineering use.
The measuring stick
Tungsten earns its place here honestly. It isn’t some remote astrophysical yardstick, it’s the metal that sat coiled inside household light bulbs for a century, chosen precisely because almost nothing else survives glowing white without turning to liquid. WASP-33 b’s modeled dayside sits roughly 1,644 °F (913 K) short of that melting point. Close a quarter of that remaining gap and this planet would be pressing directly against the temperature at which tungsten itself gives up and flows.
How we know
WASP-33 b turned up in 2010, caught by the transit method: astronomers watched its star dim on a strict, repeating schedule every time the planet crossed in front of it. The star spins so fast that its spectral lines blur together, so the discovery team, led by Collier Cameron, confirmed the planet using Doppler tomography, tracking the shadow it cast across the star’s rotating face rather than the usual radial-velocity wobble. That transit signal is the measured part: a light curve, recorded and repeated.
The 4,547 °F figure is something else entirely, a model output, not a thermometer reading. NASA’s Exoplanet Archive lists it in its composite-parameters table as an equilibrium temperature, calculated by assuming the planet reflects none of its star’s light and spreads the heat evenly across its whole surface. Change those assumptions and the number shifts: one study using the same 2010 data settled nearer 2,700 K, while a team led by Smith, observing the planet’s actual light in 2011, measured a brightness temperature closer to 3,620 K. A day-side-only redistribution estimate lands near 3,271 K. None of these are wrong, they’re answering slightly different questions about where the heat actually sits.
A 2020 study led by von Essen used secondary-eclipse data, catching the planet’s light dim as it swings behind its star, to estimate separate day and night temperatures: about 4,966 °F (3,014 K) on the day side, 2,429 °F (1,605 K) on the night. That gap is the real story. WASP-33 b finishes an orbit in under 29.3 hours, sitting so close to its star that heat has barely any time to travel from the blazing side facing the star to the one turned away.
Size says heavyweight, density says otherwise
WASP-33 b’s measured radius is about 70,770 miles (1.139 × 10^8 meters), 17.86 times Earth’s radius. A planet that size sounds like it should be crushingly massive. Its measured mass, though, is 3.973 × 10^27 kilograms, about 2.093 times the mass of Jupiter. Divide one by the other and the bulk density comes out near 642 kilograms per cubic meter, a figure worked out here from the archive’s own mass and radius, not a number listed outright, so treat it as an estimate rather than a catalog entry. Water runs 1,000 kilograms per cubic meter. WASP-33 b is lighter for its size than an ocean. Find a sea big enough to hold it, and this planet would float.
A year shorter than a day
WASP-33 b completes an orbit in just under 29.3 hours, a full year finished before a single Earth day-night cycle would be. It manages that by sitting only 0.0239 astronomical units from its star, an astronomical unit being Earth’s average distance from the Sun, about 93 million miles (149.6 million km), which works out to roughly 2.22 million miles (3.575 × 10^9 meters), on a measured, essentially circular orbit.
What’s still open
No camera has photographed WASP-33 b, and none will, not the way Cassini photographed Saturn’s rings or Juno photographed Jupiter’s clouds. Every image of this planet circulating online is an artist’s illustration, built from the light curve and the modeled numbers, not a picture of an actual disc. The host star itself sits about 397 light-years away, its distance pinned down by the Gaia satellite’s survey; a light-year is a distance, the ground light covers in a year, never a stretch of time.
Whether this planet’s atmosphere carries a stratosphere, a layer where temperature climbs with altitude, is still argued over, Haynes and colleagues first reported evidence for one in 2015, and the debate hasn’t closed. The equilibrium temperature itself keeps shifting slightly as better stellar and orbital data come in. The 2,781.7 K figure is this pass’s best estimate, not a final word on how hot WASP-33 b really runs.