A Neptune-class exoplanet has been found with its atmosphere stripped away

What happens when a giant planet gets stripped of its atmosphere? It leaves behind a giant core, rich in iron and other metals. A team using NASA’s TESS mission recently found such a remnant core, orbiting a star just 730 light-years away.

The newly discovered exoplanet known as TOI 849 b is a strange one indeed. It orbits a star, not too dissimilar from our own sun, just 730 light-years away. It was discovered with NASA’s Transiting Exoplanet Survey Satellite (TESS), the successor to the successful exoplanet-hunter extraordinaire, The Kepler Space Telescope.

TOI 849 b is strange because it’s both massive (about 40 times the mass of the Earth) but relatively small (just 3.4 times wider than the Earth), as detailed in a recent Nature article. The combination of big mass and small stature means that this planet has to be incredibly dense.

Compare those stats to something like Neptune, which is only 17 times more massive than the Earth but swells to over four times the Earth’s radius.

But whereas Neptune is made of mostly low-density gas and ices, TOI 849 b must be made of mostly iron, rock, and water – the heavy stuff in the cosmos.

It gets stranger still: TOI 849 b orbits so closely to its parent star that a “year” is a mere 18 hours long, and the surface temperatures reach a sweltering 1,500 °C. It’s not a good place for a vacation, but definitely a good place to investigate models of planetary formation.

We expect a planet with that much mass to be even bulkier, swaddling itself in layer after layer of low-density hydrogen and helium, like the gas giants of our own solar system. So either the growth of TOI 849 b was stunted early on – preventing it from acquiring the usual gaseous envelope – or it lost its gassy layers later in life, perhaps by straying too close to its parent star.

Either way, planets of this size are relatively rare, especially this close to their host star, and studying TOI 849 b in more detail can give us crucial clues on how solar systems form and evolve.