So metal: Newly discovered exoplanet is likely over 80 percent iron

Zoom / TESS Planetary Observatory.

For centuries, scientists have had only a limited number of examples to consider when it comes to understanding the formation of planets. As we’ve explored ever-increasing numbers of worlds, we’ve found many that seem like nothing we have in our solar system: hot gas giants, super-Earths, tiny Neptune, and more. Therefore, it may be a relief to find something that resembles a familiar planet, as it indicates that the processes that shaped the solar system may not be unusual.

A new discovery certainly falls into this category, with researchers announcing the discovery of what appears to be an extremely iron-rich planet which, at least in terms of composition, is very similar to Mercury. The difference is that it is almost at the top of its star and is probably hot enough that any iron on the surface can melt.

a very short year

The new planet has been found orbiting a red dwarf star named GJ 367 about 30 light-years from Earth. Red dwarfs are small, faint stars that make it easier to identify the planets around them. A planet orbiting between a red dwarf and Earth will proportionally block more of the star’s light. Because the star is low in mass, the planet’s gravity will cause it to shift more as it rotates, creating larger Doppler shifts in the light from the star.

The new planet, GJ 367b, appeared in data from NASA’s Transiting Exoplanet Survey (TESS) mission. TESS monitors droplets in starlight caused by an orbiting planet, and it does so in an impressive clip — the data used here included a new image every two minutes for two weeks. That’s more than enough to capture the signal caused by GJ 367b, which completes a trip around its star in just a third of a day.

Discovering in this way makes GJ 367b a candidate for an exoplanet; To confirm its existence, the research team turned to a ground-based telescope, which monitored the Doppler shifts in the star’s light due to the planet’s orbit. This confirmed the existence of the planet, as the one-third of a day signal was present (as was the roughly 45-day signal caused by the star’s rotation).

The planet itself is small, its radius is three-fourths the radius of the Earth. But it is relatively massive, with a mass of more than half the mass of the Earth. This results in a density of just over eight grams per cubic centimeter – which is actually heavier than iron.

infernal conditions

GJ 367b is close enough that it is gradually locked to its host star, which means it rotates once per orbit, keeping one side facing the star the entire time. and he very close to that star. This results in an estimated surface temperature of 1,745 K – very close to the melting point of iron. Of course, the outer crust is likely to be rocky. Or it would be rocky were it not for the fact that many silicon-rich rocks melt at similar temperatures as well.

Clearly anything resembling the atmosphere as we know it would have heated up a long time ago. But there is a possibility that some of the molten rocks and minerals will evaporate and create a little local atmosphere on the side facing the stars. Obviously, the far side of the planet would be much cooler, and anything in the form of vapor would end up on the planet very quickly.

The research team fed the stats to an AI trained on other planets, and the AI ​​predicted that GJ 367b has a structure very similar to Mercury: a large metal core that occupies much of the planet’s interior, stretching more than 85 percent of the way to the surface. The rest will be silicate rock. This is not unreasonable, other than the fact that the density of GJ 367b is 1.5 times that of Mercury. Therefore, there must be some important differences as well.

Anyway, we have an idea of ​​how to make Mercury so rich in iron – it’s the product of collisions that stripped away some of the rocky material. But we don’t understand how anything the size of Mercury could form near a star. So, while there is some reassuring familiarity here, there are limits to that.

Science, 2021. DOI: 10.1126/science.aay3253 (about DOIs).

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