A star eating its planet! [View All]

[LongTomH]

Shades of Saturn eating his children! This article at the Bad Astronomy blog is all about a star, 600 light years away, that seems to be 'eating' a planet in orbit. Astronomers were puzzled by the fact that the star contains more heavy elements than might be expected for it age and stellar type.

The puzzle was solved by the discovery of a heavy planet, Wasp-12b that's contributing those elements. The planet is named for the British telescope system used in its discovery: Wide Area Search for Planets. More from Phil Plait:

OK, first, the planet. Called WASP 12b, it was instantly pegged as an oddball. The orbit is only 1.1 days long! Compare that to our own 365 day orbit, or even Mercury’s 88 days to circle the Sun. This incredibly short orbital period means this planet is practically touching the surface of its star as it sweeps around at over 220 km/sec (130 miles/sec)! That also means it must be very hot; models indicate that the temperature at its cloud tops would be in excess of 2200°C (4000° F).

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Astronomers used Hubble to observe the planet in the ultraviolet and found clear signs of all sorts of heavy elements, including sodium, tin, aluminum, magnesium, and manganese, as well as, weirdly, ytterbium*. Moreover, they could tell from the data that these elements existed in a cloud surrounding the planet, like an extended atmosphere going outward for hundreds of thousands of kilometers.

That’s a long way from the planet. Any atom of, say, manganese that far from the planet would be caught in a tug-of-war between the gravity of the planet and the star… and the star would win. The gravity of the star is drawing material off the planet in a vast stream, or, in other words, the planet is getting slowly eaten by its star. If astronomers ever get around to giving this planet an actual name, I suggest Sarlacc.

This explains the peculiar high abundance of heavy metals in the star I mentioned at the beginning of this post; they come from the planet! But not for long. Given the mass of the planet and the density of the stream, it looks like it has roughly ten million years left. At that point, supper’s over: there won’t be anything left for the star to eat. In reality it’s hard to say exactly what will happen; there may be a rocky/metal core to the planet that will survive. But even that is so close to the star that it will be a molten blob of goo. The way orbits work, the way the dance of gravity plays out over time, the planet itself may actually be drawn inexorably closer to its star. Remember, too, the star is old, and will soon start to expand into a red giant. So the planet is falling and the star is rising; eventually the too will meet and the planet will meet a fiery death.

See ya, Wasp-12b! Wouldn't wanna be ya!

As Phil pointed out:

Ha! The Universe, as usual, is smarter and more clever than we are. There’s a lot of strange out there, and the more we look, the more we find.

Right you are, Phil!