Poor astronomy | Theorized exoplanet AB Aurigae b is seen directly in pictures

We now know of over 5,000 exoplanets, worlds orbiting other stars. The vast majority of them were discovered via the transit method, where the planet causes a mini-eclipse and blocks a little bit of its host star’s light. This is an indirect method; we do not see the planet itself, only the effect it has on the light of its star.

Right now, fewer than 60 exoplanets have been directly imaged; that is, seen on actual images taken by their star. This method works best in infrared light with young stars – newly formed planets glow in IR due to their own heat, while stars tend not to emit as much infrared, making the planets easier to spot. This also works best for planets that are quite far from their star – 50-300 times farther out than the Earth is from the Sun – so they are not lost in the glare.

This can also work for even younger planets. So young, they are still formed.

AB Aurigae is a young star, 2-4 million years old, and is about 560 light-years from Earth. It’s more massive, warmer and brighter than the Sun, and that’s what we call one star before the main sequencewhich does not yet fuse hydrogen and helium together in its core, but very close.

It is surrounded by the dust and gas from which it is formed. There is a ring of dust seen in millimeter wavelengths of ALMA that is about 18 billion kilometers out of the star, and a flat disk of gas much closer. The gas disk has spiral arms in it, which theoretical models show could indicate a planet or brown dwarf – an object in the mass between a planet and a star – in there, its gravity interfering with the gas in the disk. The dust ring also abruptly cuts off on its inner edge; yet another indicator of a decent solid body orbiting the star and its gravity shortening the inner ring.

With that in mind, a team of astronomers used the huge Subaru telescope in Hawaii and the Hubble Space Telescope to search for any still-forming planets, called protoplanets orbiting AB Aurigae.

And it looks like they’ve found one.

A bright spot is seen a short distance away from the star, about 90 times the Earth-Sun distance, or about 14 billion kilometers out – just over twice as far as Pluto from the Sun. It is outside the main disc of gas, but inside the dust ring.

AB Aur is slowly moving in our sky relative to more distant objects, and if this spot were a background star or galaxy, it would be in a very different position in the images after 15 years. It was seen in infrared observations taken by Hubble in 2007, and is still in almost the same position relative to the star in new observations. It is also seen in images taken with the Subaru telescope, so it is not due to some telescope or machining artifact. It’s real, and it’s moving along with the star.

Not only that, during the time between the pictures the place is seen to move a little bit around the star, in accordance with it orbiting the star at the distance clockwise. That would be true for a protoplanet as well as for just a large lump of gas, so it’s interesting but not crucial.

But better: Its position matches theoretical predictions about where a massive protoplanet should be to explain the spiral properties of the gas disk, which is intriguing.

It is clearly not a point source, not a simple dot, in the pictures either. Its size suggests that it is at most about 2 billion kilometers across, which is huge. But if a still-forming planet is at that distance from its star and has a mass of about 4 times the mass of Jupiter, it would have a disk of material around it that would be about that size. Then again, this is consistent with a massive protoplanet.

If this were just a speck of dust orbiting the star, the light we see from it would be highly polarized; that is, the waves of light would be strongly aligned. This is does not seen, indicating that the light is coming from the object itself and not just from reflected light. It is lit in other words. It is also seen in the kind of light emitted by excited hydrogen gas, which is expected for a planet surrounded by gas falling into it.

The colors of the place also agree that it is a massive but still very hot planet. Using theoretical models, the best fit the astronomers got for a young planet was about 9 times Jupiter’s mass, 2.75 times its diameter – quite large, but that’s because it’s still forming and has not settled yet – and with a temperature of about 2,000 ° C. Super hot, but again it is expected. This equates to a growth rate – how fast matter falls on it – of about one millionth of Jupiter’s mass each year. It may not sound like much, but Jupiter is a beast. That growth equals 5,000 trillion tons a day.

That’s 66 billion tons per second. About second. It’s like a 4 kilometer wide asteroid – about half the diameter of the dinosaur killer – that affects the protoplanet every second of every day for hundreds of thousands of years. No wonder it’s glowing hot.

Having said all that, this is not conclusive evidence that it really is a protoplanet that is growing, but it is pretty compelling evidence.

It makes me very happy. As I have written before, I was then working on the Hubble observations in 1999 of this star. This potential protoplanet was too close to the star to be seen in our data, but the new observations were made in a different way that allowed the planet to be teased out of all the light from the star. Impressive.

And it’s so close to us! We see stars form all over the Milky Way, all the time, and this is another piece in the galactic puzzle that shows planets forming right along with them. The universe is full of planets, worlds that cannot be counted, and we are just beginning to find them.


Tip about the dew shield to Thayne Currie, lead author on this research, for making me aware of his work.

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