Another exoplanet imaged?

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Nov. 21 2008 9:03 AM

Another exoplanet imaged?

Another planet orbiting a sunlike star has been directly detected by imaging... maybe. Here's the image:

The possible planet orbiting Beta Pictoris is labeled. Click to embiggen. Image credit: ESO.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


In this infrared image (taken at 3.6 microns, about 5 times the wavelength the human eye can see), the planet is the labeled point of light. Image processing techniques have negated the light from the star, revealing the glowing tell-tale of the planet. But, like the previous planet seen circling Fomalhaut, this image needs a bit more explanation!

The star in question is β (beta) Pictoris, or just β Pic. It's about 64 light years away, and is far younger and a little bit hotter than the Sun. It has a disk of protoplanetary debris circling it, and from our angle we see it almost exactly edge-on, so the disk looks like a thick line bisecting the star. We've known about the disk since the 1980s.

But the disk is a little weird. There are actually two disks: the main one, and a much fainter, thinner one tilted a bit -- you can just make out the edges of it in the picture, to the left of the upper part of the main disk, and to the right of the main disk in the lower part. The main disk is also not symmetric; it stretches more on one side of the star than the other. There are also some features in it, what look like they are rings of material (but we see them edge-on too, so they are difficult to discern). All of this points to a disrupting influence on the disk, and that implies a big ol' planet in there somewhere.

The problem is finding it! The star is very bright (easily seen by the unaided eye in the southern hemisphere) and so any planet would be washed out. But astronomers using the Very Large (8 meter) Telescope have found something that is suspiciously like a planet. They very carefully subtracted the image of the star from itself (there are several techniques to do this) to reveal any faint blips that might remain, and voila! There it is.

It's a strong signal, so it's definitely real. But is it a planet? It might be a foreground or background object. The best way to know is to wait a year or two and take more images. If it is a planet, it will move across the sky along with the star. If it's a galaxy, it won't. Until we take more images, we won't know for sure.

However, note that it is aligned with the disk! That's a very strong (though circumstantial) piece of evidence that this truly is a planet. If I had to bet, I'd say it's the real thing. If it is, it's about 8 times the mass of Jupiter and about 8 times farther from the star than the Earth is from the Sun (somewhat less than the distance Saturn is from the Sun). The possible mass was determined using the brightness of the object; we know the star is about 12 million years old, and any planet that young will still be glowing from the heat of its formation -- in this case, at about 1200 Celsius. It's brightness and color depends on its mass, so the mass can be determined.

If this pans out, it will be the fourth planet directly imaged around a sun-like star. Incredible!

Remember, if it is a planet, it's not Earth-like at all. It's yet another super-Jupiter, and incredibly hot. But it's a planet.

Let me reiterate what I said in my last post on this topic. Trying to see faint objects near stars in astronomical images is phenomenally tough work. I spent weeks working on Hubble data to do this very thing, and it's mind-numbing, slow, meticulous, and difficult work. Subtracting a star's light from the image is really hard (one of the best pieces of software I ever wrote figured out the exact center of a star's image so another star's image could be used to subtract it, in fact) and if you're off by even a fraction of a pixel it won't work. This potential planet is deep in the bright part of β Pic's glow, yet they were able to see it clearly after processing, and that's an achievement all by itself.

I spent many months hoping to end up with a picture like this, and it is fantastically cool to see this dream finally come true -- especially with such a famous and well-studied target like β Pic. It's a great milestone, and my congratulations to astronomer Anne-Marie Lagrange and her team who were able to tease out this prize!