Thursday, June 10, 2010

Orbit of an extrasolar planet detected!

The star Beta Pictoris  has long had a fascination for astronomers looking for planets around other stars.  Now astronomers have taken direct pictures of the first confirmed planet around that star.

In 1983, the IRAS infrared telescope discovered a disk of dust around Beta Pictoris, the first time dust had been imaged around a star other than the sun.  This dust disk has been well-studied since that time.  For example, the Hubble Space Telescope looked at the disk and found there were multiple disks, as you might expect if one or more Jupiter-sized or larger planets were tugging on the dust as they orbited the star. 

In 2003, European astronomers used the VLT telescope in Chile along with adaptive optics (which allow one to see sharper) to look at the dust disk.  A re-analysis of those images in 2008 found a small dot of light on one side of the star.  The dot was roughly how bright we'd expect a planet to be, but it was also quite possible that the dot was another star that happened to lie along the same line of sight.  This possibility has burned planet hunters before.

The way to tell if the dot was a planet is to look again later.  The star Beta Pictoris slowly moves across the sky because it actually is moving relative to the sun.  Other stars appear to move, too, but at different rates and in different directions, because they are all moving in their own orbits around the galaxy.  We call this movement "proper motion".  Stars and planets that are related to one another, like binary stars, will have the same proper motion.  So, if we see two dots move together, we can be confident that they are related.

This technique has been used before.  In the fall of 2008, two separate teams announced they had taken images of planets around the star Fomalhaut and around the star HR 8799.  In both cases, the planets had moved with the star through the sky, and had even shown a tiny bit of orbital motion.  These planets were unexpected in some ways, because they are much further from their parent stars than the planets in our Solar System are.

Back to Beta Pictoris.  In spring 2008, the European astronomers tried to take new pictures of the potential planet.  But they couldn't see it.  Either it was an unrelated star that had not moved with Beta Pictoris, or it was a planet that had moved closer to the star itself and so could no longer be seen against the glare of its parent star.  Last fall, the astronomers went back to image Beta Pictoris once again, and the spot was back.  On the other side of the star.

There's only one likely explanation for this -- the spot is a planet orbiting the star Beta Pictoris.  Through hard work and a little luck, astronomers had seen the planet complete part of its orbit where it appeared to pass close to its parent star.

For now, the discoverers can only place limits on how big the planet is and how far from its parent star it is.  Based on when and where the planet has been seen, the planet is probably about 8 to 15 times further away from Beta Pictoris than the Earth is from the Sun.  This would put it somewhere between Saturn and Uranus in our Solar System.  Based on the brightness of the planet, it is probably about 9 or 10 times the mass of Jupiter -- about as big as a planet can get (we think).  As astronomers watch the planet orbit the star, both of these numbers will get to be better known.

Beta Pictoris itself is a very young star, perhaps only 12 million years old (this is young by astronomical standards).  The fact that we see a giant planet there means that planets form quickly.  Beta Pictoris is also about 75% more massive than our sun, meaning that it burns brighter and that it won't live nearly as long as our sun, perhaps only 2 billion years (the sun will live a total of roughly 10 billion years). 

It's important to remember that this is not the first planet to be imaged around another star, and it's not the first planet to be found in an orbit similar to the big planets in our Solar System.  Beta Pictoris is one of the youngest stars with confirmed planets, and this is the closest planet to its host star of which we've been able to take a direct picture of the planet.

More planet news may be coming soon.  On June 15, the first 43 days of data from the Kepler Mission to look for Earth-like planets around other stars will be made public, along with four professional papers describing the data and a few hundred candidate planets (mostly Jupiter-sized planets very close in to their parent stars, the "hot Jupiters").  And who knows what else has been found and is being confirmed and analyzed as we speak!

To read the full press release on the new discovery around Beta Pictoris, go here!

1 comment:

  1. Fergal3:31 PM

    Kurtis, I think you're being a little generous to Mon. Lagrange et al.. This is the third paper claiming to discover this planet from these authors. The first was based on the first image of the companion (which could have been a false alarm), the second was after they looked a second time and didn't see anything, and only now do they have the evidence they need to actually make that claim.

    It is an interesting result, but it's a very bad example of how to do astronomy.