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Saturday, June 12, 2010


And it Moves! Direct imaging of Beta Pictoris b as it orbits its Sun

Planet orbiting ß Pictoris imaged by the European Southern Observatory (image credit ESO).

Planet detection has come a long way in the past few years, we have detected planets by the minute wobbles their gravity produces in their parent stars, the dimming of their parents sunlight as they pass across them, by gravitational lensing and finally we have been able to directly image a handful of them.

The problem with directly imaging planets is that their reflected light is feeble compared to that of their parent star, and we need to block out the light of the star in some way, either by placing an obstacle in the telescopes light path or using interferometry to cancel out the light of the star. Usually this means we can only see big planets far out from their parent star.

Dust disk around Beta Pictoris, with wobble. Image Credit Hubble.

We have long suspected that ß Pictoris had at least one planet around it. The star is surrounded by a dusty disk, and there were wobbles in the disk that was consistent with a planet plowing through the dust.

Then in 2008 the first image of what appeared to be a planet around ß Pictoris was reported. Now that report has been followed up with a second image that shows the dot that was reported back in 2008 really was a planet.

As seen in the top image (which is a composite of images taken in 2003 and 2009 laid over an image of the dust disk, the image of the star is blanked out) the dot had moved to the other side of the star, if it was just a back ground star that had moved due to the relative motion of ß Pictoris the dot would have been in a completely different location. So the dot is a planet.

And a very interesting planet, roughly 8 times more massive than Jupiter, it orbits roughly where Saturn would be in our solar system. This is important for two reasons. One is that it is at the so called "snow loine" where water is stable as ice. The snowline is thought to separate disk regions where rocky or gaseous/icy planets form. ß Pictoris b is at the right place to have formed by the process called core accretion (how Jupiter and Saturn are thought to have fomed. The other massive planets that have been directly imaged are too far from their parent star to have formed in situ and must have been ejected to their current orbits (such as Fomalhaut B, 115 AU away from its sun compared to the 12 AU of ß Pictoris b).

This also means it is the first time we have been able to see a directly observed planet complete such a large part of its orbit, all the others are too far away to mocve far in the time we we have observed them but in 15-20 years we could see a full orbit of ß Pictoris b (okay, so it's not like the super fats "hot Jupiters" that scream around their suns in days, but this is the most "normal", from a solar system perspective, plant that we have directly imaged).

As well, ß Pictoris b looks to be responsible for the warp in the inner dust disk, but can't be responsible for the warps in the outer disk, there must be more planets still undetected in the system.

Finally, the system is young, only 8–20 million years old, so giant planet formation must be very fast (in geological time that is). Observing ß Pictoris b and the ß Pictoris system will allow us to understand the early evolution of solar systems much better than before. A preprint of the journal article reporting this find is here.

Image of ß Pictoris b generated in Celestia, I have included the orbital elements for ß Pictoris b below. Copy the script and save it to a file called beta-pictoris-b.ssc in the extras folder.

The Bad Astronomer reports other details of ß Pictoris b and tells of his personal involment with the story.

"b" "HD 39060" # beta pictoris's HD number

# beta Pictoris b, long suspected to exist now directly imaged
# orbital movement detected 2003-2009

Texture "exo-class5.*"
NightTexture "exo-class5night.*"

Color [0.49 0.89 1]
Albedo 0.5

Mass 2542.9 # M.sin(i) = 8 Jupiters
Radius 81009.7 # rough guess
Oblateness 0.01 # guess

#InfoURL "http://exoplanet.eu/planet.php?p1=beta+Pic&p2=b"

EllipticalOrbit {
Period 6000
SemiMajorAxis 12.0
Eccentricity 0.05
Inclination 80 # rough estimate from image, doesn't transit
ArgOfPericenter 37
#MeanAnomaly 166

Obliquity 82 # guess, to match inclination
#EquatorAscendingNode 96 # guess, to match ascending node

# likely to be in captured synchronous rotation

AltSurface "limit of knowledge" "HD 39060/b"
Texture "extrasolar-lok.*"


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