Scientists from NASA and the European Space Agency have revealed that their observations of Vega indicate an outer and inner ring of asteroids – just like our own solar system.
Vega is a very bright star in our skies. Found about 25 light years away in the constellation Lyra, Vega is somewhere between 625 and 850 million years old and is (compared to our sun) something of a rock star – its burning bright and burning fast. So fast, in fact, that its pretty much hit its mid-life crisis point and will die in around 500 million years.
Its also something of a yardstick for astronomers, being one of the first stars other than our own to be photographed and is generally used to calibrate other observations. Another interesting fact is that our viewpoint of it is directly end-on to its axis of rotation and because it spins so fast (it spins at approximately 91% of the theoretical limit that it would fly apart under its own rotational speed) its equator is some 2300 degrees Kelvin cooler than the rest of the surface.
Vega has long been known for infrared signatures associated with large amounts of dust, thought to lie in a band not unlike our own Oort cloud that lies beyond the furthest reaches of Pluto. Earlier this week a team using NASA’s Spitzer telescope and the European Space Agency’s Herschel telescope revealed that infrared observations of Vega have revealed a second, warmer asteroid belt in closer orbit around Vega that wasn’t previously seen because Vega itself is so bright.
Now the reason why this news is so interesting is because Vega, along with another star called Fomalhaut (which you might recall had an exoplanet (Fomalhaut b) discovered around it in 2008) apparently follows a model that is not too dissimilar to our own solar system – a large, colder dust cloud with a smaller inner asteroid belt. The ratios of distance between the star and the belts are approximately similar to our own, so the gap between the inner and outer belts may well be filled with planets such as the large gas giants in our own solar system.
Fortunately for us, Vega has long been suspected of harbouring an exoplanet – although up until now direct observation of one has proven elusive. Various models put forward over the last decade include:
1) The shape of the asymmetric outer dust belt as being indicative of a planet twice the mass of Jupiter at about 50-60 AU from the star (N.Gorkavyi et al, 2000). Gorkavyi later did a simulation of an observed dust disk ring arc at 95 AUs, suggesting that there may be a sub-Jovian planet between 90 to 100 AUs out from Vega. The simulations indicated that one (or more) very massive planet within 50 to 60 AUs that might have destroyed the inner circumstellar dust disk by gravitational scattering.
2) A study showing that the disc shape is the result of a gas giant in a highly elliptical orbit (Millimeter-wave aperture synthesis imaging of Vega: Evidence for a ring arc at 95 AU, Koerner, Sargent, Ostroff, 2001)
3) A simulation by Wilner, Holman, Kuchner & Ho (Structure in the dusty debris around Vega) in 2002 of the outer disc indicated that the furthest extent of the orbit of a Neptune-like planet would be at about 30AU distance from Vega. Theres also an animation (apparently from the same study) by Kuchner showing how the disc conglomerates at each apex of such a giant’s elliptical orbit.
4) And another study by Mark Wyatt, (Resonant Trapping of Planetesimals by Planet Migration: Debris Disk Clumps and Vega’s Similarity to the Solar System, 2003) proposed the idea that the structure can be explained by a couple of Jupiter and Neptune sized gas giants around Vega, where the Jupiter sized planet pushed the Neptune-sized one into a highly elliptical orbit that takes it far away from Vega.
Now I’m sure at this point you’re asking a couple of questions. Questions like “All those studies are just what might have caused these shapes in the dust clouds – where are the observations of the planets themselves?”
This is where the long rambling paragraphs at the top of this article comes into play – a lot of our current methods are handicapped by either the characteristics of Vega itself or our position relative to it.
We cant find planets by transit (planets passing between Vega and Earth) because it is axis-on to us – most star systems we have seen have the axis of star rotation and the ecliptic (the plane that has roughly all the planets motion on it) at near ninety degrees, so in all likelihood a planet will never pass in front of Vega from our point of view.
At the same time, the Radial Velocity method is handicapped by the equatorial bulge of Vega because it rotates so fast. Normally when planets orbit stars, they drag a bit of stellar material around the stars equator due to gravity – the same effect of the Moon on the Earth produces our tides. We look for that eccentricity and if we find it there’s a planet in orbit around the star. If I recall correctly, the eccentricity produced is small and therefore difficult to isolate from the bulge produced by the extremely fast rotation of Vega.
I’m not saying that these things are impossible – probably just difficult. I’m sure that as time goes on, the increasingly sophisticated technology that goes into devices like the James Webb Space Telescope will reach enough sensitivity that these difficulties of detection can be overcome.
In the meantime we just have to be a little patient.