# Question about star wobble as it relates to planet discovery?

Asked by cockswain (15269) March 19th, 2010

I watched an episode of The Universe on The History Channel and the episode discussed the discovery of extra-solar planets. Basically, how much a star wobbles can be an indicator of how much mass is orbiting it. This has led to the discoveries of many Jupiter-sized planets orbiting other stars. My question is, how do they know it is just one planet and not many smaller ones that equal the mass of the larger planet that is causing the wobble? I realize this is likely best explained with equations, but if possible please use a simpler explanation.

Observing members: 0 Composing members: 0

Intuitively, I would say that several smaller planets would cause the wobble to not be as cyclical or regular as it would be with just one huge planet.

With one planet, to simplify, you would see the mass shifting regularly from left to right, while with several planets, what you would see might be more irregular (such as not wobbling as much in one direction or another).

I don’t know if I am clear…

TheBot (766)

@TheBot That’s pretty much it.

There’s also a bit of magnitude involved—most of the extrasolar planets discovered using the wobble have been significantly larger than jupiter. The jupiter-sun system actually pivots around a point on the surface of the sun.

If you look at our solar system, we have four large planets: Saturn, Jupiter, Uranus, and Neptune. None of these planets are (currently) in an orbital resonance, so we’d expect to see a series of effects on the sun’s motion.

Jupiter does the most wobbling to the sun—pushing it back and forth about 1 solar radius. Saturn has 8.74% of the effect on the sun. Neptune 0.16%, and Uranus 0.34%

So, here’s a graph, showing the effects of the various planets:
http://folio.benpeoples.com/img/v7/p977376374.png

Note that Uranus & Neptune don’t really come off the Axis—their effect is so small on the sun’s wobble, it doesn’t really show up. Saturn & Jupiter push it around, but at most distances, we’re only going to be able to see the single planet.

This make sense?

(Oh: I should note that this graph is just showing the relative magnitudes of the effects, and I assumed the orbits were circular to make the math easy. Think of it as a napkin sketch)

grumpyfish (6630)

To add to what’s already been said in answer: It’s unlikely in the extreme that even an equal (or much greater, in fact) mass of “several planets” would have the same orbital period so as to always align on one side of the star at the same time. So a bunch of planets in various non-synchronized orbits would tend to cancel out the effects of any single planet… of relatively equal size.

And the discoveries to date can’t point to “small planets”, because we don’t have ways to observe the minor gravitational fluctuations that they would cause over the great distances we’re observing over.

CyanoticWasp (20070)

Those are great answers. Thanks for the help. I particularly found the graph helpful, because it shows the total wobble varies slightly over several periods, indicative of the effect of multiple non-synchronized orbits. This pretty much answers my question. Without knowing anything about the sensitivity of the instruments detecting the wobble, I’m assuming on a distant star we are seeing just the tiniest of shifts in lightwaves as it moves around. Further, many of these large extra-solar planets are in pretty tight orbits around their star, some being only 4 days for one revolution. Do you think it is pretty safe to say wobble is only going to be able to detect one, or at best, two planets depending on their mass and distance relative to the star? Everything else would get concealed by the noise, like Neptune and Uranus?

cockswain (15269)

One other point. You can only see the wobble if the star is moving towards and away from us, so we can see the red/blue shifting. If the wobble of the star is directly perpendicular to us, we won’t see it, because we can’t observe the red/blue shifts.

Another way they have discovered planets is by observing transits. The brightness of the star decreases as the planet passes between us and the star.

Rarebear (25159)

On the other hand, that star just may need some computer spin balancing. Bring it down to the shop. We’re also having a special on shocks this month: buy 3 and the 4th is free.

CyanoticWasp (20070)

or