Question: do you think we have found all the planets in the galaxy yet?

Charlie Ryan answered on 17 Mar 2011:

hi razza2k11 great question and one that i am particularly interested in, although it’s not directly related to what i study.
Simply no! So far they have discovered in out solar system: 8 planets and five dwarf planets—Ceres, Pluto, Haumea, Makemake, and Eris (although there is some arguments about what to call these bodies).
Outside of the solar system they have discovered about 500 planets orbitting around other stars – according to wikipedia the exact number is 538! This is quite amazing since it is so hard to spot these planets as the light from the stars is much stronger compared to the reflected light of the planets.
There are various ways people use to spot a planet, but the two most sucessful are the transit and radial velocity methods.
The transit method is simple – basically astronomers set up a telescope to watch for a star’s light to become fainter very slightly. And sometimes this dimming is due to the planet going in front of the star – basically kind of like a partial solar eclipse happening in another solar system!! the planet blocks out some of the stars light, and if this happens periodically (say every month), then it may be a planet!
the radial velocity method is a bit more complicated. As a planet orbits around a star it makes the star wobble back and forth a little in space. This wobble is detectable when you look at the light spectrum of the star, which varies as the wobble takes place.
But both of these techniques work best with planets very near to the star (because the transit will block out more light, or the wobble will be greater) and the orbit is edge on to our view of the star. Also it’s much easier to do with stars near to ours.
It’s unsure how many stars actually have planets round them, because of the techniques being better at spotting large close-to-their-stars planets (hot jupiters!!). But perhaps a conservative (worst case – or best case if your worried about alien invasion!) estimate is 10% of stars have a planet round them. As there are 200 billion stars in the milky way galaxy, this works out at 20 billion planets. That’s an almost unbelievable number!! there’s got to aliens on one of them!