why are all the extrasolar planets that have been detected as massive as Saturn and Jupiter?

Answer 1

These planets are found because of their gravitational effects - and only massive planets are going to have large effects. Smaller planets are going to be much more difficult to detect.

Answer 2

Not all the extrasolar planets are as big as Jupiter---some closer to Neptune-sized or smaller. But most of them are large, and close to their stars. That's because the principal method of planet discovery is to search for stars with a little "wobble" in their velocity, where the wobble is caused by the gravitational attraction of a planet. This wobble is bigger if the planet is bigger, and it is bigger if the planet is close. So there is a "selection effect", where most of the planets found are big and close.

The surprising thing is that there are so many big, close planets. Until a few years ago, most planetary astronomers thought that such systems would be rare or nonexistent.

New methods of finding planets will likely find smaller planets in the future. The "nanolensing method" should be able to find planets that are smaller than the Earth, and far from their star.

Answer 3

this is not the case. the lightest planets that have been found are rocky, and about 7 times the mass of the Earth.

And this was using Earth-based tools, and not the best tools either.

It is very likely, actually almost certain, than soon to be launched satellites with much better tools on board (and no atmosphere to complicate things), will pretty quickly detect hundreds then thousands more exoplanets, than the mere 155 or so we know today.

of course, in the very early days, the main method was gravitational wobble (the planet pulls a bit on the star as it revolves, when the star moves it makes its light shift a bit), and you get more gravitational wobble from heavy planets orbiting close to the star.

it's a bit as if you were on the street in a city and your method of assessing your environment was bouncing very large and heavy (several tons) rubber balls off the environment. Those balls would bounce off buildings so you'd see building easily. From time to time they'd bouce off cars (though not well) so you'd "see" a few cars. They'd kill or wound people so you wouldn't "see" any people.

You could then be tempted to conclude that in cities there are numrous big buildings, very few cars, and that's it. When the reality is, of course, tons of people, many cars but much less than people, and many buildings but much less than people or cars.

Hope this helps

Answer 4

The detection of extra-Solar planets is a new science. When I was a kid it was deemed impossible to detect planets as far away as the stars. So, I would say give them a chance. The effect of an Earth sized planet on the stars motion (which is how they detect them) would be very small.

With 100 billion stars in the galaxy, this investigation has only just started.

Answer 5

they find the planets, not by seeing them, but by observing their gravitational pull on a star near them. the star moves from the pull and it can be observed and measured. a more massive planet has a bigger gravitational pull.

massive planet has big pull

small planet has little pull

its easier to notice the big pull.

Answer 6

Because the smaller the planet, the tougher it is to find..

Answer 7

I agree with "Zhimbo" -- bigger planets are easier to find. So we find then first.