Big news: first solid exoplanet found!
Astronomers have just announced the discovery of the first planet orbiting another star that is unequivocally not a gas giant: it must be a very dense, rocky-metallic object not much bigger than the Earth!
The planet, discovered by the orbiting Kepler telescope, is called Kepler-10b. The star (Kepler 10) is roughly the same mass and temperature as the Sun, and is located over 500 light years away.
The planet was detected because it passes directly between us and the star as it orbits. When it does that, it makes a mini-eclipse, blocking a bit of light from the star. By knowing how big the star is and how much light is blocked, the size of the planet can be measured (the bigger the planet, the more light is blocked). In this case, Kepler-10b is only about 1.4 times the diameter of the Earth, making it the smallest exoplanet ever found!
However, there’s more. The planet’s gravity tugs on the star as it orbits, so as the planet makes a big circle around the star, the star makes a little circle in response (I like to use the analogy of a father dancing with his small daughter; as he swings her around she makes a big circle around him and he makes a little circle, because he’s much more massive than she is). As the star moves slightly toward and away from us we can measure the change in velocity using the Doppler shift, and that in turn tells us the mass of the planet. It turns out Kepler-10b is a lot more massive than the Earth, tipping the scales at 4.6 times the Earth’s mass.
So it’s not terribly earth-like; if you stood on its surface you’d weigh almost 2.5 times what you do now!