Amazing article: http://www.universetoday.com/112769/an-earth-sized-diamond-in-the-sky-the-coolest-known-white-dwarf-detected/#more-112769
We find exoplanets in one way right now, and that’s by measuring dips of star light as the “planet” transits in front of it and alters the star’s magnitude. This technique changing as we speak, with large telescopes being constructed. We just blew a top off of a mountain for the ELT (Extremely Large Telescope.. I know, creative, right?) and the James Webb Telescope, which will replace Hubble, just passed a major construction milestone. These will block the targeted star’s light out with cool shields to view exoplanets in visible light. They actually can do this now, but only to a small degree.
There is a particular “exoplanet” they found around a pulsar. It is roughly the size of Earth and exhibited odd properties, so it has been a regular target for various astronomers. They have come to find out this thing is not an exoplanet, but a white dwarf. These types of binary systems are very, very common. However, there is a reason I am posting about it.
This white dwarf, compared to others we observe, is very unique. These dwarfs are stars similar to our Sun, but much further in their evolution. They have expanded to red giants and blown off all outer layers long ago and left a super/condensed core of a star. As I said, this star is now the size of Earth, yet started out roughly the same size of our home star. What makes it amazing is that it is most cooled down white dwarf we’ve ever measured. It has cooled to the point where the carbon it is largely comprised of has actually solidified.
This, by definition, is a star, not a planet. However, it has a solid surface of diamond.