On this day in 1995 — 20 years ago — Bellerophon (51 Pegasi b) was discovered. It was the first planet discovered to be orbiting around a Sun like star — 51 Pegasi. It was the second exoplanet ever discovered. And it is quite a planet, which destroyed some theories about solar system formation. It is a very big planet: roughly half the mass of Jupiter. But it is really close to 51 Pegasi — just 0.05 AU, which is six times closer than Mercury is to the Sun. The year on Bellerophon is four Earth days long.

Bellerophon was discovered the way that about half of all exoplanets have been discovered: with Doppler spectroscopy. I put the animation above (courtesy of Zhatt at Wikipedia) so that you would see how this works. As I’ve discussed before, Jupiter doesn’t actually orbit around the Sun; it orbits around the center of mass of it and the Sun (forget the rest of the solar system for now). But as a result, the Sun also orbits around the center of mass of the solar system.

As a result of Jupiter, the Sun wobbles — changing speed by about 12 m/s over the course of one Jupiter orbit (12 years). Since the 1990s, we’ve had equipment capable of detecting Jupiter if we were observing from another solar system. In the case of 51 Pegasi, the wobble was much greater: 70 m/s. But in the early 2000s, new systems were installed that could detect wobbles as small as 0.3 m/s. And soon, we should have systems that can detect wobbles of just 0.1 m/s. That last one is important because the Earth induces a 0.1 m/s wobble in the Sun. It’s very exciting.