Last week I gave my first proper talk to a conference of PhD students from nearby universities. Being an easily distracted man, rather than actually write my talk, I decided to spend the day before putting together an animation of the entire history of Planet Detection, from 1750 to 2015. It shows the orbital period (x-axis), planet mass (y-axis), radius (circle size)* and detection method (colour) of the 1800+ planets now known.
The idea of this plot is to compare our own Solar System (with planets plotted in dark blue) against the newly-discovered extrasolar worlds. Think of this plot as a projection of all 1873 worlds onto our own solar system, with the Sun (and all other stars) at the far left. As you move out to the right, the orbital period of the planets increases, and correspondingly (thanks to Kepler’s Third Law), so does the distance from the star. Moving upwards means the mass of the worlds increase, from Moon-sized at the base to 10,000 times that of Earth at the top (30 Jupiter Masses).
The colours are also important – dark blue shows the solar system planets (which include Ceres and Pluto for a few deacades each); In light blue are RV planets, which began the gold rush in 1995 with the discovery of 51 Peg; In maroon are Direct Imaging planets; in orange the microlensing discoveries; and in green those planets found by the transit method.
You might see a few patterns beginning to emerge:
The top left has a dense cluster of large worlds. These are the Hot Jupiters. We know of loads of these, even though they’re pretty rare, simply because they are easiest to find. Being so close to their star they produce the biggest radial velocity signals (light blue) and are most likely to transit (green). Ground-based transit surveys like WASP cant find anything beyond ~15 days, causing the sparse region to the right of this group.
The top right cluster is a population of Jupiter-like worlds that Radial Velocity is best at finding – anything beyond 10 years is too long at the moment to have a full signal.
The bottom group is from the Kepler space telescope. This clustering is the only one that’s actually real and not just a systematic effect. This is because Kepler was capable of finding every type of planet down to ~1 Earth radius. So this clustering shows that there are more Earth and super-Earth sized planets than any other. Hopefully we can begin to probe below it’s limit and into the Earth-like regime, where thousands more worlds should await!
Hope you enjoy it, and feel free to borrow it for your own use!
*Where Mass or Radius were unavailable I used the Mass-radius relations of Weiss & Marcy. Information from exoplanet.eu, so it might be a bit wrong. Thanks to Matt Kenworthy for suggestions. Pulsar planets are not plotted.