As far as detecting exoplanets go, it’s the oldest game in town…
The Radial Velocity or Doppler Wobble technique looks for rhythmic fluctuations in a star’s spectrum or colour as an indication of a planet’s gravity tugging at the star. We first saw the fruits of this labour in the 90s as the first ever exoplanet 51 Pegasi b was discovered. It rocked the foundations of contemporary thinking about how planets formed – we had assumed up until that point that our Solar System was a perfect template for all planetary systems (simply because we had no evidence to the contrary). It’s a nice, simple theory: small, rocky planets at the front, big, gaseous planets at the back. 51 Peg b changed all that – it was what we now call a “Hot Jupiter”: a gas giant planet that orbits so close to its host star that it would make Mercury seem distant. This makes them “hot” of course – the surface temperature of 51 Peg b is calculated to be 2000 degrees Celsius!
The Radial Velocity method is responsible for finding most of the 403 exoplanets known today. Of course if you’re looking for gravitational tugs from a planet, then the easiest planets to spot are the most massive and the closest (hence the propensity of Hot Jupiters detected). However, 15 years have passed since 51 Peg b, and instrumental advances have allowed the radial velocity method to be brought to bear on smaller and longer orbital period exoplanets.
Enter HARPS. The High Accuracy Radial Velocity Planet Searcher has announced 32 new extrasolar planets, bringing their grand total over their five year mission to an impressive 75. They range in mass from an almost familiar 2 Earth masses (affectionately referred to as “Super Earths”) to a bloated 10 Jupiter masses – remember that if a planet gets too massive (bigger than around 18 Jupiter masses) then it may start nuclear reactions in its core, forming a brown dwarf star. This dispels the received wisdom that radial velocity cannot compete with other detection methods (such as gravitational microlensing) in detecting low-mass planets. Admittedly, radial velocity searches get to such low masses by a careful selection of targets – by selecting M dwarf stars (cool, diminutive stars) they can discern gravitational signals from low mass planets more easily.
And we should take heart. We now see that low-mass planets are typically in multiple planet systems (sometimes with as many as five planets in one system), and that they are much more common than observations made out ten years ago. We are moving from the certainty of the pre-exoplanet era, through the concerning uncertainty of the last decade, to a less fraught middle ground, where planetary systems are thought of in terms of statistical mechanics rather than intricate, non-linear dynamics. Solar Systems like ours are probably not the norm, but we’re not too rare either, it seems. The Exoplanet Encyclopaedia is constantly being revised as new discoveries come in; each new planet hones our theory even further.
The British are no slouches in planet hunting. The WASP consortium, who find planets by the transit method, where the star is fractionally eclipsed by the planet itself, have a substantial collection of exoplanets of their own (15 so far, and many more on the way). We’ll soon get a hand in at radial velocity as well, if the UK Infrared Telescope’s Planet Finder or UPF gets the green light. Perhaps in a few years I’ll be blogging on Britain’s latest haul of radial velocity planets – perhaps even the first Earth analogue!