Strap yourself in — you're about to learn about the Rossiter-McLaughlin Effect and why it's hot news among researchers who study alien solar systems.
These days it's routine to discover planets around other stars. A quick check of the authoritative Extrasolar Planets Encyclopedia shows that the tally of extrasolar planets has risen to 446 (not counting the
nine eight in our own solar system). So to get the attention of astronomers, a new find really needs to be something special.
At the Royal Astronomical Society's ongoing national meeting, astronomers have announced the discovery of nine transiting extrasolar planets. These worlds are termed "hot Jupiters" — they are huge and very close to their host stars — and their orbits are oriented such that each crosses its star's disk as seen from Earth. Every crossing registers as a slight dip in the star's brightness. To date, 73 transiting planets have been identified, so the discovery of several more of them isn't itself particularly newsworthy.
Topping the leader board in transiting-exoplanet discoveries is a remarkably productive program known as SuperWASP. Initially called WASP (Wide Angle Search for Planets) when conceived during the 1990s by observer Don Pollacco, SuperWASP has been monitoring the sky using arrays of CCDs attached to telephoto lenses. In this way, the team has found 26 extrasolar planets to date.
The "news" in this announcement is that some of the latest batch of extrasolar planets have orbits tilted so extremely that they're moving retrograde, that is, going around in directions opposite the way their stars are spinning. It's fair to say that these discoveries have literally turned the long-held view of how planets form on its head. Theorists have long assumed that stars and their planetary companions assemble from spinning disks of gas and dust, which leads to planetary motion in the same sense as star's rotation.
SuperWASP's observers can't actually resolve a planet transiting its star, so how do they know some are moving retrograde? The answer is depicted at right. As a star spins, the spectrum coming from the limb moving toward Earth is blueshifted and that from the receding limb is redshifted. Ordinarily these spectral shifts would be equal in strength.
Throw in a transiting hot Jupiter, and the result is a subtle spectroscopic signature called the Rossiter-McLaughlin Effect. If the planet has a "normal" (prograde) orbit, it begins each transit by covering part of the star's blueshifted limb, and this creates a spectral mismatch that can be measured. Conversely, a retrograde planet begins its transit on the redshifted limb, as the plot shows, again creating a distinctive spectral signature. Still, the effect is subtle, requiring sensitive radial-velocity measurements made with a battery of spectrometer-equipped telescopes.
How these massive, close-in worlds got going backward remains a mystery. In dynamicist-speak, it's a lot easier to change a planet's orbital period or eccentricity than to alter its inclination. In a press release from the European Southern Observatory, "The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,' says Andrew Cameron (University of St Andrews), who presented the new results at the RAS meeting. Conceivably massive planets can interact in a way that causes one to reverse course, but logistically it's a stretch.
Let me point out that Sky & Telescope readers got wind of wrong-way finds by SuperWASP and NASA's Kepler spacecraft in a December 2009 article by editor-in-chief Robert Naeye. As exoplanet specialist Joshua Winn (MIT) notes there, "Now we've gotten a glimpse of the weird, wild systems we've been hoping to find all along. Theorists will have a lot of fun trying to get the planets in these orbits."