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Kepler's tally of exoplanets (not showing many 100's of worlds from other surveys) (Credit: NASA Ames/SETI/J Rowe)
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This week has seen the release of the latest set of 'confirmed' exoplanets from NASA's Kepler mission. In total, 715 worlds have been added to the list of what are thought to be genuine Kepler planet detections (previously standing at 246). If you're confused because you've heard astronomers previously throw around numbers like 1,000 or 3,600 for the number of known exoplanets, well, that's because it is a bit confusing.
Altogether, the analysis of Kepler data has thus far produced a total of about 3,600 good candidate planet detections. But those are just candidates, they are periodic dips in the light of distant stars that meet certain criteria suggesting that the cause of the dips are real planets transiting or occulting the starlight - instead of noise, stellar flicker, or companion stars.
To say that these really are planets you need corroborating evidence. Previously that evidence has typically come from the measurement of the Doppler shift in the host star's light, resulting from the wobbling motion of the star about the common center-of-mass that it shares with any planets. It's a tough piece of astronomical wizardry to do that, to say the least, and in this latest Kepler planet release (opening the cage door as it were), a different approach is used. The key is that the 715 newly confirmed planets exist around just 305 stars. In other words each planet candidate has at least one other transiting planet in its system.
That fact allows for a statistical, probabilistic, analysis to eliminate other possibilities for these transit signals, such as stellar partners or plain old noise - because if you see one transiting planet, the odds increase for you to see its neighbors. It's a bit like saying that if you see something small flapping around a flowerbed you'd be tempted to call it a butterfly, but if you see two or more things flapping around, you might confidently say they were all butterflies.
It's clever, and adds another tool to the toolbox of exoplanetary detection - which has (independently of Kepler) already found hundreds of additional worlds. But what exactly do we have in that toolbox? This science is moving fast, and it can be easy to lose track of what an exoplanetary zookeeper has to rely on. So here's a highly simplified list of some major planet detection techniques (in no particular order):
Doppler or 'radial velocity' measurements: Requires exquisitely precise monitoring of thousands of spectral features in the light of distant stars. The best levels achieved at the moment amount to registering shifts of light in a spectrograph that correspond to several dozen silicon atom diameters in the pixels of an astronomical digital camera. Ouch.
Transit measurements: This is what Kepler did (and might still do in a limited fashion) as well as what numerous Earth-bound telescopic instruments carry on doing (and this). By sheer chance a planet's orbit takes it between us and its parent star, blocking weeny bits of light (much less than 1% in most case). The way that light dips in the so-called transit-curve (light versus time) has a characteristic form, and if it repeats with the same interval (another orbit) you gain confidence that it's a planet.
Transit-timing-variation: Your lovely transiting planet seems to not transit at quite the same time in its orbit, the reason? Other, unseen, planets are pulling at it, and bingo! You can not only deduce the presence of those planets but, if the planets orbit close enough, also place constraints on the masses of all planets. It's also possible to use the variation in star on star eclipse timing to do this, if your planets happen to conveniently orbit a stellar binary.
Gravitational lensing: What happens when a star and its planets gets between you and a more distant star? Because mass warps space, the star and its planets can act as a lens, briefly magnifying the light of the background star. How this magnification plays out over hours and days can reveal the masses and orbits of those planets. It's a tricky event to a) find happening, b) analyze. But the payoff is huge, with sensitivity to planets on large orbits and even to planets that have forsaken their parents stars.
Phase curves/brightness modulation: You're staring at a star with a sensitive telescope and the light you measure has a strange, repeating, variation. What's going on? The reflected and emitted light of a companion planet, that's what. In this case the planet need not transit the star, instead you're watching the varying light as it moves from crescent illumination to disk and back again, all merged with the light of the star.
Pulsar timing: The granddaddy of planet detection techniques, if a field that's barely 22 years old can have a granddaddy. In 1992 the first evidence for planet-sized bodies outside of our solar system came from repeating variations in the measurement of the pulses from a spinning neutron star some 1,000 light years from us. Usually a pulsar is comparable to an atomic clock in its regularity, this one wasn't. The reason - 3 planet-sized bodies orbiting it closely, and pulling at it.
Astrometry: As with the Doppler method, planets cause their stars to orbit a point offset from their own centers - the center-of-mass or balance point of the system. If you can measure the location of the star to high enough precision (micro-arcseconds is handy) you can actually see the star moving around in a little jig during the course of the planetary years. ESA's new GAIA mission will, among other science, reveal this motion for perhaps a thousand exoplanet host systems.
Direct imaging: This is the biggie, the holy grail, the deluxe version. Spot the emitted and reflected light from planets directly, in an image. The first problem is that stars are bright and planets are small, dim, and appear very, very close to their stellar parents. The second problem is the nature of light, it diffracts, and even the best telescope blurs starlight and swamps out those faint planets. The solution? Block or suppress the starlight. You can do this with a coronagraph (as in this spectacular success last year), or by 'nulling' the starlight - arranging your optics so that the starlight interferes destructively and zeroes out, but the planet light doesn't. A possible, and promising, space-based option is to do coronagraphy on steroids - build an occulting spacecraft, put it thousands of kilometers away from your telescope, and move it to block out stars...a starshade revealing all the planets in all their glory. Got a spare $1B ?