"There are now, depending on how confirmed you want a planet to be, up to over 1,000 confirmed planets," Stanford's Bruce Macintosh told an audience at the recent meeting of the American Association for the Advancement of Science. That's in part because of the ease of the transit method of exoplanet detection, which watches for changes in light as the planets pass between their host star and us. "With a sufficient camera, you can do this with a tiny telescope," Macintosh said, pointing to the HATNet system of 11cm telescopes.
But the transit method also tells us next to nothing about the planet, simply its size relative to the star it's orbiting. Radial velocity measurements, which look at the planet's gravitational influence on its host star, can tell us the mass. Combined, the two can tell us the density, which can provide some hints as to what the planet's composition is.
That's very little information to go on if we're trying to assess things like planet formation models or habitability. To really understand a planet, we have to start looking at the composition of its atmosphere, and there are only a handful of planets we've been able to do that with. And all of them have relied on a technique called adaptive optics. These systems correct for the distortion of the atmosphere by using a mirror that can be deformed to compensate for it.
To help explore the atmospheres of exoplanets, Macintosh has helped build the Gemini Planet Imager (GPI), the latest in adaptive optics systems. It includes hardware that diverts some of the light to hardware that analyzes it for distortion, as well as a deformable mirror in the light path. The mirror is not the main one; instead, it's a smaller reflector. This makes the adaptive changes more manageable and allows the hardware to be upgraded in the future. Since it has been installed, planets that had taken an hour to image can be detected in about a minute.
Right now, things like the GPI are limited to imaging young planets that are still glowing from the heat of their gravitational collapse. This imaging can tell us lots of things about the planets; lack of methane, Macintosh said, indicates that it's being destroyed as it circulates through a hot atmosphere. The carbon to oxygen ratio tells us a bit about how the planet probably formed (either through the accretion of icy bodies or by drawing in gas).
But to some extent, that's secondary to being able to directly see planets. While showing an image of the HR8799 system, which has three planets orbiting it, Macintosh said, "Kepler's laws work; the fact that they work on other planets is not a surprise. But it's probably one of the three most amazing things I've seen in my scientific career."
Right now, adaptive optics systems only work for young planets that are roughly Jupiter sized and above—they're big enough and hot enough to stand out above the light of the host star. But within a decade, there will be 30-meter telescopes that will allow us to see down into the sub-Neptune/super-Earth range of planets. In the meantime, the GPI is expected to have imaged 600 stars and identified 50 to 60 new planets.
While adaptive optics were developed for imaging through the Earth's atmosphere, Macintosh also suggested that they'd be finding a home in space, installed in one of the orbiting Hubble-class telescopes given to NASA by the intelligence community. In this case, they'd be used to fix any distortions caused by imperfections in the primary mirror—something that required astronauts to do for the Hubble.
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