“There are now, depending on how confirmed you want a planet to be, up to more than 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 partly due to the convenience of the transit method of exoplanet detection, which watches for changes in light as the planets pass between their parent stars and us. “With a good enough camera, you can do this with a small telescope,” Macintosh said, pointing to the HATNet system of 11cm telescopes.
But the transit method also tells us next to nothing about the planet, just its size relative to the star it orbits. Radial velocity measurements, which look at the planet’s gravity on its host star, can tell us its mass. Combined, the two can tell us its density, which may provide some hints about the planet’s composition.
That’s very little information to go on when 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 that we’ve been able to do that with. And they all relied on a technique called adaptive optics. These systems correct the deformation of the atmosphere by using a mirror that can be deformed to compensate.
To help explore the atmospheres of exoplanets, Macintosh helped build the Gemini Planet Imager (GPI), the latest in adaptive optics systems. It includes hardware that redirects 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 most important; instead it is a smaller reflector. This makes the adaptive changes more manageable and allows the hardware to be upgraded in the future. Since it was installed, planets that took an hour to image can be detected in about a minute.
Right now, things like the GPI are limited to imaging young planets still glowing from the heat of their gravitational collapse. This imaging can tell us a lot about the planets; lack of methane, Macintosh said, indicates it is being destroyed as it circulates through a hot atmosphere. The ratio of carbon to oxygen tells us something about how the planet probably formed (either through the accretion of icy bodies or through the aspiration of gas).
But to some extent that is secondary to being able to see planets directly. 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 no surprise. But it’s probably one of the three most amazing things I’ve seen in my scientific career.”
At the moment, adaptive optics systems only work for young planets about the size of Jupiter and larger – they are large enough and hot enough to rise above the light of the parent star. But within a decade there will be 30-meter telescopes that will allow us to see into the sub-Neptune/super-Earth range of planets. In the meantime, the GPI is expected to image 600 stars and identify 50 to 60 new planets.
While adaptive optics were being developed for imaging through the Earth’s atmosphere, Macintosh also suggested they find a home in space, installed in one of the orbiting Hubble-class telescopes donated to NASA by the intelligence community. datum. In this case, they would be used to correct any distortions caused by imperfections in the primary mirror – something astronauts had to do for the Hubble.