The New Wave in Exoplanet Astronomy: Near-Earths in the Near-Visible
Finding Exoplanets with Earth-Based Telescopes Using New Adaptive Optics
Until very recently, there was a strict limit to the type of planets we could look for from Earth. The technology we possessed limited us to seeking out massive, young gas giants – hot planets we could pick out by their infrared signals.
Thanks to a team of astronomers from University of Arizona, we have inched closer to being able to detect Earth-like planets in visible wavelengths, using Earth-based telescopes.
“Our eyes see visible light because they evolved under the light of our Sun,” said paper co-author Katie Morzinski, a Sagan fellow at the University of Arizona, “The Sun emits most of its energy in the visible wavelengths, and therefore the Earth reflects visible light. Thus, to find an Earth analog, we would need to look in the visible.”
It is far easier to capture images of planets in the infrared. Infrared wavelengths are longer than visible wavelengths. As such, infrared images are bent less by turbulent air. Shorter wavelengths, such as visible light, are bent more. The resulting images of planets taken in the visible tend to be blurry.
To overcome this issue, Professor Laird Close at University of Arizona developed an new adaptive optics system. Magellan Adaptive Optics, or MagAO, uses a thin shell of glass suspended over the telescope mirror by magnets. While the image of a young planet is being collected, the 1.6-mm thick glass surface is pulled and pushed in hundreds of places. This creates a rippled surface. The rippled glass counteracts the light distortion from the atmosphere, producing a crisp final picture of the observed planet. MagAO can find young planets, some of which are still forming in disks around nearby stars, at wavelengths closer to the visible.
“Advances in adaptive optics mean that the technique has improved and we now have second-generation systems on the sky,” said Dr. Jared Males, first author on the paper. “MagAO is the first of these to successfully produce extremely good images in the visible wavelengths.”
MagAO is being used to find new planets as well as study previously known ones. MagAO discovered its first planet, HD 106906 b, in December 2013. HD 106906 b is eleven times the mass of Jupiter, circling further from its star than we believed planets could: more than 650 times the distance between Earth and our Sun. To test MagAO’s ability to capture clear images of more Earth-like planets in the near-visible wavelengths, Chile’s Magellan 6.5-meter telescope was trained on Beta Pictoris b. Beta Pictoris b was discovered using the European VLT (Very Large Telescope) in Chile in 2008. It is only nine times the Earth-Sun distance from Beta Pictoris: a hot, bright star 63.4 light years away. MagAO overcame both the star’s intrinsic brightness and the planet’s nearness to take an image closer to the visible spectrum of a planet from an Earth-based telescope than has previously been done.
“A G2 star like our Sun is brightest in the visible. That means planets like the Earth will also tend to be bright in the visible because we will some day be imaging them in reflected light,” said Males, “So if we can work in the visible [sic] we can get smaller (meaning better) angular resolution for a given telescope diameter. Earth-like planets are very close to their stars, so we need very good angular resolution to be able to see them.”
While this technique has accomplished the previously unattainable by un-distorting a near-visible planetary image, it remains to be seen whether it can see a much cooler planet in the habitable zone: one Earth-sun distance, give or take 40 million miles.
“We know for a fact that a planet at 1 AU from a G2 star can be habitable, because the Earth is,” said Males.
Searching out such planets would take months of telescope time. “We cannot do a survey to find habitable planets, because we cannot get months of time per year on our instrument,” said Morzinski, “The Magellan telescopes are shared by a partnership between the University of Arizona, Carnegie Observatories, Harvard, MIT, Michigan, Chile, and Australia.” Still, space-based telescopes are challenging to launch and even more challenging to get time on. The possibility remains real that this technique-a combination of mobile glass, magnets and a highly sensitive charge coupled device (CCD) attached to a large telescope-will grant us our first glance of another Earth-like planet, as we look up from the ground for a light we can see with our own eyes.