New Method Could Photograph Earth-Like Planets

Compared to current methods that can only photograph very large planets beyond our solar system, a new way of directly imaging planets orbiting other stars could be used to discover alien worlds that are smaller, more Earth-like, closer to their suns and further away from our own solar system.

Several methods for detecting extrasolar planets exist, including ones that use powerful, large telescopes to directly image the light coming from these distant worlds. The new method, which uses a device called a 'vortex coronagraph,' allows astronomers to search out these alien worlds using just a small portion of a telescope.

To show that the device worked and could detect exoplanets, Gene Serabyn, of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues used it with a 1.5-meter (5-foot) portion of the Palomar Observatory?s Hale Telescope, located north of San Diego. With it, they detected three previously discovered exoplanets that orbit the star HR 8799.

The three planets, called HR8799b, c and d, are thought to be gas giants similar to Jupiter, but more massive. They orbit their host star at roughly 24, 38 and 68 times the distance between our Earth and sun, respectively

These planets, among the first to be directly imaged, were originally detected by two much larger telescopes: one of the two 10-meter (33-foot) telescopes of W.M. Keck Observatory and the 8.0-meter (26-foot) Gemini North Observatory, both on Mauna Kea in Hawaii.

"We managed to see these planets with a telescope that's smaller than one panel on the Keck telescope," Serabyn said.

Seeing these planets with a much smaller telescope suggests that if this device is used on bigger ground-based telescopes, astronomers will be able to find planets around stars that are further away from us, that orbit closer to their star, and/or that are smaller and more Earth-sized (most of the exoplanets that have been discovered to date are much larger, Jupiter-sized worlds).

"What this does is it allows you to consider using a much smaller telescope, and something that's much more affordable, to look for Earth-like planets," Serabyn told SPACE.com.

The device could also be put on space telescopes, which would currently have to be very large and likely cost-prohibitive to do exoplanet detection like this.

Serabyn estimates that even on a 2-meter space telescope (one with an opening just over 6 feet wide), this device could help detect a planet within one astronomical unit (that's the distance between the Earth and the sun, or about 93 million miles, or 150 million kilometers) out to a distance of about 30 light-years. (A light-year is the distance that light can travel in one year, or about 6 trillion miles, or 10 trillion kilometers).

The new method works by combining adaptive optics ? sets of mirrors that bend and re-bend light thousands of times per second to remove the distortion effects of the Earth's atmosphere ? with a coronagraph, a telescope attachment that uses a black dot to block out direct starlight so that nearby objects, which would normally be obscured by the starlight, can be resolved.

But the problem with a traditional coronagraph is that some of the starlight leaks around the black dot obscuring the light of possible planets, especially those that orbit close to their stars. To solve this problem, Serabyn and his colleagues used "a spiral pattern" that would shift the light in such a way that "it pushes all the starlight outside of the beam of light and it leaves the planet light, which didn't actually go through this central vortex, just going straight through," Serabyn explained.

Serabyn plans to discuss using this device on larger telescopes in the near future. "At the moment what we have is good enough to use on any ground-based telescope," he said.

The device and the detection of the three exoplanets are described in the April 15 issue of the journal Nature.

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