Quest for Earth-like planets becoming more sophisticated

Sen—Is there another Earth out there in the Universe, orbiting in the habitable zone of a planet like our Sun? This question has been preoccupying astronomers for decades. Nearly 2,000 exoplanets have already been discovered, and the quest to find Earth-like planets is getting ever more sophisticated.

A new collaboration led by the University of California, Berkeley, aims to bring separate telescopes together into a family, united by the single goal of seeking habitable worlds.

The instruments include the Gemini Planet Imager, a powerful ground-based camera designed to take pictures of exoplanets.

Astronomers also plan to use a robotic telescope called the Automated Planet Finder, at Lick Observatory. It will examine the 100 nearest stars every night to search for any Earth-size planets orbiting them.

"The key work is to measure the Doppler effect nightly and search in those nightly Doppler shifts for periodicities that indicate the star is being pulled to and fro by the gravitational pull of the planet," UC Berkeley exoplanet-hunter Geoff Marcy told Sen in an email. "We can measure the speed of a star, using the Doppler effect, to within plus or minus 2 metres a second."

Most planets today are found using two methods: the radial velocity one that looks at the "wobbles" in a star when a planet orbits them, and the transit technique that monitors how a star's light dims when a planet passes across its face. NASA's Kepler telescope, a program of which Marcy is a part, has found more than 1,000 confirmed exoplanets with the transit method in a small region of Cygnus.

Marcy, however, is a pioneer of radial velocity and has found more than 100 of the first exoplanets using this technique. In the past 20 years, planet-hunting has progressed from discovering "hot Jupiters" close to their parent stars, to finding planets that are as small as the Moon or Mars and much further away.

For this survey, Marcy said the interest is in habitable planets, but "the key isn’t speed, but rather the ability to discover and measure properties of planets that were never detectable with previous techniques." The team recently received $3.25 million from NASA for the next four years, allowing collaborations between several institutions. It will be led by UC Berkeley's James Graham.

For the moment, GPI is only able to image planets that are about the size of Jupiter, but Marcy said understanding these gas giants is key to figuring out solar systems generally.

"The gas giants, with masses comparable to that of Jupiter, can migrate in close to the star, or be pulled by other giant planets out far from the star. As a giant planet migrates inward or outward it will gravitationally perturb the smaller planets, including the Earth-size planets," he said.

This introduces several effects, such as flinging the planets into non-circular orbits, or yanking around asteroids and comets that can fly out of the system or crash into planets. "So," Marcy added, "Jupiter-size planets can affect the suitability of a planetary system to support life."

The techniques used for imaging with the GPI will also help with the development of future instruments that can search for Earth-sized planets, added collaborator Bruce Macintosh, a physicist at Stanford University, in an email to Sen.

Eventually, the astronomers plan to link in telescopes that use adaptive optics to shift their mirrors, compensating for any blurriness in the atmosphere. This would include the active Keck telescopes in Hawaii and the Thirty Meter Telescope (TMT), which was supposed to begin construction in Hawaii this month but has been delayed by local protests. TMT will be able to detect Earth-sized planets when completed, Macintosh pointed out, as well as NASA's proposed WFIRST-AFTA telescope that could fly in the mid-2020s.

When asked how far he wants to push the science of searching for another Earth, Marcy responded, "We will simply do the best we can!" The program will form a part of the Nexus for Exoplanet System Science, a multi-university research institution and NASA centre collaboration to better understand the nature of exoplanets generally.

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