Finding Another Earth Within Reach

From EsoCast. Planet hunters unveil the tricks of the trade for finding planets around nearby stars and scanning them for signs of life.

Are we alone? It’s the biggest question ever. And the answer is almost within reach. With so many galaxies, and each with so many stars, how could the Earth be unique?

In 1995, Swiss astronomers Michel Mayor and Didier Queloz were the first to discover an exoplanet orbiting a normal star. Since then, planet hunters have found many hundreds of alien worlds. Large and small, hot and cold, and in a wide variety of orbits. Now, we’re on the brink of discovering Earth’s twin sisters. And in the future: a planet with life — the Holy Grail of astrobiologists.

Michel Mayor’s team found hundreds of them from Cerro La Silla, ESO’s first Chilean foothold. Here’s the CORALIE spectrograph, mounted on the Swiss Leonhard Euler Telescope. It measures the tiny wobbles of stars, caused by the gravity of orbiting planets.

ESO’s venerable 3.6-metre telescope is also hunting for exoplanets. The HARPS spectrograph is the most accurate in the world. So far, it has discovered more than 150 planets. Its biggest trophy: a rich system containing at least five and maybe as many as seven alien worlds. But there are other ways to find exoplanets. In 2006, the 1.5-metre Danish telescope helped to discover a distant planet that is just five times more massive than the Earth.

The trick? Gravitational microlensing.?The planet and its parent star passed in front of a brighter star in the background, magnifying its image. And in some cases, you can even capture exoplanets on camera.

In 2004 NACO, the adaptive optics camera on the Very Large Telescope took the first image ever of an exoplanet. The red dot in this image is a giant planet orbiting a brown dwarf star. In 2010, NACO went one step further. This star is 130 light-years away from Earth.? It is younger and brighter than the Sun, and four planets circle around it in wide orbits.

NACO’s eagle-eyed vision made it possible to measure the light of planet c — a gas giant ten times more massive than Jupiter. Despite the glare of the parent star, the feeble light of the planet could be stretched out into a spectrum, revealing details about the atmosphere.

Today, many exoplanets are discovered when they transit across their parent stars. If we happen to see the planet’s orbit edge-on, it will pass in front of its star every cycle. Thus, tiny, regular brightness dips in the light of a star betray the existence of an orbiting planet. The TRAPPIST telescope at La Silla will help search for these elusive transits.

Meanwhile, the Very Large Telescope has studied a transiting planet in exquisite detail. Meet GJ1214b, a super-Earth 2.6 times larger than our home planet.

During transits, the planet’s atmosphere partly absorbs the light of the parent star. ESO’s sensitive FORS spectrograph revealed that GJ1214b might well be a hot and steamy sauna world.

Gas giants and sauna worlds are inhospitable to life. But the hunt is not over yet. Soon, the new SPHERE instrument will be installed at the VLT. SPHERE will be able to spot faint planets in the glare of their host stars. In 2016, the ESPRESSO spectrograph will arrive at the VLT and greatly surpass the current HARPS instrument.

And ESO’s Extremely Large Telescope, once completed, may well find evidence for alien biospheres. On Earth, life is abundant. Northern Chile offers its share of condors, vicuñas, vizcachas and giant cacti. Even the arid soil of the Atacama desert teems with hardy microbes.

We’ve found the building blocks of life in interstellar space. We’ve learnt that planets are abundant. Billions of years ago, comets brought water and organic molecules to Earth. Wouldn’t we expect the same thing to happen elsewhere?

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