How scientists will explore exoplanets for signs of alien life

Leading experts have been talking about the latest findings in the quest to discover signs of alien life, at science festival Starmus IV, in the Norwegian city of Trondheim.

An artist’s impression of the view from one of the planets found orbiting the red dwarf star Trappist-1. Image credit: ESO/M. Kornmesser

Michel Mayor, with Didier Queloz, discovered the first planet orbiting a normal star beyond the Sun, in 1995. The star, 51 Pegasi, lies about 50 light-years distant.

Mayor, from the University of Geneva, set the scene by saying that around 4,000 so-called exoplanets have since been detected, with several hundred awaiting confirmation.

He described the two basic techniques which revealed their existence – Doppler spectroscopy which measures a star’s wobble, and the transit method where starlight dims as a planet passes in front of it, blotting out part of its light.

Mayor said challenges facing planet hunters using ground-based telescopes included “noise” in the data, from disturbances in Earth’s atmosphere to activity on the surface of target stars.

But ever more sensitive equipment attached to telescopes such as the European Southern Observatory’s Very Large Telescope (VLT) in Chile, and ESO’s forthcoming much larger 39-metre Extremely Large Telescope (E-ELT) would find many more and smaller worlds, including Earth-sized rocky bodies.

Something in the air

Professor Sara Seager, of the Massachusetts Institute of Technology, was next up, speaking about her mission to study the atmospheres of exoplanets in a bid to find another world like Earth.

Seager said she wasn’t looking for monsters, or intelligent humanoids, but rather gases in atmospheres that did not belong – in other words, biosignature gases indicating the presence of life.

The oxygen that now makes up 20% of Earth’s own atmosphere was produced by photosynthetic bacteria in the first few million years of the planet’s existence. It nearly wiped them out.

Sara Seager tells Starmus how detecting the make-up of an exoplanet’s atmosphere can reveal signs of life. Image credit: Max Alexander/Starmus

Today, oxygen is astrobiologists’ favourite biosignature gas, Seager said. Methane, nitrous oxide and other simple gases were also byproducts of microbial energy extraction.

But life produces more complex molecules too, and life on Earth produced thousands of different molecules.

Seager explained that the composition of exoplanets’ atmospheres will be studied by analysing the light from their home star as they transit in front of them. Analysis of the starlight using a spectroscope showed dark lines of varying intensities which marked out different elements.

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Sun-like stars were too bright for such signals to be measured from rocky, Earth-sized planets, Seager said. So astronomers were first focusing on planets orbiting cooler, red dwarf stars.

Such stars had traditionally been considered unlikely locations for life because of the violent outbursts of radiation that red dwarfs emit. Bit they were the easiest to search.

Next, Seager talked about an upcoming NASA mission called the Transiting Exoplanet Survey Satellite (TESS) which is due to be launched in March, 2018. It will survey the whole sky looking for planets crossing the faces of small, nearby stars.

And the James Webb Space Telescope (JWST), set for launch in October 2018, will be capable of studying nearby rocky planets and detecting atmospheres around them, she said.

Finishing off, Seager said that the number of exoplanets found so far in our own region of the Milky Way suggested that there could be 100 billion billion Earths out there in the Universe.

Billions of Earth-like worlds in the Milky Way

And she said it would not be long before we would be able to look up, with the family, and point at a star in the night sky, and say: “That star has a planet like Earth with signs of life on it”.

Taking it to extremes

Final speaker on the theme of looking for alien life was Dr Lynn Rothschild, who addressed the most extreme environments where life might be found.

Rothschild, an evolutionary biologist and astrobiologist at NASA’s Ames Research Center, said that the only life we knew about was on Earth. Organic life, based on carbon and needing water to survive.

To search for life on other worlds, we had to know what we were looking for.

Rothschild said regions on Earth which were considered extreme for life included those with a lack of water, high salinity, extreme pressure at the seabed high levels of radiation, chemical extremes and extremely high or low temperatures.

Yet life had been found in several such extreme environments.

An artist imagines how a robotic probe might penetrate the ice of an outer moon, such as Europa or Enceladus, and release a hydrobot to look for alien life. Image credit: NASA

She said some scientists had suggested that simple life might have travelled from Earth to the inhospitable planet Venus, which has been described as Earth’s evil twin, where it might be surviving in the upper clouds. But she cautioned that we knew little yet about whether life existed in Earth’s own clouds.

Rothschild said possible locations to look for life beyond Mars included Ceres, with its salt deposits, and Jupiter’s moon Europa where we might drill through to its subsurface ocean. Further out, there was Saturn’s largest moon Titan, with its Earth-like landscape, albeit with liquid methane rather than water, and Enceladus, which seems similar to Europa.

NASA’s New Horizons probe had shown that dwarf planet Pluto was a world that might hold liquid water. And other Trans Neptunian Object were also potential bodies to study.

Rothschild concluded her talk by saying: “Are we alone? I suspect we’re not.”

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The Starmus festival, combining science, art and music, is being hosted in Trondheim for the first time by the Norwegian University of Science and Technology, following three previous events in the Canary Islands.

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