A nearly 100-year-old spectroscopic recording of starlight has recently been recognised as the earliest evidence of exoplanets that orbit other stars. The ancient spectra on silver plate kept at the Carnegie Institution in Washington DC, US – record the light of a white dwarf star known as van Maanen’s star.
Stars such as our Sun fuse hydrogen in their cores into helium, but white dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel. Re-analysing the vintage van Maanen’s star spectrum, researchers have found evidence of calcium in the star’s outer layers. In a ‘dying’ white star, all elements heavier than hydrogen and helium should normally be buried deep in the core and invisible to spectroscopy.
But experts are now confident that the heavier elements observed in such ‘dirty’ white dwarf stars are evidence of planets. The existence of such stars is explained by the fact the heavier elements must come from orbiting exoplanets – planetary systems not yet observed by other methods.
So spectra of van Maanen’s star count as recorded evidence of exoplanets has remained hidden in the archives since 1917 – fully 78 years before Michel Mayor and Didier Queloz discovered the Doppler shifts in the light of the star 51 Pegasi, which launched the current hunt for further exoplanets.
Any extraterrestrials inhabiting the planets orbiting van Maanen’s star won’t have survived the fiery demise of their star, but there are more than 3300 confirmed exoplanets around other stars. Around half of them were discovered by the successful planet-spotting spacecraft Kepler, which was launched into an Earth-following orbit around the Sun in 2009. For some of these planets, spectroscopic measurements can now unveil details that may ultimately lead to the discovery of extraterrestrial life.
Atmospheric signatures
The majority of the exoplanets known today have been discovered due to their transits – they pass through the line of sight between their star and us and thus dim the starlight measurably.
In 2002, David Charbonneau from Harvard University, US, succeeded in using such transit observations made with the Hubble Space Telescope to detect spectroscopic signatures during transit that are absent in the pure starlight and thus presumed to be due to the filtering effect of the atmosphere of the transiting planet.
Charbonneau and colleagues first detected sodium signals, but other atoms and molecules have since then also been spotted in the atmospheres of transiting exoplanets, including carbon monoxide and water vapour.
Earlier in 2016, researchers from University College London, UK, along with an international team, reported the first atmospheric studies of an exoplanet from the size range known as ‘super-Earths’, ie only a few times heavier than Earth, but still much smaller than the gas giants in the outer reaches of our Solar System.
The researchers used the Hubble Space Telescope to study the atmosphere of the planet 55 Cancri e, which is around 40 light years away from Earth. It is unusually close to its star, taking only 18 hours to complete an orbit. Temperatures on the planet surface are therefore exceedingly high, at around 2000°C. The spectroscopic analysis discovered unusual molecules including hydrogen cyanide, pointing to a high ratio of carbon to oxygen in the composition of this planet.
Ultimately, the coveted prize in this field is to reveal similarly detailed chemical information about Earth-like planets orbiting in the ‘Goldilocks zone’ of their systems, at a distance from their star suitable for liquid water to exist on the planet’s surface. Discovering a chemical imbalance in the atmosphere of such a Goldilocks planet, such as the simultaneous presence of methane and oxygen, could yield a strong indication of life. Sophisticated instruments currently under development will soon boost this line of research.
The Transiting Exoplanet Survey Satellite (TESS), led by George Ricker at Massachusetts Institute of Technology (MIT) and developed by NASA’s Goddard Space Flight Center, is due to launch in August 2017. During its two-year mission, it will monitor 200,000 bright stars for indications of planetary transits, covering the entire sky, with the aim of discovering a range of exoplanets including Earth sized ones as well as gas giants. The broader sweep will make up for the lower sensitivity, compared with Kepler. The planets it will discover will be easier to study with ground-based telescopes.
The James Webb Space Telescope, a large infrared telescope with a primary mirror of 6.5m diameter, is a collaborative project of NASA with the Canadian and European space agencies, due to launch in October 2018. Among other astronomical investigations, it will use infrared spectroscopy to study the atmospheres of known extrasolar planets.
And finally, the European Space Agency’s PLATO (Planetary Transits and Oscillations of stars) mission will scan a million stars for indications of habitable planets. Set for launch by 2024, PLATO will scan a wider field of view than Kepler with almost equally good sensitivity.
Direct observation
Analysing the tiny fraction of a star’s light filtered through the atmosphere of a passing planet is challenging, but picking up direct emissions from exoplanets is even harder. However, it has recently become possible thanks to advanced observation devices like the Gemini Planet Imager, a near-infrared instrument installed at the Gemini South Observatory at Cerro Pachón, Chile.
Gemini can detect thermal emissions from Jupiter-like planets, regardless of the distance from their stars. Being able to discover gas giants in Jupiter-like orbits, this method fills a gap left by the Doppler and transit observations, which both rely on shorter orbits. The first exoplanet discovered by this programme, 51 Eridani b, was reported in August 2015.1 This planet is of a similar size as Jupiter, but around 2.5 times more distant from its star. Spectroscopic analysis of its infrared emission revealed a higher concentration of methane than in any other exoplanet so far. Its spectrum is in fact more similar to Jupiter’s than to most other exoplanets studied.
In December 2015, researchers using this device reported the discovery of a planet that has virtually been ejected from its system, orbiting 16 times further from its star than Pluto is from the Sun.2
SPHERE – the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument – is installed on the European astronomical research organisation ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile and began observations in June 2014. SPHERE uses several advanced techniques, including adaptive optics compensating the effects of the Earth’s atmosphere, a coronagraph to black out the light of the exoplanet’s star, and differential imaging based on the differences in colour and polarisation between starlight and the emission of the planet. By combining these techniques, SPHERE can image exoplanets and protoplanetary disks at a higher contrast than all instruments previously.
Listen for life
While spectroscopists may one day discover chemical imbalances indicative of life, other researchers are still listening out for deliberate communication attempts, an endeavour known as Search for Extraterrestrial Intelligence, or SETI for short. In July 2015, a stellar cast of scientists including astrophysicist Stephen Hawking announced an ambitious new SETI project. Funded with $100m for the next ten years by the Russian internet entrepreneur, Yuri Milner, Breakthrough Listen started operating in January 2016. The funding is split into three roughly equal parts: for instrument time; equipment and method development; and for academic staff and graduate students.
The project targets the 1m stars closest to Earth, as well as the centre and the plane of the Milky Way. Beyond our own galaxy, it will also listen out for possible messages from the 100 closest galactic neighbours.
The project aims to surpass all previous SETI efforts in scale. It will not only cover a larger part of the sky, but will also scan a much wider part of the radio spectrum, covering the entire ‘quiet spot’ between 1GHz and 10GHz, which is regarded as a promising area for interstellar communication, as it is free from interfering emissions from natural sources. And all this, the initiative promises, will be done 100 times faster than by previous efforts.
For the radio search, the project is due to buy vast amounts of instrument time on two of the world’s largest radio telescopes, namely the 100m diameter Robert C. Byrd Green Bank telescope, part of the National Radio Astronomy Observatory at Green Bank, Virginia, US, and the 65m Parkes telescope near Parkes, New South Wales, Australia.
In addition, the programme will search for optical signals with the automated planet finder at the Lick Observatory, near San Jose, California, which could detect an ordinary 100 Watt laser across interstellar distances.
For data analysis, Breakthrough Listen has agreed a partnership with the pioneering distributed computing project SETI@home, which farms out analysis to millions of computers via a screensaver. All software developed for the programme will be open source, and all data made freely available. On 12 April 2016, the project made the first batch of observation data accessible via its website and via SETI@home.
Data recorded so far include most of the stars within 16 light years of Earth, including stars such as 51 Pegasi that are known to host extrasolar planets, as well as a sample of stars between 16 and 160 light years away. This population of targets includes nearby sun-like and giant stars as well as numerous binary stars. The search also targeted 40 of the nearest spiral galaxies. Stars within 16 light years that are accessible only from the Southern Hemisphere, such as Alpha Centauri, will be observed by the end of 2016 with the Parkes Telescope.
On the air
‘Breakthrough Listen is officially on the air and scanning the skies for signs of intelligent life,’ Milner said. ‘It is a comprehensive effort, made possible by the tremendous scientific and technological advancements we’ve witnessed since the early days of similar efforts. Now, we join our trailblazing colleagues and ask people worldwide to review our collected data and explore the Universe with us.’
In April 2016, Milner and Hawking announced the Breakthrough Starshot programme to construct a gram-scale nano-spacecraft that could use light for propulsion and reach 20% of the speed of light, enabling it to travel to Alpha Centauri within 20 years.
From the thousands of exoplanets that have already been confirmed, we can extrapolate that there are as many planets as stars in the Milky Way – around 100bn – and there are as many other galaxies in the Universe. Given that life on Earth began to thrive almost as soon as the late heavy bombardment with asteroids subsided, some 3.8bn years ago, it’s highly likely that many of the other planets have also evolved life. However, a recent theoretical study suggests that very few of the initially habitable planets can keep their climate habitable in the long term, while many more suffer a runaway climate catastrophe and end up too hot like Venus or too cold like Mars.3
Whether any of the surviving habitable planets host intelligent life with a technology advanced enough to send radio signals that Milner’s project could pick up is a more open question. One key parameter in the probability considerations first formalised by American astronomer Frank Drake in the 1960s is the survival time of civilisations with advanced technology. Will they use the technology to self-destruct, eg in a nuclear war, or by depleting their planetary resources within a few centuries, or will they learn to live with the technology on a longer timescale, long enough to overlap and communicate with civilisations in other planetary systems?
The remainder of the 21st century will answer that question for our own civilisation, while Breakthrough Listen should give us an idea of the survival statistics of others in our cosmic neighbourhood.
References
1 B. Macintosh et al., Science, 2015, 350, 64.
2 P. G. Kalas et al., Astrophys. J., 2015, 14, 32.
3 A. Chopra and C. H. Lineweaver, Astrobiology, 2016, 16, 7.
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