Human Universe (12 page)

This space we declare to be infinite …

In it are an infinity of worlds of the same kind as our own.

Giordano Bruno, 1584

The existence of alien worlds has been speculated about for many centuries. Ever since Copernicus began the process of demoting our solar system from its preferred place in the cosmos, it has been natural to assume that at least some of the stars in the sky must have planetary systems. Yet despite this seemingly common-sense conclusion, reached by virtually every right-thinking astronomer from Giordano Bruno onwards, the existence of other planets remained nothing more than an educated guess well into my lifetime. The vast distances between the stars and the limitations of technology locked us inside our own solar system with no way of seeing beyond. Throughout the nineteenth century a number of astronomers claimed to have detected distant planets, but all these observations proved to be flawed.

Today the picture couldn’t be more different; the night sky is known to be awash with worlds. One of the more enticing of the known solar systems is located around a slightly smaller, cooler version of our Sun called Kepler-62. About 1200 light years from Earth in the constellation of Lyra, the system has been widely studied because it has at least five planets. Two of them, Kepler 62-e and Kepler 62-f, are particularly interesting because they are Earth-like in both size and distance from the star. Bathed in Kepler-62-shine, these worlds may, if they have the right atmospheric conditions, support oceans of liquid water on their surfaces. We will discuss the significance of this in the context of life later on.

The discovery of extra-solar planets has been possible due to the rapid development of precision astronomical instruments, both space-based and terrestrial, that allow us to see beyond the bright glare of stars to the worlds that lie in the shadows. Imagine looking at our solar system from the nearest star system to Earth, Alpha Centauri. The system is 4.37 light years away, and consists of two sun-like stars – one slightly more massive than the other – orbiting each other with a period of approximately 80 years. The red dwarf Proxima Centauri is probably a distant gravitationally bound component of the system, making it a loosely bound triple star. Looking back towards Earth from 40 trillion kilometres with the naked eye, our sun would look like any other solitary star. Detecting exoplanets is no easy task because planets are vanishingly small and faint, masked by the brightness of their parent stars, and directly imaging them remains a major technical challenge.

To step out of the glare has required the development of indirect methods of detection based on surprisingly sensitive technologies. On 21 April 1992 the first conclusive detection of an exoplanet was made by radio astronomers Aleksander Wolszczan and Dale Frail, working at the Arecibo Observatory in Puerto Rico. They were hunting for planets around a pulsar known as PSR 1257+12, located 1000 light years from Earth, using a delicate method of indirect observation known as pulsar timing. Pulsars are spinning neutron stars, some of the most exotic objects in the universe. PSR 1257+12 is 50 per cent more massive than our Sun, but has a radius of just over 10 kilometres. It is, in effect, a giant atomic nucleus, spinning on its axis every 0.006219 seconds – that’s 9650 rpm. As you may gather from this rather precise
statement, it is possible to measure the spin-rates of pulsars with great precision by timing the interval between pulses of radio waves emitted from the stars like a lighthouse. Wolszczan and Frail reasoned that if a large enough planet was orbiting a pulsar, the gravitational tug should shift the arrival times of the radio pulses by enough to be detectable. And sure enough, they found two planets orbiting PSR 1257+12, and measured their masses and orbits. Planet A has a mass of 0.020 times the mass of Earth and orbits the star once every 25.262 days. Planet B is 4.3 times the mass of Earth, and orbits once every 66.5419 days. Subsequently, a third planet has been discovered, with a mass of 3.9 times that of Earth and orbiting every 98.2114 days. Pulsar astronomy is indeed a precision science.

This was an historic observation, but of limited direct interest to SETI since there is absolutely no chance that life could survive the hostile environment around such a violent astronomical object. It was, however, an existence proof – the first discovery of planets beyond our solar system, and a surprising one at that.

To search for Earth-like planets around Sun-like stars required the development of different but equally beautiful methods of observation. The first of these to be deployed was the radial velocity method. A star doesn’t sit still at the centre of a solar system with planets orbiting around it. Rather, the star and planets orbit around their common centre of mass. The centre of mass of a solar system with a single star will always be inside the star itself, because it carries virtually all of the mass, but the star will still wobble around the centre of mass of the system as seen from Earth.

This planetary-induced wobble is small but measurable. In our solar system Jupiter causes our Sun to wobble backwards and forwards with a velocity change of approximately 12.4m/s across a period of twelve years. The Earth’s effect is minute in comparison, inducing a velocity change of just 0.1m/s over a period of a year.

In the 1950s, future Green Bank pioneer Otto Struve suggested that such a planetary-induced wobble could be detected using the Doppler Effect. When a star moves towards the Earth, its light is shifted towards the blue part of the spectrum, and when it moves away from the Earth its light is shifted towards the red part of the spectrum. By making measurements of the specific frequencies (i.e. colours) of light absorbed by chemical elements in the star’s atmosphere, and measuring how much these are shifted relative to the known frequencies as measured here on Earth, the motion of the star backwards and forwards can be determined over a period of time, and this can be used to calculate the orbital period of the planet and to estimate its mass. If there is more than one planet, the motion of the star will be more complicated, but since the orbital periods of the planets are regular, the contributions of the different planets to the star’s wobble can be figured out.

The radial velocity method measures the variation in the wavelength of the radiation transmitted by a star. The variation is due to the star ‘wobbling’ as the exoplanet rotates around it, causing the distance from us to the host star to vary minutely. The dedicated planet hunter – the Kepler Space Telescope – uses the transit method (see
here
).

Struve was one of the first respected scientists to publicly state his belief in extraterrestrial life. In the 1950s, however, the spectrographs used to measure red and blue shift were only able to detect velocity changes of a few thousand m/s, and at the Green Bank meeting he could only speculate that his technique would one day confirm his prejudice that planetary systems are common. Struve didn’t live long enough to see his method applied, dying just two years after Green Bank, long before technology caught up with his ambition. It took until 1995 for two Swiss astronomers, Michel Mayor and Didier Queloz, to detect a planetary-induced Doppler shift using the Observatoire de Haute-Provence in France. The team discovered a planet orbiting the Sun-like star 51 Pegasi, located 50.9 light years from Earth.

This planet is named 51 Pegasi b, but its nickname is Bellerophon, after the mythological Greek hero who rode Pegasus, the winged stallion. Since its historic discovery, Bellerophon has been observed and examined in quite some detail, and it is no second Earth. It is a deeply hostile world, orbiting its parent star every four Earth days on a trajectory that takes it far closer than Mercury approaches our own Sun. Unlike Mercury, Bellerophon is a gas giant planet with a mass 150 times that of the Earth and a surface temperature approaching 1000 degrees Celsius. Although only half the mass of Jupiter, it may have a greater radius because the high surface temperature causes it to swell. Such exoplanets are known as Hot Jupiters – big enough and close enough to cause a significant wobble in their parent stars, which is why these types of worlds were discovered first by the early planet hunters.

The first evidence of a potential Earth-like planet arrived in 2007, when Stephan Audrey and his team at the European Southern Observatory in Chile announced the discovery of a planet around the red dwarf star Gliese 581, just over 20 light years from Earth. This was the second planet to be discovered in this system, but Gliese 581-c made headline news around the world because of its apparent Earth-like qualities. This planet is a rocky world, about five times as massive as Earth, and possibly the right distance away from its parent star to support liquid water on the surface: the stuff out of which science-fiction dreams are made. Further research has cast doubt on the idea that Gliese 581-b might have the necessary conditions to support life, but in March 2009 the second-Earth hunters got their own dedicated scientific instrument, and with it a cascade of new data became available.