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.
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KEPLER-62
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THE HABITABLE ZONE
The most important requirement for the evolution of life as we know it is liquid water. This can only exist on the surface of a planet if that planet is far enough away from the star at the centre of its planetary system: too close and the surface is too hot, resulting in any water boiling off into space; too far away and the surface is too cold and the water will exist only as ice. The too hot/too cold scenario is what is known as the Goldilocks Zone. The distance and width of the Goldilocks Zone also depend on the size and temperature of the central star – it is further away from large, hot stars and closer in systems with small, cold stars. Using the Hertzsprung-Russell diagram and the known size of the star allows the calculation of each system’s Goldilocks Zone, thus allowing us to determine whether the observed planets are likely to have liquid water and are therefore candidates for the evolution of life.
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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.
One of the most exciting areas of current astronomical research is the hunt for planets around other stars – known simply as exoplanets – which are potential homes for extraterrestrial life. Until recently, such a search would have been impossible, as planets are too faint to see over interstellar distances. However, thanks to new instrumentation, we are now able to detect the telltale signals of exoplanets using two main techniques: the radial velocity method and the transit method.
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RADIAL VELOCITY METHOD
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).
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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 di
scovery 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.
The Kepler Space Telescope has transformed our knowledge of the distribution of planets in the Milky Way. Kepler is not a general-purpose instrument with multiple detectors and myriad ambitions; the telescope was designed for one purpose: to look for Earth-like planets. Free of the distorting effects of the Earth’s atmosphere, Kepler carries a high-precision photometer, an instrument that has measured the light intensity from over 100,000 stars considered stable enough to support life on planets around them. Kepler searches for planets using a technique known as the transit method. If a planet passes across the face of a star as seen from Earth, the observed brightness of the star will drop by the tiniest of margins. Kepler’s photometer is so sensitive it can measure changes in brightness (to use precise astronomical language we should say changes in the apparent magnitude) of less than 0.01 per cent. Observing repeated dips in brightness allows the orbital period of the planet to be measured, and the details of the changes in the brightness, combined with knowledge of the orbit, allow the size and mass of the planetary candidate to be estimated. The transit method has been extremely successful in the hunt for exoplanets, but the technique is not entirely reliable, often throwing up false positives. Once a promising candidate is found, the location is passed to ground-based telescopes for further analysis, and, if confirmed, the planets are classified as discoveries. Kepler has used the transit method of planet hunting on a quite extraordinary scale since it became fully operational in May 2009. As I write in July 2014, NASA’s Exoplanet Archive lists 1,737 confirmed planets, over 50 per cent of which have been discovered using the Kepler data. This number is all the more staggering because Kepler is only capable of detecting a very small number of the planetary systems in our galaxy. Kepler views around 0.3 per cent of the sky in the constellations of Cygnus, Lyra and Draco, and even in this small patch, the telescope can only detect planets that pass directly in between their parent star and Earth. If the plane of the planetary orbits is orientated at the wrong angle, which is more likely than not, Kepler will not see any planets. Furthermore, Kepler only observed for four years, and because it has to see more than one transit to measure an orbit, it is blind to planets that orbit with periods greater than four years – which is the case for all the outer planets in our solar system. And finally, Kepler only sees stars out to a distance of approximately 3000 light years, whilst our galaxy has a diameter of 100,000 light years. Kepler’s data set, then, contains only a tiny fraction of the planetary systems out there. All of these losses can be corrected for in a statistical sense, and when the numbers are crunched we have a reliable observation-based number to put into the Drake Equation. The fraction of stars that have planetary systems is close to 100 per cent! On average, there is at least 1 planet per star in the Milky Way galaxy, and we can insert the second term with confidence: fp = 1.
The extraordinary Kepler mission was expected to last until 2016, but technical malfunctions may mean the telescope has now finished its planet-hunting activity. Even so, the huge volume of data is still being worked through and indications suggest it may have captured evidence for up to 3000 more planets circling distant stars.
This is encouraging for SETI enthusiasts, but in the hunt for civilisations, it’s not the number of planets out there that really matters; rather, it is how many of these planets are capable of supporting life. This is the next term in the Drake Equation – the average number of planets per star that has planets that can support life – ne. This is sometimes referred to as the Goldilocks question: how many of those billions of planets are not too hot and not too cold, but just right to allow life to exist on their surface?
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TRANSIT METHOD
The transit method of exoplanet identification depends on the measurement of the brightness of the light emitted by a star. This is very slightly dimmed as a planet passes between the star and the telescope. The Kepler Space Telescope can measure a variation of less than 0.01 per cent and has discovered 1,737 planets since its launch in May 2009.
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THE RECIPE FOR LIFE
Why Earth? What is it about our planet that makes it a home for life? In 2008 NASA brought together a team of scientists to define in the most basic terms the properties a planet needs to have a chance of supporting life, given our current scientific knowledge. Top of the list was liquid water – an ingredient virtually every biologist would agree is necessary for life. Water is a uniquely complex liquid, with its simple H2O molecules forming great complexes held loosely together by hydrogen bonds. It forms the scaffolding around which biology happens, holding molecules and orientating them in just the right way for chemical reactions to take place. It is a superb solvent, and remains a liquid over an unusually large range of temperatures and pressures. It has been said that we will never truly understand biology until we understand water, such is its role in the chemistry of life on Earth. Fortunately, water is abundant in the universe. Hydrogen is the most common element, making up 74 per cent of the matter in the universe by mass. Oxygen is the third most abundant, at around 1 per cent, and these two reactive atoms combine to form water whenever they can. Water has been present in the universe for over 12 billion years, which we know because we’ve seen it. In July 2011, a giant reservoir of water was detected around an active galaxy known as APM 08279+5255. The cloud contains over 140 trillion times the amount of water in Earth’s oceans, and is over 12 billion light years away, having formed less than 2 billion years after the Big Bang. So water is necessary for biology and, fortunately, extremely common throughout the universe.
Earth is unique in the solar system, however, because it is currently the only place where the surface conditions are right for water to exist in all three of its states: solid, liquid and gas. There are ice sheets at the poles and on the summits of the highest mountain peaks. In the atmosphere, clouds of water vapour form and fall as rain and snow, flowing back through rivers into the oceans that cover over 70 per cent of the surface. Mars has water, but on the cold red planet it can only be found as ice trapped in the poles and deep below ground and, just possibly, as sub-surface liquid lakes. Venus may once have been wet, but its proximity to the Sun and runaway greenhouse effect boiled any primordial oceans off into space long ago. This appears to suggest that it is Earth’s distance from the Sun that defines its suitability for life. Drag the Earth closer to the Sun and the temperatures would rise, the oceans would evaporate into the atmosphere, and if things got too hot the water molecules would escape into space, leaving Earth a dry, Venusian world. Drag the Earth further out towards Mars, and temperatures would drop until eventually the surface water would freeze.
Extended regions of liquid
water, conditions favourable
for the assembly of complex
organic molecules, and energy
sources to sustain metabolism.
NASA, 2008
It might appear tempting, therefore, to look for planets at roughly the same distance from their stars as Earth in the search for living worlds. This would be oversimplistic, because things are a lot more complicated. The conditions on the surface of a planet depend on many factors, the distance to the star being only one. The mass of the planet determines the gravitational pull it exerts on the molecules in its atmosphere, and this determines which atmospheric molecules it can hang on to at a given tempera
ture. This is important because the atmosphere plays a critical role in setting the surface temperature of a planet. Venus has the hottest surface in the solar system other than the Sun because of its greenhouse gas-laden atmosphere, despite being much further away from the Sun than Mercury. The Moon, on the other hand, has very little atmosphere due to its small mass, and even though it is the same distance from the Sun as the Earth, its surface temperatures range from over 120°C in direct sunlight to below -150°C at night. NASA’s Lunar Reconnaissance Orbiter measured the coldest temperature ever recorded in the solar system, -247°C, in the limb of a crater at the Moon’s North Pole, which never receives sunlight because the Moon’s spin axis is almost perpendicular to its orbital plane. The composition of the atmosphere is determined in part by the geology of the planet; on Earth, plate tectonics play an important role in regulating the amount of carbon dioxide in the atmosphere. CO2 is a greenhouse gas, and higher concentrations of such gases raise the temperatures. The presence of sulphur dioxide in the atmosphere from volcanic eruptions can cool the surface of a planet, however, because sulphate aerosols reflect sunlight back out into space. The Mount Pinatubo eruption in June 1991 cooled the Earth’s surface by up to 1.3 degrees for the three years following the eruption. And we shouldn’t forget that life itself alters the composition of planetary atmospheres quite radically. Earth’s atmosphere today is a product of the action of living things; before photosynthesis evolved, there was very little free oxygen in the atmosphere, and plants play an important role in removing CO2 and locking it up in biomass. The planet’s mass, spin axis, orbit, geology and atmospheric composition all conspire in a complex way to set the average surface temperature and atmospheric pressure, which ultimately determine whether liquid water can exist on the surface. And if life gets going, its effects have to be folded in as well.
Human Universe Page 9