Human Universe (13 page)

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:
f
_p
= 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 –
n
_e
. 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?

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.

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 H
₂
O 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.

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 temperature. 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. CO
₂
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 CO
₂
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.

Beyond the planet, a vitally important ingredient for producing a potentially living world is, of course, the parent star itself, and all stars are most definitely not alike. There are over two hundred billion stars in the Milky Way galaxy. The largest known supergiant stars are over 1500 times the diameter of our Sun. If such a star were located at the centre of our solar system, it would engulf Jupiter. At the other end of the spectrum are tiny red dwarfs, with diameters from around half that of our sun to as small as a tenth of it. The smallest known star at the time of writing goes by the name of 2MASS JO5233822-1403022, which shines eight thousand times less brightly than our sun and is smaller (but denser) than Jupiter.

As with virtually everything in physics, a good way to make sense of this stellar menagerie is to draw a graph. The most famous graph in all of astronomy is known as the Hertzsprung-Russell diagram, after astronomers Ejnar Hertzsprung and Henry Norris Russell, who drew it independently in 1911. They plotted the surface temperature of the stars (which is directly related to their colour – hot stars are blue or white hot, cool stars are red) against their brightness. It is immediately obvious that the stars are not distributed randomly on the diagram. Most lie on a sweeping line ascending from the bottom right to the top left. This line is known as the Main Sequence. Our yellow sun lies around the middle of the main sequence, and all the stars on this line are generating their energy in the same way – by fusing hydrogen into helium in their cores. These are the ‘standard stars’, if you like, although their masses, lifetimes and suitability for the support of living solar systems are very different.

The basic physics underlying the Main Sequence line is simple. Stars are clouds of hydrogen and helium, which is pretty much all there is in the universe to a good approximation, collapsing under their own gravity. As the cloud collapses, it heats up. This is not surprising – all gases get hot when they are compressed – try pumping up a bicycle tyre. Eventually, the collapsing ball of gas gets so hot that the positively charged hydrogen atoms overcome their mutual electromagnetic repulsion and fuse together in a nuclear reaction to make helium. This releases a tremendous amount of energy, which further heats up the gas, increasing the rate of nuclear reactions and continuing to heat the gas. Hot gases want to expand, and so ultimately a balance will be reached between the crushing force of gravity and the outward pressure exerted by the nuclear-heated gas. This is the current state of our Sun, happily converting 600 million tonnes of hydrogen every second into helium to counteract the inward pull of gravity. For less massive stars, the equilibrium will be reached at a lower temperature because the inward pull of gravity is weaker. Having a lower surface temperature, these stars will be redder than our sun, and also less luminous. These are the dim, red stars at the bottom right of the diagram, known as red dwarfs. We’ve already met an example of a red dwarf – our nearest stellar neighbour, Proxima Centauri. Red dwarfs also have the longest lifetimes of the stars on the Main Sequence, simply because they have to burn their fuel at a lower rate in order to reach a stable equilibrium with gravity.

At the other end of the Main Sequence are the massive blue stars. Ten times the mass of our Sun or more, the inward pull of gravity is strong, and they have to burn their hydrogen fuel at a profligate rate to resist collapse. This makes them hot, and therefore blue, but also short-lived. The largest Main Sequence stars will use up their nuclear fuel in ten million years or less, at which point they will move off the Main Sequence to become red giant stars. The red giants, like the famous Betelgeuse in the constellation of Orion, are stars nearing the end of their lives. Starved of hydrogen in their cores, they begin to fuse helium into heavier elements like carbon and oxygen. These stars are the origin of most of the heavy elements in your body. Their cores become superheated in their ultimately futile battle against gravity, causing their outer layers to expand and cool. This is why the red giants sit at the top right of the Hertzsprung-Russell diagram. They are vast, and therefore bright, but their cool surfaces cause them to glow a deep red. Red giants will last for only a few million years before they run out of nuclear fuel, at which point they shed their outer layers, forming one of the most beautiful sights in nature – a planetary nebula. It is these clouds, rich in carbon and oxygen, which ultimately distribute the building blocks of life into the galaxy. Your building blocks are likely to have been part of a planetary nebula at some point over five billion years ago. Cooling at the heart of the nebula is the fading core of the star, exposed as a white dwarf. These stars populate the bottom left of the Hertzsprung-Russell diagram.