Our consideration of Pluto shows that life might exist in remarkably unlikely places, and my list in Table 1 may be too conservative. Many of the cold outer Solar System objects used to be dismissed as potential habitats. But perceptions have shifted. It’s possible that there are subsurface ammonia-rich oceans on Saturn’s moon Rhea, Uranus’s largest moons Titania and Oberon, and perhaps some large KBOs, such as Sedna, which is similar in size to Pluto yet 100 AU from the Sun. Given that so many objects in the Solar System are potential refuges for life, billions of similar possibilities surely exist throughout our galaxy.
Chapter 7
Far-off worlds, distant suns
The hunt for exoplanets
Beyond the Solar System, astronomers have discovered over 3,400 exoplanets, including candidates and confirmed bodies. It’s easier to detect large ones, so nearly all are bigger than Earth and some are rather exotic. Hot Jupiters are Jupiter-size planets within 0.5 AU of their parent stars, some orbiting in only a few days. Then there are planets that sound like Earth on steroids, the Super-Earths. These are up to ten times the mass of our planet. Although harder to find, it’s clearly just a matter of time before many Earth-sized exoplanets become known. How many will be habitable or even inhabited?
Of course, before identifying a habitable exoplanet, you have to find exoplanets. With the vast distances involved, the search is difficult but nonetheless astronomers have developed two classes of methods. The first, indirect detection, looks for stellar properties, such as position or brightness, which are affected by the presence of unseen planets. The second is direct detection of a planet with an image or a spectrum of its light.
The four indirect detection methods are: astrometry, stellar Doppler shift, transits, and gravitational microlensing. The key to the first two is that a planet and star orbit a common centre of mass. For example, if a planet and star had exactly the same mass, they would orbit a point halfway between them. In reality, a planet is smaller than a star, so the centre of mass is closer to the star and perhaps inside it. But in all cases, the star will follow a little orbit around the centre of mass and ‘wobble’ even when its planets can’t be seen. In our own Solar System, Jupiter’s twelve-year orbit causes twelve-year wobbles of the Sun’s position. Saturn adds in another smaller twenty-nine-year wobble, corresponding to its twenty-nine-year orbit around the Sun.
Astrometry measures the motion of a star in the sky using telescopes. It’s sensitive to big planets far away from their star so it could be used to find planetary systems similar to our own. But you have to wait many years or decades to track the effects of planets far from their star.
The second technique, the Doppler shift or radial velocity method, relies on the fact that when the light of a star is split into all its colours, the spectrum has dark bands like a bar code. The elements in a stellar atmosphere absorb photons coming from the star’s interior, which cause the dark lines. If the star moves toward or away from the Earth, the lines shift to higher (bluer) or lower (redder) frequencies, respectively. Everyone has experienced the Doppler effect in sound. When a wailing police car approaches, the sound waves are bunched into a high-frequency squeal, but after the car passes by, they become a low-frequency drone. A similar frequency shift occurs with light. A rhythmic red shift and blue shift of lines in a stellar spectrum shows that the star is wobbling and has a planet around it. The size of the shift indicates the mass of a planet, while the pacing gives the time for the planet to complete an orbit.
In 1995, Didier Queloz (then a student) and his mentor, Michel Mayor, from the University of Geneva, detected the first planet around a Sun-like star using Doppler shift. It was a planet of at least half a Jupiter mass orbiting a star in the constellation of Pegasus every four days. It was a total surprise. No one expected a giant planet so close (0.05 AU) to its parent star because giant planets shouldn’t form there. Now we understand that some extrasolar planets undergo planetary migration early in their history (Chapter 3) and we end up seeing what survived the jostling. In 2012, Doppler data suggested a possible Earth-mass planet just 4.3 light years away, orbiting the star Alpha-Centauri B. The planet, if present, lies 0.04 AU from Alpha-Centauri B, which is so close that its surface is probably molten rock.
One subtlety with the Doppler shift technique is how we view the planetary system. If the orbit is face-on (described as an inclination of zero degrees from a plane perpendicular to the line of sight), there’s no to-and-fro Doppler shift. The more edge-on the orbit, the greater the Doppler shift. It becomes maximal at an edge-on inclination of 90 degrees. There’s often no way of knowing the inclination, so a measured Doppler shift might be smaller than the ideal edge-on case, allowing us only to infer a minimum planetary mass. The Doppler technique is most sensitive to big planets close to their star, so that’s why most of the exoplanets initially detected were hot Jupiters. But we now know from the transit method that hot Jupiters are actually only a tiny minority of exoplanets, around 0.5 to 1 per cent.
The transit method measures the decrease in starlight when a planet crosses the face of a star, which can happen if we’re lucky enough to view an exoplanet system virtually edge-on. Such geometry is statistically rare, but there are so many stars that if you gaze widely and long enough, transits will be seen. From the dip in starlight, it’s possible to determine a planet’s diameter if the star’s size can be estimated. In turn, the planet’s orbital period and distance from its star can be calculated from the cycling of the dimming. NASA’s Kepler mission is a telescope that operated from 2009 to 2013 from an orbit around the Sun staring continuously at 145,000 main sequence stars in the constellation of Cygnus (the Swan) to look for transits where most of the stars are 500–3,000 light years away. Kepler has found over 3,200 exoplanet candidates, which are confirmed by checking for periodic dimming or using another detection technique, such as Doppler shift. In 2017, the Transiting Exoplanet Survey Satellite (TESS) telescope will begin examining two million stars for transits.
The fourth indirect method is microlensing. When an object passes between a distant star and us, Einstein’s Theory of General Relativity predicts the bending of light by the object’s gravitational field. The foreground object effectively acts as a lens, focusing the light and making the distant star appear gradually brighter. But if a planet orbits a lens star, the background star can brighten more than once. This sensitive technique can find Earth-mass planets at orbital distances of 1.5–4 AU around a star. Unfortunately, once the alignment has happened, it’s virtually impossible to follow up with more detailed measurements because the objects are typically very distant. However, microlensing can gather statistics. Large planets drifting alone in interstellar space can also act as lens objects, and one result is that there appears to be many unbound Jupiter-mass planets floating between the stars. These lonely worlds were presumably ejected from their extrasolar systems.
For astrobiology, the most important techniques are direct detections that capture light from a planet. Direct detection is challenging because a planet is a dim body close to a vastly brighter star. Nonetheless, telescopes in space, such as the Hubble Space Telescope, and extremely large telescopes on the ground have accomplished direct detection using a coronograph, which is a mask to block the starlight. The term ‘coronograph’ comes from techniques that were originally used to block out sunlight to study the Sun’s corona, which is the wispy halo seen in a solar eclipse. Ground-based telescopes also use adaptive optics, which are procedures to sense and correct the distortions caused by the shimmering of the Earth’s atmosphere.
A space-based telescope has the advantage of avoiding the blurs from the Earth’s atmosphere, but there’s the question of which part of the spectrum to examine. In general, planets emit mostly infrared radiation and reflect visible starlight. If we looked at our Solar System from afar, the Sun, being hot and big, would outshine the Earth by a factor of about ten million in the infrared and ten billion in visible light. So looking for a distant Eart
h in the infrared gives a thousand times better contrast than in visible light. Unfortunately, light also spreads and blurs (diffracts) when it encounters any object such as a telescope and this is worse with infrared light than with visible. Fortunately, a technique called interferometry can cancel unwanted light. Noise-cancelling headphones use the same principle to silence undesirable sound waves.
Light waves have crests and troughs like water waves. Nulling interferometry uses more than one telescope mirror to precisely align light waves arriving from a point in the sky so that wave crests cancel troughs and light is turned into darkness. In this way, starlight can be ‘nulled’ to see planets. Both NASA and the European Space Agency have studied space telescopes, called Terrestrial Planet Finder and Darwin, respectively, which use interferometry to capture images and spectra of exoplanets around nearby Sun-like stars.
The results of exoplanet surveys to date generally show that the number of exoplanets increases at lower masses, so rocky, Earth-sized planets should be common. Density estimates for some exoplanets also allow assessment of composition. Density has been inferred using mass from the Doppler method and size from transit methods. Also, if a transiting Earth-sized planet has at least one Neptune-like sibling, a method called transit timing variations can provide the mass of planets and thus density. Even if the larger planet doesn’t transit its parent star because of its orbital inclination, it can cause slight variations in the smaller planet’s transit times that allow planetary masses to be calculated. With density measurements, there are increasing clues about which exoplanets are made of gas, rock, water, or some mixture.
The habitable zone
When we were considering life in the Solar System, we concentrated on bodies with liquid water because we’re sure that’s a requirement for Earth-like life, and the same idea applies to exoplanets. Whereas dry planets with subsurface oceans are worth investigating with space probes within our Solar System, they generally wouldn’t be targets for exoplanet astrobiology. There’s no prospect in the near future of sending spacecraft to visit exoplanets, so everything must be done with telescopes looking at light from afar. Consequently, we care most about exoplanets with liquid water right at the surface. These have the chance of a biosphere that pumps lots of gases into an atmosphere. In principle, such biogenic gases are detectable in the light from the planet and might indicate life.
In 1853, William Whewell noted that Earth’s distance from the Sun allowed liquid water between what he called a ‘central torrid zone’ and an ‘external frigid zone’. In the 1950s, the American astronomer Harlow Shapley (1885–1972) (who discovered the dimensions of our galaxy) also talked about a zone around stars where planets could have liquid water on their surface. If a planet is too far away from its host star it ices over, and if it’s too close it’s too hot for liquid water. Nowadays, the term habitable zone (HZ) refers to the region around a star in which an Earth-like planet could maintain liquid water on its surface at some instant in time. We specify a particular time because stars age and brighten or dim, so the HZ moves. In contrast to the HZ, the continuously habitable zone (CHZ) is the region around a star in which a planet could remain habitable for some specified period, usually the star’s main sequence lifetime.
The width of the HZ around different types of main sequence stars, including the Sun, has been estimated. The inner edge is set by a planet’s susceptibility to the runaway greenhouse effect, while the outer boundary is usually determined by the failure of greenhouse warming when carbon dioxide is cold enough to condense into clouds of dry ice or dense enough to scatter away sunlight. The latter was a potential problem for early Mars (Chapter 6). For the Sun, the HZ’s inner edge is around 0.85–0.97 AU, while the periphery is 1.4–1.7 AU. The spread reflects the uncertain effects of water and carbon dioxide clouds at the inner and outer borders, respectively. For example, clouds might cool a close-in planet by reflecting sunlight, so the inner edge might be 0.85 instead of 0.97 AU. For the most optimistic outer boundary, Mars resides within the HZ. However, Mars’s small size has led to a thin atmosphere that can’t sustain liquid water. In other words, if Mars had been as big as the Earth, perhaps it might be habitable today. So planet size matters and the HZ is not the whole story for habitability but just a first guide.
Around other stars, the HZ is closer in or further out depending on whether a star is cooler or hotter than the Sun. For example, a K star, which is cooler (Fig. 1), would have a tight habitable zone from about the orbit of Mercury to that of the Earth. An even cooler M-type red dwarf would have the centre of its habitable zone at only 0.1 AU or so, well within the 0.4 AU orbit of Mercury.
In fact, the HZ of faint M dwarfs is so snug that planets will experience tidal locking, which occurs when gravity sets the rotation period. The same phenomenon makes one face of the Moon point toward the Earth. The Moon is tidally locked and spins on its axis once for every orbit around the Earth. The match between orbital and rotation periods, called synchronous rotation, is no coincidence. The Moon elongates slightly in the Earth–Moon direction, and if this orientation deviated, the Earth’s gravity would twist the Moon’s skewed bulges back into alignment.
A synchronously rotating planet in the HZ of an M dwarf would have one side in sunlight and the other in perpetual darkness. However, this doesn’t make such planets uninhabitable. Although the night side would be cold, a moderately thick atmosphere can transport enough heat to warm the night side while cloud cover can cool the day side. Also, a large moon orbiting such a planet would have variable sunlight. So tidal locking in the habitable zones of M dwarfs is not a showstopper for life. That’s encouraging because M dwarfs are the most common type of stars, making up about three-quarters of the total.
In recent years, assumptions defining the HZ have been questioned. Conventionally, the outer edge is determined by carbon dioxide condensation. But some large, rocky exoplanets might have thick hydrogen-rich atmospheres. We know from studying Titan and the giant planets that hydrogen and methane behave as greenhouse gases, and these condense at far colder temperatures than carbon dioxide. Also, on the inner edge, a water-poor planet with polar lakes rather than oceans might not succumb to a runaway greenhouse because it wouldn’t have enough water to generate an atmosphere completely opaque to infrared radiation. Thus, the HZ might be wider than we think.
Is there a galactic habitable zone?
Some astronomers also argue that there’s an optimal region in the galaxy for habitable planetary systems called the galactic habitable zone (GHZ). They note that the Sun is two-thirds of the way from the centre of the Milky Way, whereas planetary systems near the densely populated centre would be perturbed by supernovae or passing stars. At the other extreme, stars near the galaxy’s edge are very poor in elements other than hydrogen or helium, and this might curtail planet formation. Astronomers have an odd convention (which sickens chemists!) of calling all elements other than hydrogen or helium ‘metals’; the ‘metal’ content of a star is called its metallicity. Stars with giant exoplanets tend to be metal rich, and since giant exoplanets were the ones discovered first, it was initially thought that high metallicity was needed for planets to form in a nebula. More recently, no correlation with metallicity has been found in stars of Sun-like mass that have Super-Earths.
Another problem with the GHZ is that stars don’t stay put. Recent research shows that stars wander across the galactic disc as a result of gravitational scattering by spiral arms. In any case, the exoplanets that will be scrutinized for signs of life in the foreseeable future will all be close, within a hundred light years of us. So whatever the validity of the GHZ, it’s not a practical consideration for finding habitable planets any time soon.
Biosignatures, or how we find inhabited planets
Of course, the central question is whether we can find life. In 1990, NASA’s Voyager 1 spacecraft was heading out of the Solar System and looked back at the Earth from 6.1 billion km away. The famous picture it took is known as the �
�Pale Blue Dot’, in which Earth is so small that it’s a single bluish pixel. If we knew nothing else, what could we deduce? If we said that the planet’s colour was a mixture of blue oceans and white clouds that would just be guesswork. How can we do better?
Generally, to determine an exoplanet’s properties and look for life, we need a planet’s visible or infrared light spectrum. Different molecules and atoms absorb different frequencies in spectra. Thus, we can look for atmospheric gases such as oxygen or methane that can be produced by life. Such planetary features that indicate life are biosignatures. In fact, in the same year as Voyager 1’s ‘Pale Blue Dot’, the Galileo spacecraft, which was headed for Jupiter, obtained spectra of Earth. It was a sort of dry run for exoplanets and showed that you could detect Earth’s atmospheric oxygen, methane, and abundant water. The simultaneous presence of oxygen and methane is evidence for life because these two gases should quickly react with each other unless a biosphere is producing great amounts of them, which prevents equilibrium. Thus, we say that Earth’s atmosphere is in chemical disequilibrium. That’s the kind of biosignature we would like from exoplanets.
Already, some information has been collected about exoplanet atmospheres by looking at differences in spectra when transiting exoplanets pass behind or in front of a star. The spectrum in the primary transit when a planet crosses the face of a star includes light passing through a ring of atmosphere around the planet. So subtracting the spectrum of just the star after the planet has passed by can isolate the spectrum of the planet’s atmosphere. Alternatively, you take a spectrum when the planet is behind the star in its secondary transit and you subtract that from a spectrum of the planet plus star when the planet is beside the star. This produces the spectrum of the whole planet. In fact, this technique has been used to obtain infrared spectra of hot Jupiters with the Spitzer Space Telescope, which is a telescope orbiting the Sun.
Astrobiology Page 12
