Chapter 7 Exoplanets

  There is no longer any doubt that planets are common around other stars. Until comparatively recently, this was a matter for speculation,but by 2010 the number of stars proven to have at least one planet orbiting them had exceeded 400. Allowing for how diffi cult it is to make these detections, it is clear that the majority of Sun-like stars must be accompanied by planets. To avoid confusion, professionals usually refer to them as 『extrasolar planets』 or 『exoplanets』. The exoplanet tally excludes exotic dim objects exceeding 13 Jupitermasses,which is the threshold above which nuclear fusion of deuterium (heavy hydrogen) can occur. Those are called 『brown dwarfs』 and are regarded as more star-like than planet-like.

  Detection methods

  Evidence that most young Sun-like stars have a surrounding ring of dust began to accumulate in the late 1970s. Initial clues came from the infl uence of dust on a candidate star’s infrared spectrum,then images of dust discs began to be obtained in the 1980s.Irrespective of whether these discs are like the solar nebula before planets formed or are remnant dust surviving in the equivalent of a star’s Kuiper belt, their mere existence showed that there ought to be plenty of planets out there too. The fi rst defi nite exoplanet discovery was made in 1995, after which discoveries gathered pace year by year.

  Radial velocity

  The fi rst discovery, and the majority ever since (more than 300 by2010), was made by detecting slight changes in a star’s radial velocity. This is the speed at which a star is travelling towards or away from the Earth, irrespective of any movement across the line of sight. Radial velocity changes can be determined to a remarkable precision of one metre per second by measuring shifts in the exact wavelength at which characteristic absorption lines appear in a star’s spectrum. These shift to shorter wavelengths (『blue shift』) if the star is moving towards us and to longer wavelengths (『red shift』)if the star is travelling away, in a phenomenon called the Doppler effect. Variations in radial velocity had long been used to measure orbital speeds (and hence to infer masses) of double stars, but the tiny infl uence of a much less massive exoplanet on a relatively much more massive star requires very sensitive modern instrumentation.Radial velocity changes caused by the Earth’s own orbital motion have to be accounted for before the more subtle changes attributable to the tug of the exoplanet on its star become apparent.

  The gravitational attraction between star and exoplanet depends on the sum of their masses. Fortunately, for Sun-like stars there is a well-understood relationship between stellar spectral type and mass. Knowing this, we can use the period and magnitude of radial velocity changes to determine the mass of the exoplanet responsible for the forward and back motion of the star. Usually,there is no independent measure of the orientation of an exoplanet’s orbital plane, and unless the orbital plane is exactly edge-on to our line of sight, the true change in velocity must be greater than what we detect. However, statistical arguments(based on assuming randomly oriented orbital planes) show that the majority of masses can be no more than twice the estimate based on assuming that the orbit is edge-on to us.

  The radial velocity method works best for large planets orbiting close to their star, because large mass and close proximity both lead to the greatest changes in the star’s radial velocity. Thus it should have been no surprise that the fi rst exoplanets to be detected tended to be more massive than Jupiter but orbiting only a fraction of an AU from their star.

  Discovery of these so-called 『hot Jupiters』 caused quite a stir,because they are well inside their stars』 ice line and cannot have formed where we now see them. It is now accepted that they grew further and then migrated inwards, and this has reopened the debate about the extent of planetary migration in our own Solar System’s early history. If Jupiter’s migration had continued inwards, it would have either destroyed or scattered each terrestrial planet in turn. For a while, 『hot Jupiters』 opened the prospect that such an outcome was normal, and that planetary systems like our own are exceptionally rare. However, improved and additional techniques for exoplanet detection have begun to fi nd rocky planets, showing that the preponderance of 『hot Jupiters』 in the early discoveries was merely a selection effect resulting from ease of discovery.

  Transits

  

  The second most prolifi c method for discovering exoplanets, likely soon to outpace the radial velocity method, is to search for 『transits』,when a tiny fraction of a star’s light is cut off during passage of an exoplanet in front of it. Most transits are discovered by repeated scans of likely stars using automated telescopes, originally from the ground but now also by dedicated telescopes in space.

  A transit can happen only if the exoplanet’s orbital plane lies almost exactly in our line of sight, which statistically should apply to only about half a per cent of all exoplanetary systems. The dimming of the starlight is slight, but is greatest for the largest exoplanets and occurs more often (and so is more likely to be detected) for exoplanets orbiting close to their star. Once again,discovery of 『hot Jupiters』 is favoured over any other kind of planet. The exact amount by which the starlight dims can be used to deduce the size of the planet compared to its star. The duration of the transit gives us clues to orbital speed and orbital radius, but follow-up radial velocity measurements can better characterize the system. Because a transit demonstrates that the orbital plane lies in our line of sight, masses derived by the radial velocity method are true values rather than minimum estimates.

  Imaging and other methods

  Exoplanets are exceedingly challenging to image, because they are so much fainter than their stars. Single exoplanets have been imaged at only a handful of stars. As you might expect, these are all Jupiter-sized or larger, mostly orbiting at tens or even hundreds of AU. In 2008, an adaptive optics image obtained from infrared telescopes in Hawaii showed three exoplanets orbiting a young Sun-like star (catalogued as HR 8799) at 24, 38, and 68AU. Beyond them is a dust disc at about 75 AU.

  Another method for exoplanet detection, called 『astrometry』, has great potential for the future. This is based on very precise measurement of a star’s position in the sky. Any unseen orbiting companion will tug the star from side to side. Astrometry seeks to detect this, instead of radial velocity changes along the line of sight. The motion is greatest if caused by a massive planet in a large orbit, so is complementary to methods more sensitive to small orbits. The fi rst confi rmed success of the astrometric method was in 2002, when the Hubble Space Telescope documented sideways wobble of the star catalogued as Gliese876, refi ning what we knew about a 2.6 Jupiter-mass planet orbiting at 0.20 AU already detected by radial velocity changes.The fi rst astrometric discovery of a previously unknown exoplanet came in 2009, when a red dwarf star catalogued as VB10 was found to be dancing out of position because of a 6Jupiter-mass exoplanet.

  A wholly different technique takes advantage of random (and never to be repeated) exact alignment between a foreground star and a background star. The foreground star acts as a 『gravitational microlens』 that amplifi es the light from the background star. The detected brightness of the background star rises and then falls over a duration of several weeks. If the foreground star has a fortuitously placed exoplanet, this will cause a brief spike in the brightness (lasting a few hours or days) superimposed on the slower rise and fall. By 2010, microlensing had discovered a total of ten exoplanets.

  Naming exoplanets

  Names are not given to exoplanets. They are identifi ed by adding lower-case letters after the name or catalogue designation of their star. The fi rst to be discovered is b, the second is c, and so on (a is not used). Thus, the fi rst exoplanet of Gliese 876 is Gliese 876 b,and two subsequently discovered exoplanets in the same system are Gliese 876 c and Gliese 876 d. This convention is messy, with letters bearing no relationship to the positions of exoplanets in multiple systems. However, it works, and perhaps it is wise for us not to impose names. Maybe the natives already have perfectly good names for their homes.

  Multiple exoplanet systems

  Multiple exoplanets are known orbiting nearly 50 stars.Sometimes a combination of detection techniques provides this information, but radial velocity alone can do the job: it is just a matter of unravelling progressively more subtle periodic variations. Table 8 lists some of the larger multiple systems.Among these, the system of Gliese 581 (a red dwarf about 20.5light years away) is particularly noteworthy. It includes the least massive known exoplanet, Gliese 581 e, which could be only 1.9Earth-masses (and is almost certainly less than 4 Earthmasses),and also what is possibly a (large) ocean-covered terrestrial planet, Gliese 581 d, of more than 7 Earth-masses.Gliese 581 e is far too hot to support life, or even to retain an atmosphere, but Gliese 581 d appears to lie in its star’s habitable zone.

  Table 8　Some multiple exoplanetary systems. Some of themasses quoted are minimum estimates. MJ = Jupiter-mass

  The nearest star known to have an exoplanet is epsilon Eridani,which is only 10.5 light years away. Epsilon Eridani b, discovered by the radial velocity method, is a Jupiter-mass giant in a 3.4 AU orbit. Infrared telescopes show that the star is accompanied by zones of rocky debris (asteroid belts) centred at about 3 AU and 20AU, plus an outer dust disc extending from 35 to 100 AU. Structure in the dust disc has been cited as evidence for an unconfi rmed one-tenth Jupiter-mass planet, epsilon Eridani c, at about 40 AU.

  Study

  We have little direct information about any exoplanet. If we determine mass (by radial velocity or astrometry), we can infer size by assuming a likely density. A transit will reveal size, which can also be deduced by imaging (based on brightness and assumed albedo). From size, we can deduce mass if we assume a density. Distance from its star gives us a fair idea of surface (or atmospheric) temperature, but this depends also on albedo and the mixture of greenhouse gases in any atmosphere, so there is a considerable margin for error.

  The next major advance in the study of exoplanets will probably come as we improve our ability to analyse their atmospheric composition. This is best done by telescopes in space, capable of isolating and analysing the visible and infrared spectra of individual exoplanets – most importantly terrestrial ones. Several abundant atmospheric gas species can be identifi ed by their characteristic absorptions. Detection of a pair of gases that ought not to co-exist under conditions of simple chemistry, such as oxygen and methane, may be the fi rst evidence we obtain of life affecting an exoplanet’s atmosphere in the same way that the Earth’s atmosphere has been radically changed.

  Life on exoplanets

  There are about 10,000 million Sun-like stars in our galaxy (about1 in 10 of the total stars). Exoplanets must be abundant, having been found orbiting about half of the adequately studied Sun-like stars. Most discoveries so far have been giant planets, because those are the easiest to fi nd, and there is no proof yet that terrestrial planets are common. Planetary systems are clearly diverse, and no terrestrial planet is likely to have survived the inward migration of a 『hot Jupiter』 such as ψ And b, currently orbiting less than 0.06 AU away from its star ( Table 8 ). However,because we are beginning to fi nd terrestrial planets, it is likely that they occur in a signifi cant proportion of exoplanetary systems.

  The question of how many exoplanets might host life is a vexing one. Let’s take a very conservative estimate that on average only1% of Sun-like stars have a terrestrial planet orbiting in a long-duration habitable zone. That would give 100 million habitable terrestrial planets in our galaxy, and there are probably at least as many habitable satellites orbiting giant exoplanets.

  The next step in the chain of logic is far less certain. Given the conditions required for life, how likely is it that life will begin? The building blocks for life are not a limiting factor. We know that space is awash with organic molecules, and that water is abundant too, so most exoplanets in a habitable zone will have all the necessary ingredients for carbon-based life. That means the Star Trek clichéof 『life as we know it』, without delving into speculation about other forms of life that are reliant on exotic chemistries.

  The ease or diffi culty with which life can spontaneously arise is a big gap in our understanding. Many (myself included) hold that with countless trillions of appropriate organic molecules in an exoplanet’s ocean, and with millions of years to play with, life will inevitably start. Once life has spread, it is hard to see how it can be completely eradicated, but if it was, it could presumably restart just as readily.

  We know that it took life on Earth less than 500 million years to establish a permanent footing. The abundance of life in the galaxy(and, by implication, in the cosmos beyond) will remain unproven until we detect signs of life on exoplanets. Even if we were to fi nd current (or past) life on Mars, Europa, or Enceladus, we could not leap to the conclusion that life had begun there independently,because objects in the Solar System are not totally isolated from each other. Microbes could survive transport from one to another inside fragments of impact ejecta. Europan life could have come from Earth; it is conceivable that life on Earth arrived on a meteorite from Mars.

  Is anybody out there?

  If there is life around other stars, what about intelligent life? Let’s speculate rationally. So far as we know, biological intelligence requires multicellular life. If microbial life begins, how likely is it that subsequent evolution leads to multicellular organisms? You can take your pick on this issue. It took a couple of billion years to happen on Earth.

  After multicellular life appears, will competition for survival drive Darwinian evolution as on Earth? Intelligence is one factor that confers an advantage, so how inevitable is intelligence?

  Even on my conservative fi gure of 100 million habitable terrestrial planets in the galaxy, plus a pessimistic view that life has only a 1in 100 chance of starting, that still leaves a million worlds with life, of which the Earth is one. It would be strange (and aweinspiring)if Earth were the only or fi rst planet out of all that number ever to host intelligence. But if life is so abundant, and if intelligence is a common outcome of life, then where is everybody? Unless it arises exceedingly rarely, or does not last long (for example, our own civilization could succumb to wars,various natural disasters, or self-made climate change), then the galaxy ought to be teeming with intelligence.

  Intelligent life would not have to be indigenous to where we fi nd it. Although the distances between stars are vast, it is perfectly feasible to travel between them. You do not need faster-than-light travel – all you need is determination and patience. Imagine a spaceship big enough to house hundreds of people, which would take 100 years to travel to a habitable exoplanet of a star 10 light years away. We could build such a ship ourselves, using technology foreseeable in the next few decades. One or two generations of the crew would live and die en route (unless some kind of suspended animation is used), and it would be very much a one-way trip. If we were to send such colonists to all the nearby habitable exoplanets (we expect to identify and characterize these by the end of the century), it would not be long before successful colonies had the capacity to launch their own colony ships, and so on. The galaxy is 100,000 light years across. Even if a wave of colonization takes 1,000 years to spread 10 light years, the entire galaxy could be colonized in only 10 million years. Catastrophes wiping out whole worlds or failures of individual colony ships would be insuffi cient to derail the process once it was underway.

  The galaxy is more than 10 billion years old. If intelligent life is abundant, there has already been ample time for countless previous species to have colonized the galaxy. This is the Fermi Paradox, named after comments made by the American physicist Enrico Fermi in 1950. Extraterrestrial civilizations ought to be numerous, but there is no sign of them: no artifi cial signals detected from space (despite scans of the sky by teams working under the banner of SETI – Search for Extra-Terrestrial Intelligence), no sign of great works of astronomical engineering,and no credibly documented alien visitors to Earth. Is intelligent life rare, after all, or are we too stupid to see the evidence? One day, I hope we will fi nd out.