In the 1960s it became clear that there was no advanced life elsewhere in the Solar System, so attention turned to life on hypothetical planets in other stellar systems. However, it is prohibitively expensive in energy costs to travel to other star systems, and, relativity being what it is, it takes a long time, too. Therefore, the focus turned to the idea of detecting life outside the Solar System. Giuseppe Cocconi and Philip Morrison pointed out in 1959 that radio telescopes could detect radio or microwave signals sent by an advanced alien civilization from a distance of more than 10 light years away [60]. Now, the 1960s were the era of the radio telescope. It was during this period that radio telescopes detected the background heat of the Big Bang and pulsars, among a host of other discoveries. In fact, in 1962, when Jocelyn Bell Burnell detected radio signals from the first discovered pulsar, the periodic signal was originally thought to be from aliens contacting Earth; the signal was originally given the name LGM-1, standing for “Little Green Man.” I call the years from 1959 to 1993 the SETI period because that is when the search for extraterrestrial intelligence flourished. I devote chapter 12 to the subject of the detection of alien intelligence in detail; for now, I will simply mention that several searches for alien intelligence, initially organized by Carl Sagan and Frank Drake and later funded by Steven Spielberg and others, looked for but did not find any alien races attempting to contact us over a radio or microwave channel.
In the 1960s it was impossible to detect exoplanets—planets outside the Solar System—using optical telescopes. This is why scientists wanted to look for intelligent life signaling us; it was the only way we could detect it. But things have changed: in 1993, ground-based optical telescopes detected the first planets circling other stars, and since then over 700 exoplanets have been confirmed, with over 2,000 other candidates; the planets within our Solar System are now in the minority by a huge margin. Most planets have been discovered using space-based instrumentation, first the Hubble Space Telescope and more recently the Kepler mission instruments, but a host of other Earth- and space-based detectors have found them as well.
Even with modern technology, few exoplanets have been directly imaged. The reason is that stars are large and bright, whereas planets are small, dim, and close to the star. If the star is D light-years away and the planet is d astronomical units away from the star, the angular separation between them as seen from Earth is given by
One arc-second (″) is 1/3,600 of a degree; this is the apparent size of a quarter seen at a distance of fifty football fields. This telescopic resolution is not impossible: the best telescopic angular resolution depends on the wavelength of light and the diameter of the telescope. Most exoplanet searches are done using telescopes designed for work in the near infrared or visible region of the spectrum where the wavelength of light used is near 1 µm (= 10−6 m). If A is the diameter of the aperture of the telescope and λ the wavelength being used by the telescope, in the units given above the smallest angle between two objects that the telescope can possibly resolve is given by the Rayleigh resolution criterion:
From this criterion a telescope with a 1 m diameter operating at a wavelength of 0.5 µm (in the middle of the visible spectrum) could resolve this angular separation for a planet 1 AU away from its star at a distance of 26 light-years. Ideal conditions are rarely met on Earth, where atmospheric turbulence reduces telescope resolution considerably, but telescopes in space can operate with near ideal resolution. The Hubble Telescope, for example, has a 0.0403″ resolution in the visible region of the spectrum (0.5 µm) and a 0.026″ resolution in the ultraviolet region of the spectrum [7].
Even with a telescope with sufficient resolution, a planet is still difficult to find because it is very dim compared to the star it orbits: it will reflect a certain amount of light from the star in the visible region of the spectrum and emit light in the infrared because of its own temperature, but those amounts will be very small compared to the star’s. The luminosity of the planet in the visible and the infrared region is given by the formulas
and
where V and I stand for visible and infrared, respectively, L is the luminosity of the star, a is its average albedo in the visible region of the spectrum, d, as above, is the distance of the planet from the star, and r is the radius of the planet. The distance and radius can be in any units you want so long as they are the same units.
For example, the Earth has a radius of 6,500 km and a distance from the Sun of 1 AU, or 1.5 × 108 km; from this, its visible luminosity is only about 1.4 × 10−10 of the Sun’s and its infrared luminosity is only 3.4 × 10−10 of the Sun’s.1 Despite these problems, a few exoplanets have been directly imaged by both space-based and ground-based telescopes using various clever techniques. The first exoplanet imaged was a planet orbiting the bright, close star Fomalhaut; the planet, known as Fomalhaut b and discovered in 2008, is about three times the mass of Jupiter and orbits at a distance of 115 AU [5].
Direct imaging works best for large planets far away from their star. Because of the difficulties in seeing such small objects so far away, only twenty-nine planets have been detected by direct imaging out of more than 700 confirmed exoplanet finds and nearly 2,000 more candidates [4]. Most planets outside the Solar System have been detected by indirect means. There are two major indirect methods for finding exoplanets: by Doppler wobble and by transits. The physics behind each of these methods is interesting enough that we should look at them in detail.
15.2 DOPPLER TECHNIQUE
In earlier chapters we discussed Kepler’s laws of planetary motions and went over the fact that planets orbit their stars under the influence of gravity on elliptical orbits. According to Newton’s third law, this isn’t the entire story: for every action there is an equal and opposite reaction. The star itself is influenced by the gravitational attraction of the planet. In fact, the planet doesn’t really orbit the star; both star and the planet orbit a common center of mass (which is located quite close to the star). The orbit of the planet is big, and the planet travels it with a high velocity; the motion of the star is small, and the star orbits the center of mass with a low velocity. But the star’s motion is measurable.
Let’s say that we have a star of mass M with a planet of mass m at distance d from the star. To make things simple, we’ll consider just the case of circular orbits: if the star’s orbital speed around the center of mass is V and the planet’s speed is v, then conservation of momentum tells us that
or
Since m is much smaller than M, the planet’s speed will be much bigger than the star’s speed. As an example, Jupiter, the largest planet in our Solar System, has a mass 318 times the mass of the Earth, which is just about 1/1,000 the mass of the Sun. Since its orbital speed is 13 km/s, the orbital speed of the Sun is a mere 13 m/s, and the effects of Jupiter will dwarf the effects of the other planets in the Solar System.
The Doppler technique gives two key pieces of information:
1. The speed of the star as it circles the center of mass, and
2. The period of the planet as it circles its star. The issue here is that using the Doppler effect we can see whether the star is moving away from us or toward us. As the planet circles its star, the star is sometimes moving toward us, sometimes away from us; the time it takes to make one complete period is the same amount of time it takes the planet to circle its star.
From these pieces of information we can get the speed of the planet around the star, the distance of the planet from the star, and the mass of the planet. Here’s how it works:
If Y is the amount of time it takes for the planet to circle the star (measured in years), d is the average distance from the star (measured in astronomical units), and M is the mass of the star (measured relative to our Sun), then Kepler’s third law tells us
But we know M because (on the main sequence) there is a strong correlation between spectral class and stellar mass (as detailed in the last chapter), and the spectral class of a star is easy to determine. The speed of a
planet on a circular orbit around its star is given by the formula
which can also be expressed as
So we know the distance of the planet from the star because we have measured M and Y:
We also know v:
From v we get V, and now we know the mass of the planet. Because Jupiter’s mass is almost exactly 1/1,000 the mass of the Sun and 1,000 meters = 1 km, we can write a simple formula for the mass of the exoplanet in terms of Jupiter’s mass:
Again, to be very clear, to use this formula, express the planet’s mass in units where the mass of Jupiter equals 1, express the mass of the star in units where the mass of the Sun equals 1, express the planetary speed in kilometers per second, and express the speed of the star in meters per second.
Actually, this technique really only gives a lower limit to the mass of the planet: you can only use the Doppler effect to measure the motion of the star either toward or away from you, but the plane of the orbit can be at any orientation with respect to the telescope. Because of this, what we are really measuring is m sin i, where i is the inclination of the orbit: i = 90 degrees means that the orbital plane is in line with the telescope, while i = 0 degrees means that we are looking at it perpendicularly.
15.3 TRANSITS AND THE KEPLER MISSION
The other commonly used technique to detect exoplanets is the transit method. If the planet’s orbit is at an inclination near 90 degrees, the planet will pass almost directly in front of the star as seen through a telescope on Earth. When the planet passes in front of the star, the star will dim by a tiny amount. The fractional dimming is equal to
where r is the planet’s radius and R is the star’s. Since Earth’s radius is only 1/100 the radius of the Sun, aliens trying to see the Earth through transits would need to measure a fractional dimming of the sun of (1/100)2, or one part in 10,000. Jupiter, on the other hand, is about 10% the size of the Sun, so we would need to measure a fractional dimming of only one part in 100. This isn’t great, but it beats the one part in 10 billion accuracy needed to directly image the planet. One can potentially measure three things from this technique:
• The radius of the planet, which can be calculated from the fractional dimming.
• The period of the planetary orbit. Each time the planet orbits the star, the star is dimmed, so watching the star for a long time tells you how long it takes for the planet to circle the star.
• The orbital inclination, which can be derived from how long it takes the transit to occur and the size of the planet.
You can’t get the mass directly from this, but if you can measure the mass using the Doppler technique as well, you can determine the composition of the planet in a crude way because knowing the size and mass allows one to determine an average density. The Kepler mission uses a specially dedicated space telescope to look for such transits; it has found more than 2,300 planet candidates, with 74 confirmed planets.
Both Doppler and transit techniques work better if the planet is large and close to its star. For this reason, the planets found to date tend to be the size of Jupiter or larger, and also closer to the parent star. However, as of December 2011 the Kepler mission had found several planets about the same size as Earth and one “super-Earth” in the habitable zone of its star [6].
15.4 THE SPECTRAL SIGNATURES OF LIFE
Once planets in the zone are detected, can we find life on them from a distance of light-years? The answer depends on whether we can measure the composition of any atmosphere the planet has in a transit. This is a daunting task. Earth’s atmosphere, for example, extends only about 100 km above the surface of the planet. If the Earth were shrunk to the size of a basketball, the atmospheric layer would extend only about one-tenth of an inch above it. The atmosphere is mostly transparent; changes in the spectrum of the star owing to the atmosphere will only be a tiny fraction of the changes owing to the transit itself, which is only a tiny change in the star’s brightness to begin with. However, these changes have been detected. In 2002 a team led by David Charbonneau measured the presence of sodium in the atmosphere of a planet orbiting the star HD 209458; the planet has a radius about 1.35 times Jupiter’s. The fractional change in the spectrum owing to the atmosphere was only about two parts in 10,000 [49].
Since then, astronomers have detected atmospheric constituents on more than a dozen exoplanets [213]. The transit technique works best in detecting atmospheres for hot Jupiters as they are large and close to their star, making the signal relatively large and the probability of a transit high. Atmospheric water vapor, carbon monoxide, carbon dioxide, and methane have all been seen as atmospheric constituents for these types of planets. To date, no atmospheres for terrestrial planets or super-Earths have been detected, but it is only a matter of time. The hope is in the detection of either oxygen or ozone (O3) for a terrestrial planet in the habitable zone around a star; because oxygen is highly reactive, its presence in an atmosphere is almost a guarantee of the presence of life on the planet. Recent studies indicate that both should be detectable in exoplanet atmospheres [214]. There are other signatures of life as well, however. Some are discussed in the next section.
15.5 ALIEN PHOTOSYNTHESIS
David Gerrold’s The War against the Chtorr series is one of the most interesting perspectives on alien invasion novels that there is. In the novels, the aliens aren’t invading Earth with a direct display of technological force; instead, there is a wholesale displacement of Earth’s ecology by a variety of alien plants and animals, each one worse than the last. The perhaps dominant Chtorran species are large wormlike beings, covered in purple-orange “fur,” that eat anything that moves. Apart from Gerrold’s tendency to polemicize endlessly, the books are fascinating and show the real potential of science fiction as a literature of the intellect. (If you are reading this book, David, please finish the series before you die!) The books are wonderful in that humanity has no idea whether there is a dominating intelligence behind the invasion that hasn’t shown itself yet or whether the alien ecosystem as a whole is invading out of some collective unconscious action.
I want to take exception with Gerrold on one matter: in the first novel in the series, A Matter for Men, the hero, Jim McCarthy, experiments with several samples of captured Chtorran life and finds that they are much more responsive to reddish light than to typical daylight:
“The point is, the atmosphere is thick and the primary is dim, but how much of each I don’t know—oh, but I can tell you what color it is.”
“Huh?” Jerry’s jaw dropped. “How?”
“That’s what I’ve been working on.… The star is dark red. What else?”
Jerry considered that. His face was thoughtful. “That’s fairly well along the sequence. I can see why the Chtorrans might be looking for a new home; the old one’s wearing out.” [92, p. 151]
Later on this piece of evidence is used to deduce that the Chtorran ecology may be more advanced than Earth’s by half a billion years because of the advanced age of its star [92, p. 211]. I want to dispute Jerry’s and Jim’s conclusions here: what they have implicitly stated is that the Chtorran star was originally a star like our Sun but is old enough to have evolved off the main sequence and into a red giant. However, this inference is completely unwarranted: when our Sun evolves off the main sequence, it is likely to sterilize life on Earth. The Chtorrans would have had to have left their planet much earlier than that to avoid being destroyed. In fact, life on Earth may only be possible for another billion years before increasing luminosity from the Sun creates a runaway greenhouse effect like that on Venus, long before Sol evolves off the main sequence [136].
It is much more likely that the Chtorran homeworld circles an M-class main-sequence star: such stars are very common, their dominant radiation is in the red or infrared region, and they are extremely long-lived, meaning that life on these worlds could have existed for a much longer time than on Earth. Another interesting point in the book is that Chtorran “vegetation” such as “red kudzu” is, as the
name implies, bright red. This raises an interesting question: what color would alien vegetation be?
Vegetation on Earth is green because it dominantly absorbs light in the red region of the spectrum (because there are a lot of photons there) and in the blue region of the spectrum (because they are high energy); the middle region of the visible spectrum, in the green, is reflected dominantly, leading to the plant’s colors. However, the chemistry and physics of photosynthesis are very complicated, as no less than eight photons are involved in each photosynthetic reaction [245]. N.Y. Kiang and her colleagues at NASA considered the twin questions of why photosynthesis on Earth evolved to absorb in the spectral region it does and what photosynthesis might be like on a planet circling another star [138] [139]. Kiang also published a less technical version of these two articles in Scientific American [137]. To quote from the latter article:
The range of M-star temperatures makes possible a very wide variation in alien plant colors. A planet around a quiescent M star would receive about half the energy that Earth receives from our sun. Although that is plenty for living things to harvest—about 60 times more than the minimum needed for shade-adapted Earth plants—most of the photons are near-infrared. Evolution might favor a greater variety of photosynthetic pigments to pick out the full range of visible and infrared light. With little light reflected, plants might even look black to our eyes [137].
So red kudzu is definitely possible, but black kudzu might be more likely. One issue with vegetation around M-class stars is that they have frequent UV flares, which have the potential of preventing life from establishing itself on land, confining it to a region below 9 m beneath the surface of the water.
Wizards, Aliens, and Starships: Physics and Math in Fantasy and Science Fiction Page 25
