Solution Planetary Systems Are Rare

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A time will come when men will stretch out their eyes.

They should see planets like our Earth.

Christopher Wren, Inaugural Lecture as Professor of Astronomy, Gresham College

Anthropic arguments are rather abstract. There have been many more tangible suggestions as to why ETCs might not exist. For example, perhaps there is no place for them to develop.

A common assumption is that complex life requires a planet — preferably Earth-like — on which to originate and evolve. A technologically advanced species may one day decide to move away from planet dwelling, of course, but the evolutionary ancestors of those species must have began as planet dwellers. (Some SF writers have explored the possibility of life evolving in more exotic locales, including the surface of a neutron star and a gas ring around a neutron star.183 Although these fictional accounts are often surprisingly plausible, it remains far easier to imagine such possibilities than it is to demonstrate convincingly and in detail how complex life could originate and evolve anywhere other than on a planet.) When Sagan arrived at his figure of 1 million ETCs in the Galaxy, he assumed there might be as many as 10 planets per star. But perhaps planetary systems are rare, and thefp term in the Drake equation is small. If fp were small enough, this alone could explain the Fermi paradox.

Not so long ago, astronomers were still not certain how planets formed. There were two competing scenarios. In the first, a planetary system like ours was pictured as forming in a catastrophic event. In the second, planetary systems were thought to condense out of nebulae.184

The nebular hypothesis feels like the most "natural" explanation, but it seems to possess a fatal flaw. If the Sun, for example, formed from the collapse of a rotating cloud of dust and gas, then calculations show that it should now rotate extremely quickly. The Sun should contain most of the angular momentum of the Solar System. And yet it does not. In fact, the Sun rotates rather sedately — its equatorial regions rotate once in about

24 days, while its polar regions rotate once in about 30 days. This observation led many astronomers to prefer models of planetary formation based on catastrophic events. The most popular model had a star almost colliding with the Sun; tidal effects pulled a gaseous filament from the Sun, and the filament later broke up and condensed to form the planets.185

If planets really did form in stellar collisions, then the outlook for finding ETCs would be bleak. The density of stars in space is quite low, so collisions would be infrequent; one early estimate put the number of planetary systems formed in this way at just ten per galaxy! In a lecture in 1923, James Jeans said: "Astronomy does not know whether or not life is important in the scheme of things, but she begins to whisper that life must necessarily be somewhat rare." Jeans clearly thought he knew the resolution of the paradox, and the paradox had not yet been formulated.

However, the nebular hypothesis never went away. Theories of planetary formation based upon collisions also possessed problems. The collision theory could not explain many observed properties of our Solar System. Furthermore, the major difficulty with the nebular hypothesis — namely, explaining how the bulk of the angular momentum of the Solar System resides in the planets — was eventually resolved. It happens that the young Sun did rotate at high speed, but the rotation generated a strong magnetic field. Magnetic lines of force stuck out into the solar nebula, like spokes from a hub, and dragged the gas around with it. This "magnetic braking" effect slowed the Sun, and transferred angular momentum to the gaseous disk. Astronomers observe direct evidence for this: young stars rotate up to 100 times as quickly as our Sun, whereas old stars rotate more sedately. Few astronomers now doubt that the planets in our Solar System formed when small planetesimals condensed out of a disk-shaped cloud of dust and gas; in gentle collisions, these planetesimals stuck together and gradually formed the planets we see today. If this theory is correct, then the same process should occur around other stars. Planets should be common, as Sagan believed.

Astronomers have even photographed protoplanetary disks, which has lent credence to their theory of planetary formation. But it is one thing to photograph a disk of gas that may one day become a planetary system; it is quite another to photograph a planet.

It is not feasible, at least at present, to see planets around distant stars. Planets shine only by reflected light, so taking a photograph of an extrasolar planet is rather like trying to observe the light of a firefly next to a thermonuclear explosion. Recent advances in observational astronomy, however, have made it possible to infer the existence of planets around other stars by the gravitational pull they exert on their parent stars. The gravitational attraction of a large planet on a star causes the path of the star figure 45 A protoplanetary disk.

to "wobble." By measuring the wobble, astronomers can not only determine the mass of the planet but also its distance from the star. The first planet detected by this technique was only found in the mid-1990s; but the technique is so successful that there are already more than 60 known extrasolar planets (the precise number depends on how you choose to define a planet), and more are being found each month.186

Clearly, then, it is simply wrong to attempt to explain the Fermi paradox by stating that the number of stars with planetary systems — and thus the total number of planets — is small. We now know of too many planetary systems to accept this argument.

And yet... so far astronomers have found only giant planets — planets with a mass similar to that of Jupiter. This is not surprising: using the technique described above, astronomers can only find giant planets. But of the stars tested to date, less than 10% of them have detectable planets. This could be because detectable Jupiter-sized planets are relatively rare — but it could mean planets in general are quite rare; certainly, not every star has a planetary system. Furthermore, as we shall discuss in later sections, the Jupiter-sized planets found to date tend to be either extremely close to their sun or, if they orbit at larger distances, they have extremely elliptical orbits. In either case there is little chance of a habitable Earth-like planet existing in these systems. A "Jupiter" close to its star will destroy rocky Earth-like planets, while a "Jupiter" in an elliptical orbit will disrupt the orbits of smaller planets, either casting them out into space or throwing them into the central star.

Personally, I believe thefp term in the Drake equation will turn out to be smaller than the early optimists believed — but by itself it will still be far too high to permit a resolution of the Fermi paradox. Fortunately, this will soon cease to be a matter of belief; rapid advances in observational astronomy mean that within a few years we should have a clear understanding of the number and type of extrasolar planetary systems.

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