Solution Jupiters Are Rare

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What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?

Richard Philips Feynman, The Feynman Lectures on Physics

Since the first discovery in 1995 of extrasolar planets, or exoplanets, astronomers have found 60+ more planets beyond our Solar System. Many of these are Jupiter-sized objects orbiting in nearly circular orbits close to the parent star. (Consider the planet orbiting Rho CrB, for example. Of all the exoplanets yet discovered it is closest in mass to Jupiter, being only 1% less massive than Jupiter. However, whereas Jupiter orbits the Sun at about 5.2 AU (an astronomical unit being the Earth-Sun distance, which is a convenient distance measure for planetary systems), the massive planet around Rho CrB has a nearly circular orbit at just 0.224 AU. This means it is far closer to its star than Mercury is to our Sun; Mercury orbits at 0.387 AU. It is not surprising that massive planets orbiting close to a star should have circular orbits: tidal forces from gravitational interaction with the star will cause the orbit to become circular even if the orbit began as an ellipse. Nor is it surprising that astronomers can detect large planets orbiting close to a star: our present techniques for detecting planets work best on precisely such objects. The surprise is that so many Jupiter-sized planets exist in orbits so close to a star. These planets should not exist at all!

Our theories of planetary formation imply that gas-giant planets like Jupiter cannot form within 3 AU of a star like our Sun. This limit is called the snow line. So what are these so-called "hot Jupiters" doing so far within the snow line? With some detective work we can rule out one possibility; namely, that these are not actually gas giants. The Doppler motions that enable astronomers to infer the existence of the planets also give us enough information to deduce their masses; and in a few cases, measurements of the parent star during transits enable us to estimate the diameters of the planets. These two pieces of information directly give us the planets' densities — and they are certainly gas giants. A second possibility — namely, that our models of planetary formation are wrong — cannot be dismissed. However, there is a lot of evidence to support the models, and there is nothing to replace them; so astronomers are loath to accept this possibility. Which leaves a third possibility: the planets formed outside the snow line and later migrated to their present positions close to their parent stars.

The orbital decay of Jupiter-type planets cannot happen once a planetary system is established, so we need not worry about a Jovian threat in our own Solar System. But the decay can happen early in the development of a planetary system. If a gas giant migrates from outside the snow line to an orbit close to the star, then the outlook for any inner terrestrial planets is bleak. Simulations show that smaller planets are forced into the star, or ejected from the planetary system altogether. Stars with "hot Jupiters" are unlikely to possess viable planets.

Not all exoplanets are hot Jupiters. Some of them are outside the snow line, where we would expect them to be. An example is the planet around Epsilon Eridani. (This is one of the closest Sun-like stars, and one that Frank Drake observed when he carried out the first search for extraterrestrial signals.) The planet, designated Epsilon Eridani b, orbits at 3.36 AU and is 0.88 times as massive as Jupiter. The problem with objects like these is their large orbital eccentricity. For example, the eccentricity of Epsilon Eridani b is 0.6 (compared with 0.048 for Jupiter). In other words, our Jupiter has an almost circular orbit, whereas Epsilon Eridani b orbits in an ellipse. In fact, the average eccentricity of the exoplanets discovered to date is 0.28 (with the eccentricities ranging from 0, for the hot Jupiters in perfectly circular orbits, through to 0.93, for a planet around the star HD80606). Compare this with the average eccentricity of planets in the Solar System: 0.08 (or 0.06 if we discount Pluto). Our Jupiter has a stable, nearly circular orbit — and it permits Earth to have a stable, nearly circular orbit too. If Jupiter were in a highly eccentric orbit, which seems the norm for a large-mass object orbiting more than about 0.2 AU from its star, then Earth might not exist.

figure 47 A comparison of the orbits of Jupiter and Epsilon Eridani b, drawn to the same scale. (Jupiter orbits the Sun with a semi-major axis of 5.2 AU; Epsilon Eridani b orbits its star with a semi-major axis of 3.36 AU.) Jupiter's orbital eccentricity is 0.048, although at this scale it appears to be circular. The orbital eccentricity of the planet orbiting Epsilon Eridani is 0.6 — which is noticeably elliptical.

So had our Solar System contained either a "hot Jupiter" or an "eccentric Jupiter," the chances are high that Earth could not have sustained life for nearly 4 billion years. Earth's orbit would have been altered catastroph-ically. It is worth stressing, once again, that our observations are significantly biased. The Doppler techniques we use to discover other planetary systems are most effective precisely at finding (i) large-mass planets orbiting very close to the parent star and (ii) large-mass planets with highly elliptical orbits. Those objects provide the largest effects for our Doppler techniques to work on. A Jupiter-mass planet in a circular orbit at 5 AU from the parent star will — for the moment — be undetectable. We cannot yet deduce from these statistics that "good Jupiters" are rare. On the other hand, it is possible we were lucky; we found ourselves with a "good Jupiter" — one possessing a stable, circular orbit. Perhaps most planetary systems are not so lucky; perhaps "bad" Jupiters are the norm?

What about planetary systems with no Jupiter — neither good nor bad — at all? It is not clear whether planetary systems can form without also forming massive gas giants like Jupiter. Even if such systems can form, they may be no more conducive to life than are systems containing a "bad Jupiter." Our Jupiter has played two roles vital to life on Earth: that of deflector and that of water provider.

In its first role, Jupiter's great mass causes stray objects in elliptical orbits, which might otherwise hit Earth, either to be ejected from the Solar System or to have their orbits made circular and therefore less dangerous. And if neither of these things happens, Jupiter itself is the biggest target for rogue objects. In 1994, for example, comet Shoemaker-Levy 9 hit Jupiter; had it hit Earth, life on our planet would now be rather different.

In its second role, which it fulfilled early in the history of the Solar System, Jupiter caused asteroids to accrete into Mars-sized planetary embryos

figure 48 In 1994, comet Shoemaker-Levy 9 hit Jupiter. Had it hit Earth, the devastation would have been immense.

with unstable elliptical orbits. Solar System objects in elliptical orbits are more likely to collide with objects in circular orbits; and some of the proto-planets collided with Earth. If such collisions were to happen now, the results would be cataclysmic. Back then, though, the results proved ultimately beneficial. The Moon may have been the result of one such collision, and our oceans may have been the result of other collisions. If recent work suggesting Earth's oceans came from asteroids is correct, then it implies that, without a Jupiter at the right distance to toss water-bearing asteroids our way, Earth might now be a desert.196

Computer simulations indicate that a Jupiter-mass planet forming in the very distant regions of a planetary system allows an Earth-mass planet to form with plenty of water — but only at 4 or 5 AU, which is far outside the habitable zone. So it seems that a planetary system needs not only a "good Jupiter," but one at exactly the right distance, otherwise the system's water is either trapped in an asteroid belt or is frozen on terrestrial planets.

And as far as we know, if a planet has no liquid water, then it has no life.

So does the existence of Jupiter, our "Big Brother," explain the Fermi paradox? As an explanation on its own, I doubt it — though of course it may yet be another factor causing life to be rare. My guess is that, as more data arrive, we will discover many planetary systems with "good Jupiters." And even if "good Jupiters" are rare, surely it is stretching matters to go from saying Jupiter played a beneficial role in the development of the Solar System to saying a Jupiter-sized planet at about 5 AU is essential for life to exist on a terrestrial planet. Perhaps other arrangements of objects in a planetary system may lead to habitable zones. Our failure to discover these arrangements may simply be a failure of our imagination.

On the other hand, we see several pleasant coincidences in our Solar System — and Jupiter plays a part in most of them. Perhaps we have to thank Jove for a lot of things! The next section describes another reason why advanced life on Earth might not have developed without Jupiter.

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