IN THE search for habitable planets, we thought we knew what to look for: foreign solar systems resembling our own. We were wrong.
Our home system, it turns out, is a place of exceptional order, with its neat arrangement of small rocky worlds close to the sun, and far-flung gas giants. When we stare beyond its confines, we come across nothing else quite like it. Instead, we consistently find systems where gas giants and rocky planets are mixed together higgledy-piggledy. If we assume that such an arrangement is the upshot of a long history of instability, it severely limits the time that life would have had to get started in most of the cosmos.
But we are now beginning to question a number of long-held assumptions about what makes a planetary system amenable to life. In particular, the role of gas giants like our very own Jupiter are under scrutiny. Maybe life can find a foothold in systems where these giants are not quite so placid after all.
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Jupiter, a vast gas ball less than a light hour from us, dominates the planets of our solar system with its sheer bulk: its mass is more than double that of all the other planets combined. We鈥檝e long thought its whopping gravitational influence was a significant factor in enabling life to get started on Earth. Shortly after the planets formed, so the traditional story goes, Jupiter began tossing comets out into distant orbits 鈥� meaning they would only occasionally return to menace Earth. Now there is an alternative view, backed by : that Jupiter fired comets into the inner solar system, where they delivered essential starter chemicals for life by crash-landing on Earth. Either way, Jupiter鈥檚 presence seems to have been key to the habitability of our planet (see 鈥�Help from Jupiter鈥�).
Our standard picture of how solar systems form had convinced us that we should find enormous gas planets in roughly the same position as Jupiter everywhere we look. As a primeval cloud of gas and dust contracts to give birth to a star, the remaining material forms rocky planets in tight orbits. Gas giants form further out 鈥� where things get chilly enough for volatile compounds such as water, carbon monoxide, carbon dioxide and ammonia to condense into ices. This provides more bulk than simple rocks and metals, so the outer planets naturally grow large.
鈥淲hy didn鈥檛 the gas behemoths swallow all the rocky planets as they glided inwards?鈥�
Unfortunately, nature hadn鈥檛 read the script. In 1995, Michel Mayor and , then at the Geneva Observatory, Switzerland, discovered 51 Pegasi b, a gas giant circling a sunlike star. Its orbit was not the gigantic one of Jupiter, but a mere 8 million kilometres in diameter, taking it once around its star every 4.2 Earth days.
It was the first example of what we now call a hot Jupiter, and the more we looked, the more of them we found. No one could explain their existence until computer models showed that under certain circumstances an ageing gas giant could spiral inwards towards its star.
But as one problem dissolved, another appeared. If these gas behemoths had glided inwards, wouldn鈥檛 they have knocked the rocky planets in the way into their star, or else ejected them from the system altogether? That conclusion seemed hard to avoid, and made planet-hunters wring their hands in despair of habitable worlds. In 2012, of the Fermilab Center for Particle Astrophysics, Chicago, seemed to have the confirmation when he looked at 63 hot Jupiter systems and . Other discoveries over the last few years added to the pessimism (see 鈥�Beware, superflares鈥�).
That conclusion may have been premature, however. When astronomers switched to looking at systems with gas giants in slightly larger orbits or with slightly less mass, there was evidence of other planets. In 2015, . It contains a hot Jupiter, a rocky planet 1.8 times the diameter of Earth and a Neptune sized-planet all in tight orbits lasting less than 9 days. The rocky world would be far too hot to host life, but the discovery swung the mood back towards optimism. Maybe rocky planets could exist happily next to gas giants after all.
How, though, are rocky planets able to survive the attentions of nearby hot Jupiters? Clearly, the computer models were lacking something. 鈥淭he trouble is that we do not yet have a coherent view of planetary formation,鈥� says of University College London.
Indeed we don鈥檛. Astronomers think that there are two ways gas giants might be created, depending on the density of the material they form from. In the first scenario, collisions between comet-like and asteroid-like fragments build up what will become the core of the gas giant. Once it reaches three to five times the mass of Earth, it pulls a dense cloak of gases around it, becoming a gas giant. This is known as core accretion, and it鈥檚 a slow process, taking a million years or so to complete.
In the alternative scenario, known as the disc instability model, the planet-forming cloud is so dense that certain areas collapse into an almost fully formed gas giant without a well-defined solid core. This process takes perhaps a thousand years.
The problem is, we can鈥檛 easily tell which way a gas giant formed, not even Jupiter and Saturn. Resolve this quandary, and it would help crack the conundrum of how the rocky planets and gas planets manage to coexist in systems like WASP-47.
We are finally about to have a shot at it. If all goes to plan, NASA鈥檚 Juno mission will arrive at Jupiter on 4 July. 鈥淛uno is all about the formation and evolution of Jupiter. This is the key theme of our mission,鈥� says of Cornell University in Ithaca, New York. The spacecraft will perform a number of close orbits to study the details of Jupiter鈥檚 gravitational field and show whether the planet has a dense core. If it does, it would validate the core accretion model.
But Juno has a second objective that will shed light on gas giants in distant planetary systems too. It will measure how much water there is in Jupiter by peering below the clouds using a microwave-sensing radiometer. With that information, planetary scientists can calculate its oxygen abundance and compare this with its carbon content, measured in 2003 when NASA鈥檚 Gallileo probe made a kamikaze dive into Jupiter鈥檚 atmosphere. Lunine calls these measurements the 鈥渒ey ingredient鈥� in understanding where the planet formed: computer models indicate that the abundance of oxygen and carbon is set by the distance from its star at which the planet formed.
In the case of Jupiter, most astronomers believe that although it may have wandered a little as it formed, it has ended up pretty much back where it began. Once we know the water content of Jupiter, we can treat that value as being characteristic of a gas giant that formed roughly where Jupiter did. We can then measure the water in hot Jupiters, by sensing the light that passes through the planets鈥� clouds and seeing how much of the characteristic wavelengths absorbed by water are missing. Compare those measurements with those of Jupiter, and we鈥檒l know where the foreign gas giants must have formed. 鈥淛uno鈥檚 measurements will be a nice ground truth to compare to exoplanets,鈥� says at the University of Exeter, UK. 鈥淲e can use this to start to see the variety of where the exoplanets formed.鈥�
Sing has already made a start. Together with colleagues, he targeted 10 nearby hot Jupiters with NASA鈥檚 Hubble and Spitzer space telescopes and calculated their oxygen concentrations. Armed with data from NASA鈥檚 James Webb space telescope, set to launch in 2018, he will be able to make more accurate measurements. 鈥淭hen we鈥檙e going to really be able to ask those questions about where did the exoplanets form,鈥� says Sing. Observations from the new telescope should let us work out exactly how gas giants formed and migrated, if at all, in different systems. That should help solve the riddle of how rocky planets survived with gas giants apparently pinballing around nearby.
One plausible solution is that gas giants do indeed migrate, with rocky planets forming only after this shuffling has subsided. There are already hints that this is possible. Theorist Alessandro Morbidelli at the C么te d鈥橝zur Observatory, France, helped develop the so-called Nice model of planetary formation, which involves gas giant migrations. Recent improvements he has made suggest that a sufficiently dense planet-forming disc could survive the passage of a migrating planet. Enough stuff would be left behind for rocky planets to form in its wake (see 鈥�Friendly giants鈥�).
One thing is already obvious. 鈥淢igration is a dramatic thing,鈥� says Tinetti. 鈥淏ut a migrating planet no longer seems to be a showstopper for having other planets in the system.鈥�
That can only be good news for those hunting habitable planets. It means we no longer need look for systems that match our own. Nature it seems, is far more egalitarian than that. Even in the most unusual cosmic neighbourhoods, there is potential for life.
Beware, superflares
THERE are signs that all sorts of planetary systems can be just as amenable to life as our own (see main story). But that life could still be snuffed out at any moment.
We have known that enormous ejections of material from stars are possible since at least the 1980s, but only recently have we begun to observe stars continuously and see how frequent they are. In 2012, Hiroyuki Maehara at Kyoto University in Japan looked at 120 days of observations of 83,000 sunlike stars and saw 148 of them produce whopping flares.
Superflares can be thousands of times more powerful than any flare our sun has produced in a century. The ensuing storm of radiation would be enough to fry our satellites and melt nuclear power stations.
If a superflare hit a habitable planet, it could eviscerate its atmosphere, allowing ultraviolet rays to rapidly scorch life from the surface.
This article appeared in print under the headline 鈥淓lephant in the room鈥�
