Modeling Other Earths
1 Feb 2007
(Source: Astrobiology Magazine)
More than 200 extrasolar planets have been found so far, but our knowledge about those distant worlds is very limited. In most cases we haven?t seen the planets; we only know they exist because of the effect they have on the star they orbit.
But we do know that most of those planets are gas giants like Jupiter. Such massive worlds exert a much greater and therefore more detectable effect on a star than a tiny planet like Earth does. Yet the dream of astrobiologists is to find many rocky Earth-like planets orbiting distant stars.
According to models created by Barrie Jones, Emeritus Professor of Astronomy at The Open University in the UK, the galaxy should be well populated with such Earth-like worlds. In this interview with Astrobiology Magazine, he explains what factors affect the formation of Earth-like worlds, and the development of life as we know it, in other solar systems.
Astrobiology Magazine (AM): Not much is known about the conditions for planets and the possibility for life in other solar systems. Can you speak about your work modeling extrasolar planetary systems?
Barrie Jones (BJ) : Extrasolar planets are mainly detected by the effect they have on their star. As a planet goes around its orbit, the star responds by moving around a little, and we can detect the star?s motion. But for very low mass planets like the Earth, at this stage we just cannot detect the very tiny motion they induce in their star. So people like me who make computer models are free to indulge our imaginations when modeling extrasolar planetary systems.
I came to this work shortly after the first extrasolar planetary system was discovered in 1995. The problem I decided to tackle was related to the habitable zone around a star, which is the range of distances from a star within which water on the surface of an Earth-like planet is liquid. If the planet is too close to the star the water is vaporized, and if it is too far away the water is solid ice. So the habitable zone is where conditions are just right for water to be liquid, which is why it?s sometimes called the Goldilocks zone.
I wondered, if we put an Earth in the habitable zone of another planetary system, could it survive there? Or are there giant planets too close to the habitable zone, so that an Earth-sized planet would be thrown out into interstellar space by the gravity of the giants? In our own solar system this hasn't happened, of course, because Jupiter is parked safely away from our habitable zone. But with other planetary systems, we cannot be certain.
To my surprise, very little work had been done on this question, even though it seemed like a very obvious question to ask. And people had asked it, but no one had done much computer modeling on it. So, with two research students, I've been modeling extrasolar planetary systems to identify which of them have habitable zones that could harbor Earths. The result on the planetary systems discovered to date is that in half of them you could have an Earth in a stable orbit in the habitable zone. That?s quite an encouraging result.
But for the time being, I?m setting that work aside. I?ll come back to it in a few years time when there are many more systems known, using a rapid way of evaluating the habitability of habitable zones that I developed.
AM: Will that be known as the Barrie Jones equation?
BJ: (laughs) It?s certainly the work I?m known for. It?s an algorithm but it?s never been named. With this fast method, we can work through a lot of systems.
AM: Most of the planets found so far are gas giants like Jupiter, which is 318 times the mass of the Earth. Haven't there been recent detections of less massive planets though?
BJ: Yes, the smallest is about five times the mass of the Earth. It was not discovered by detecting the motion of the star, but how the planet affected the apparent brightness of a background star ? a technique called gravitational microlensing. Still, detection of a star?s motion has yielded almost all the extrasolar planet discoveries, and so far the minimum mass is about seven times the mass of the Earth. At present, the smaller mass planets are being detected around M dwarfs. These stars have less mass than the Sun, and therefore move more than a solar-mass star would. For solar-mass stars, it?s going to take a few years before we can detect an Earth-mass planet.
AM:You mean until we get better technology for detection, such as the forthcoming Darwin mission? Can you tell me about your role in that mission?
BJ: My role in the Darwin mission is going to be to analyze all of the extrasolar planetary systems known at that time, and come up with a list prioritizing which ones are more likely to have Earths in their habitable zones. It's a target-selection process, a bit like what Jill Tarter and the SETI Institute have done in looking for planets that might support intelligence.
By the time we come up with a list, we should have the instrumental capability with Darwin to detect whether Earths are there or not. But which stars do you look at to see if you can find an Earth? You can't search everything, because it will take a long time to observe any particular planetary system for long enough to see if an Earth is there. If we can show that for some systems there's no chance or little chance of having an Earth in the habitable zone, but in other systems there's a much better chance, then that would be an obvious way to focus your efforts.
AM: Can you talk more about the role of gas giant planets in allowing Earth-sized planets to exist in a habitable zone?
BJ: In our solar system, Jupiter is well beyond the habitable zone. But in other solar systems gas giants are much closer to the habitable zone, and they can cause problems because of their strong gravity.
Many other planetary systems have giant planets closer to the star than the habitable zone. There's no way those giant planets could have formed so close to their star, because this region would have been too hot during the planetary formation phase. On the basis of models, we understand that these giants must have formed farther out and then migrated inwards, traversing the habitable zone, to end up parked quite close to the star.
Planetesimals in the habitable zone clump together to form rocky planets like the Earth, but giant planets form more quickly than the rocky planets. If gas giants migrate through the habitable zone, they scatter the planetesimals around and even eat some of the material. So the conventional wisdom, before anyone looked at this in detail, was that there was no way rocky planets could form after such an event, because there wouldn't be enough material left over. But recent models have shown that even if you have giant planets crashing through the rocky-planet-forming region in the habitable zone, you can still form Earths afterwards.
If Earths can form after the giants migrate toward the star and become ?Hot Jupiters,? then about 50 percent of the planetary systems could have Earths in the habitable zone. These Earths wouldn't have been ejected subsequently by the giant?s gravity. However, if we rule out the possibility of forming Earths after migration of the giants across the habitable zones, then we end up with 16 or 17 percent. There's a huge drop because so many of the systems discovered to date have Hot Jupiters! So understanding whether terrestrial planets can form after migration is important.
Therefore, the next phase of our work will address whether such Earths can form, and we will collaborate with Queen Mary University in London to explore this possibility as thoroughly as we can.
AM: Are you also involved in modeling planetary atmospheres?
BJ: No, but there is some work I'm planning to do that is relevant to atmospheres. I want to see to what extent giant planets shield atmospheres from excessive bombardment from space.
In our solar system, Jupiter in particular is thought to shield us from excessive bombardment. Jupiter does this because you've got comets coming in from the outer solar system. They get caught in Jupiter?s gravity, and there?s a tendency for them to be swung around and cast out into the depths of space rather than come repeatedly through the inner solar system.
In order to model the solar system, we?ll need to have thousands of cometary bodies peppering the inner part of the system, and then follow the fate of these bodies for millions of years of simulated time. The problem is it takes computers a long time to do these numeric integrations. If we could get the computing time assembled we could do a reliable job. The problem with less computing power is that you have to manage with a small population of bodies and you can?t be quite certain if the statistics are good.
But I think we can model the solar system. Then we?ll want to fiddle with it because there are lots of other systems out there. So we can use our model to figure out what happens if we change the mass of Jupiter, or change its position. We can see how the shielding changes for different types of planetary systems that are out there.
AM: What do you think of the idea that Jupiter?s shielding has allowed complex life to develop on Earth?
BJ: People often assert that Jupiter has shielded the Earth from excessive bombardment, by comets particularly, to the extent that it's been possible to develop an advanced biosphere. I don't believe that such shielding has been necessary.
I believe that life is so adaptable that you can have collisions much more often than the Earth has experienced and the biosphere will survive. It'll be a different biosphere from the one we've got, because extinctions like the dinosaurs -- which an impact probably caused or had a big role in -- are going to be much more frequent, and so there would be frustrated evolution. You've got big creatures on land that get wiped out often, so they can't evolve. But the oceans are much more protected from the effect of impacts. So you could have simple life on the surface of the Earth and more complex life underneath the surface and in the oceans. So I'm an optimist when it comes to the survivability of life.
There are some scientific grounds to this. Every conceivable habitat on Earth is occupied: hot, cold, acidic, alkaline, high pressure, and so on. There?s even a bacterium called Deinococcus Radiodurans that likes living in a high-radiation environment. Multi-cellular organisms like us don?t necessarily live in extreme environments, but bacteria have no problem. They will inherit the Earth if the worst happens.
But even for larger creatures at the surface, I'm optimistic that some forms would survive more frequent impacts. They may not be as big as us, but they could survive in underground habitats that protect them from the worst climate changes. If you get a giant impact the main problem is climate change. In the case of the dinosaurs, it's thought that there was something like a nuclear winter. There was so much dust in the atmosphere that there were decades when the Earth was quite cold. But small creatures survived. If a giant impact had happened again a few million years later, they likely would have come through it again.