Planets--Good News, Bad News

One fascinating new development in exoplanet research is that we can now create complex models of planetary system evolution using supercomputers. Edward Thommes, Soko Matsumura and Frederic Rasio published their findings, "Gas Disks to Gas Giants: Simulating the Birth of Planetary Systems," in the August 8, issue of Science magazine. The computer simulations were done using a cluster of supercomputers operated by Northwestern's Theoretical Astrophysics Group. Some of what they discovered is a little surprising.

We think we have a pretty good handle on how planetary systems develop, and we have assumed all along that many of them would resemble the solar system. But, with the list of known extra-solar planets now approaching 300, we haven't really found systems like ours. Instead, we've discovered some surprising forms of planets orbiting stars in ways we didn't predict, like 'hot Jupiters', gas giants orbiting in close proximity to their stars.

Is what we are finding what is really out there, or is it merely a selection effect caused by the technology and methods we use to detect extra-solar planets? These are some of the questions the computer simulations might help answer.

"We now know that these other planetary systems don't look like the solar system at all," said Frederic A. Rasio, senior author of the Science paper. "We now better understand the process of planet formation and can explain the properties of the strange exoplanets we've observed. We also know that the solar system is special and understand at some level what makes it special."

What they found is that our solar system represents the rare cases, where gas giants form, but do not migrate to the inner planetary system, and the final orbits of the planets in the system are fairly circular and stable. In many simulations, lots of gas giants formed in chaotic environments with collisions and eccentric orbits. In other simulations, plenty of smaller rocky planets formed, but hardly any gas giants materialized out of the proto-planetary disk. Only under specific, unique conditions do planetary systems like ours evolve.

Image copyright Edward Thommes, 2008, used with permission.

Apparently, earth-like planets are fairly common, or as Edward Thommes says in the NSF interview, "they're almost like weeds, they'll sprout up under almost any conditions." It's our system's gas giants, Jupiter and Saturn, that seem to be the exception to the rule. Most are not gentle giants, peacefully lumbering around their stars in distant, decades long orbits. Their research suggests that as we look for more planets farther away from their stars we are likely to find Jupiter and Saturn type giants, but these systems may also contain gas giants orbiting as close as Mercury orbits our sun.

It's been argued that having a large planet outside the orbit of Earth has protected us from comets and asteroids hurtling toward the inner solar system from the depths of space. Does having a big brother gas giant orbiting further out help protect smaller planets from bombardment by comets and other bodies, enhancing the chances for life to take hold and thrive? Maybe so, but as Ed points out, "it could also be argued that Jupiter put those comets there (in the Kuiper belt) in the first place."

This leads to some other interesting questions. What happens to the rocky planets in the habitable zone as gas giants migrate in towards their star? Can life exist in systems that only contain small rocky planets and no gas giants at all?

Understanding the connection between the make-up of the planetary system and the habitability of the planets in it is still a ways off. But as computing power increases we will be able to model more of the weird systems we may encounter in our search. As this study shows, our system may actually be the 'weird' one in the bunch.

The good news is, there are plenty of systems out there and we are finding them. The bad news is, if we were expecting to find more solar type planetary systems, earth-like environments, and ultimately extra-terrestrial life, the odds don't look as good today as they did before.