NEW YORK -- Though researchers find more and more distant planets revolving around alien suns, the discoveries highlight that Earth and its solar system may be an exceptionally rare place indeed.
That was the consensus here Wednesday evening among five planetary science experts who spoke at the 5th annual Isaac Asimov Memorial Panel Debate held at the American Museum of Natural History.
Neil DeGrasse Tyson, the Frederick P. Rose Director of the Hayden Planetarium, moderated the informal discussion. At issue was whether our solar system is special, why it looks the way it does, and how others thus far detected differ. The debate took place between theoretical and observational scientists on the different aspects of detecting and categorizing alien solar systems. About 700 people attended the event.
Prior to the discovery of planets around stars other than our sun in the 1990's, scientists thought that alien solar systems must look something like our own. They presumed that just like our solar system, there would be small rocky planets like as Earth close to their host stars and large, low density ones a little farther out. But what they discovered were solar systems unlike ours with big Jupiter-like planets close to their host star.
Of the 150 alien planets found, none of them resemble our own. "So maybe it's not the enigma of other solar systems, it's the enigma of our solar system," Tyson said in opening the debate.
The trouble with understanding planets outside of our solar system is that they are typically hard to see because of their bright host star, explained Paul Butler, co-discoverer of two-thirds of the known extra solar planets. However, even with these constraints, indirect methods allowed scientists to detect planets as massive as 300 times the Earth and ones as small as 15 times the mass of the Earth outside of our solar system,
As it turns out, the mass of a planet is its most important characteristic for comparative astrometry, the measurement of star positions. The mass determines if a planet is a gas giant or a rocky formation. "If it's a rocky planet, like Earth or Mars, then one can focus on its atmosphere and learn more about its characteristics," said Fritz Benedict of the University of Texas.
Typically, the most sought after characteristic of a planet is its habitability. A habitable planet has liquid water on its surface, explained Margaret Turnbull of the Carnegie Institute of Washington. Thus far, 90% of all detected alien planets have host stars that can flare and sterilize the surface of the planet. Furthermore, planets, which are that close to their host star, would be in a synchronous orbit. This means that only one side of the planet would face the host star and all potential water on that side would evaporate and go to its "dark" side.
While theorists such as Peter Goldrich of Caltech and Scott Termain of Princeton University did not predict finding solar systems with Jupiter-like planets so close to their orbit stars, they did theorize their dynamics. As early as the1980's, they showed that planets such as Jupiter could be very mobile, moving rapidly, and changing angle and momentum to switch orbits and migrate closer to their parent stars. "Planetary system is not static, it's continually processing. Orbits jiggle around," said Termain.
At the end, all agreed that there are still discoveries to be made before we can know if our solar system is special or unusual amongst the universe. But speculations varied.
"I have a problem referring to our own solar system as unusual, because we haven't done that experiment yet, we haven't searched for our own solar system yet," said Turnbull Thus far, the kind of data obtained and the type of observations made are tuned to search for Jupiters and not Earths, therefore that's what we find. "The experiments were designed for that," she explained.
But with the vast majority of the alien planets found in eccentric orbits, Butler has a different view. "I think with the data at hand, we can say that our solar system is rare. Eccentricity dominates," said Butler. "It's just a matter of how rare we are," he added.
And Benedict agrees. "The older I get, the less likely it seems to me there'd be a bunch of places like our solar system," he said. Or as Tyson added, "There's no place like home."