There is ongoing theoretical debate regarding how and where to spot other worlds circling distant stars. And there are new ground and in-space observational tools that are locking into real-time data.
Presently, 200-plus known extrasolar planets have been found — mostly huge gas giants like Jupiter within our own solar system of Sun-orbiting planets.
Given these discoveries — just within the last 10 years or so — under what conditions can we expect terrestrial planets to crop up? Moreover, just how common are habitable planets in the universe?
Planet scouting scientists met here January 26-28 at a media workshop sponsored by the University of Colorado's Center for Astrobiology to share theories as well as new observational information.
While the planetary plotting thickens, it's also a stew of opposite conclusions, assumptions, talk of new or weird physics, along with downright uncertainties.
It all adds up to a collegial clash between those that predict contrasted to observational findings.
Debate revived and revitalized
What's now taking place is that extrasolar planet researchers are shifting into high gear given ground and space-based tools. That being the case, will theories on spotting Earth-like worlds be overtaken by actual observation?
"Absolutely," responded Alan Boss, a research staff member at the Carnegie Institution's Department of Terrestrial Magnetism in Washington, D.C.
"Theorists spent several decades debating the formation of our solar system, where the basic physical characteristics had been known for centuries ... number of planets, masses, separations, etc. That debate has now been revived and revitalized in the last decade by the ever-increasing information we are learning from extrasolar planetary systems," Boss explained to SPACE.com.
Boss said that the focus of the debate right now is on giant planets — because those are the ones that have been found to date in greatest abundance.
Theorists are parasites
But last December's liftoff of Europe's COnvection, ROtation and planetary Transits (COROT) telescope — along with next year's slated takeoff of NASA's Kepler mission — signal near future discoveries of hot and warm super-Earths and habitable Earths, Boss suggested.
"We can expect an equally contentious debate over how to explain their formation and orbital evolution, though perhaps a debate that is not quite so contentious as the current debate over giant planet formation, given the two wildly different formation mechanisms being considered for them," Boss said.
A theory that explains the previously known might be plausible, Boss continued, but unless it can predict the unknown, it is of little value — and even then may eventually be proven incorrect by further observations.
"Theorists truly are parasites ... and derive their sustenance from the growing body of observational evidence about other planetary systems," Boss concluded.
"The standard theorist line is never believe in an observation unless it has been confirmed theoretically," said Jack Lissauer, a space scientist in the Planetary Systems Branch at NASA's Ames Research Center near Silicon Valley, California. "I'm trained as a theorist ... but I'm interested in observations."
Lissauer is co-discoverer of the first four planets known to orbit about faint M dwarf stars. He also co-discovered two faint outer rings and two small inner moons of the planet Uranus.
To correspond to the real universe, researchers must be constrained by observations, Lissauer pointed out.
But in order to guide observations and have some idea of where to look, as well as how best to design planet-hunting instruments, that's where theorists come in.
"It's synergistic," Lissauer said. "You have to have both to be able to understand your observations theoretically and confirm your theories through observation."
"Both theory and observations are key," concurred Sean Raymond, a NASA postdoctoral researcher here at the University of Colorado's Center for Astrobiology and Center for Astronomy and Space Astrophysics.
"Observations make discoveries, but theory is needed to interpret them," Raymond said.
For instance, "hot Jupiters" were discovered starting in 1995, Raymond emphasized.
Most of the exoplanets identified to date are these gas giants in a stable orbit very close to their parent star.
"It was certainly strange to have giant planets so close to their stars," Raymond told SPACE.com.
Theory is what probed the origin of hot Jupiters, he said, and came up with the idea of planet migration, the current model being that these planets form father out and migrate in to their current locations.
Cosmic cooking class
Consider it a lesson bubbling out of cosmic cooking class 101.
"It's like having dinner at someone's house. Observations can tell you the specific ingredients that went into the meal. But theory can figure out exactly how the pieces go together ... how the dish was cooked," Raymond explained.
Raymond projected outward over the next decade. He foresees the finding of Earth analogs.
"We're going to be seeing all these little small guys. That's going to be pretty cool," Raymond said.
Furthermore, more and more of them will be found in unexpected and varied environments, and around different types of stars once thought as impossible locales.
"Anytime anybody predicts, 'ooh, this could never happen' ... then you'll find somebody to disprove it," Raymond advised.
Light of a living planet
Margaret Turnbull, an astronomer at the Space Telescope Science Institute in Baltimore, is on a pursuit to look for life elsewhere.
She has created a catalog of nearby stars — dubbed "HabCat" — that could host life forms similar to those on Earth. This catalog is now the target list for the search for extraterrestrial intelligence.
At the Space Telescope Science Institute, Turnbull is also the principle investigator of a proposed mission to deploy a small telescope on the Moon.
That instrument would be designed to look back at the Earth and provide insight into how the light of a living planet changes as continents and oceans pass in and out of view, as weather patterns move and as seasons change.
The quest for extrasolar worlds is cutting-edge research and therefore is contentious, Turnbull told SPACE.com.
"It is because you are really operating at the edge of human knowledge and human understanding," Turnbull noted. "Every new observation we make to some extent overthrows what we thought that we knew."
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