Exoplanets and the limits of human imagination

circumbinary system

Our solar system is in no way typical of the diversity out there. The number of known planets orbiting other stars now stands at more than 200. The Kepler satellite mission has identified some 3,500 planet candidates awaiting confirmation. A lot of these exoplanets are proving to be weirdly unlike any in our solar system. The extreme and unexpected variety of planets is forcing researchers to rethink and expand ideas about the conditions favorable to life on other worlds.

Some planets orbit two stars locked in a binary embrace. Some are gas giants similar to Jupiter except they’ve ballooned in size beyond what anyone expected from models of planet formation based on our solar system. At least one star has five planets swinging around it in an orbit closer than Mercury’s, our sun’s nearest planet. There are rocky worlds out there with surfaces roasting at over 3,100º F, hot enough to liquify a planet’s rocky crust.

Exoplanet HD149026b has a mass 20 percent greater than Saturn but its radius is 22 percent smaller, which implies that it contains more heavy elements than can be found on all eight of the full-fledged planets circling our sun, according to David S. Spiegel and co-authors in a special section on exoplanets published in the journal PNAS.

Around distant stars, the most abundant planets are smallish: in the range of 1 to 4 times the diameter of Earth. That’s surprising because our solar system has no planets that are larger than Earth but smaller than Neptune (with a diameter 3.9 times larger than Earth). That means there is no local model to study and compare with this bounty of newly discovered planets.

“A picture is forming in which small planets abound and close-in giants are few, in which the [habitable zones] of cool stars are heavily populated with terrestrial planets and the diversity of systems challenges preconceived ideas,” says Natalie M. Batalha, in her review of Kepler mission findings. The data from Kepler suggest that every late-type, main-sequence star has at least one planet, Batalha says, and that planets potentially hospitable to life seem fairly common.

By one estimate, one in five Sun-like stars should host an Earth-sized exoplanet in the star’s habitable zone. About 11 percent of Sun-like stars have planets that are equal or double the size of Earth and bathed in stellar light roughly comparable in energy to that reaching Earth. “If life is prevalent in our neighborhood of the Galaxy, it is within our reach to be the first generation in human history to finally cross this threshold and learn if there is life of any kind beyond Earth,” says Sara Seager.

But Geoffrey Marcy and co-authors say current speculations about habitable zones are severely limited by lack of evidence and little understanding of what alien life might require:

We have no evidence of microbial life at any orbital location within our solar system beside the Earth. We have no empirical information about microbial life as a function of orbital distance from our Sun or from any other star. We also have no evidence of multicellular life around any other star, nor evidence of intelligent life. Thus, we have no empirical knowledge about the actual domain of habitable zones, for any type of life, around any type of star. Moreover we have virtually no theoretical underpinnings about exobiology. We still do not know how biology started on Earth. We do not know the mechanisms that caused a transition from chemistry to biology, nor do we know the biochemical steps that spawn proteins, RNA, DNA, or cell membranes… Indeed, we still have a poor definition of life.

The figure below compares planets by radius (relative to that of Jupiter) and the rate of energy received from the star they orbit, often 10,000 times greater than the solar energy reaching Earth.

Planets by solar flux
Source: David S. Spiegel, Jonathan J. Fortney, and Christophe Sotin, PNAS (2014)
  • Colored circles = confirmed exoplanets
  • Small white circles = candidate planets identified by the Kepler mission (KOI)
  • Red stars = the eight planets of our solar system

The next figure compares planets by radius and mass, which ranges from less than Earth’s to more than 10 times that of Jupiter.

Planets by mass
Source: David S. Spiegel, Jonathan J. Fortney, and Christophe Sotin, PNAS (2014)


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