A team of researchers from MIT and Aarhus University, Denmark, have discovered that Earth-sized exoplanets orbit their parent stars in the same way that our planet orbits our own Sun – maintaining a roughly equidistant circular orbit. The discovery further narrows the characteristics of worlds that could potentially play host to extraterrestrial life.
Astronomers have long wondered whether the highly-structured orbital trend displayed in our solar system was simply the norm, or the result of an amazing coincidence. A new study that examined the orbits of 74 exoplanets orbiting 28 distant stars appears to put the question to rest.
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The team created model orbits for the exoplanets by observing the characteristics of specially selected parent stars with predetermined characteristics. By having a knowledge of the mass and radius of a host star, the researchers could extrapolate the speed at which a potential Earth-like planet would travel around it, assuming that its orbit was circular.
The team then used NASA's Kepler space telescope to determine the actual orbital periods of the exoplanets. Subsequent observations found that all 74 of the exoplanets matched up well with the predicted models, meaning that the orbits were essentially circular. This orbital pattern represents a marked contrast to that of giant exoplanets, which are prone to highly eccentric orbits that often bring them into extremely close proximity with their parent stars.
The discovery will likely have far-reaching implications on the search for extraterrestrial life. There is now solid evidence that Earth-sized planets hold a stable orbit around their parent star, and this means that the conditions on those planets will be more or less steady over time. In contrast, larger exoplanets with more eccentric orbits will have wildly varying planetside conditions, thanks to their varying proximity to their host stars.
However, whilst the results of the study were overwhelmingly positive in their support of Earth-sized planets undertaking circular orbits, it must be noted that 72 stars represents a tiny focus group in the context of the countless planetary systems in our galaxy. Further sampling will be necessary if astronomers are to determine whether the properties of our solar system are indeed the galactic norm.