Astronomers with the University of Washington’s Virtual Planet Laboratory have devised a new habitability index meant to determine an exoplanet’s likelihood to support life.
So far, thousands of extrasolar planets have been discovered, and it would be futile to investigate all of them, especially if their environmental conditions are inhospitable.
This metric helps create a hierarchy among planets that orbit other stars, so that further attention can be given to those that can actually offer a glimmer of hope regarding alien life.
The new indicator, called the “habitability index for transiting planets”, takes into account observational data, and prioritizes it so that researchers can more easily select from a multitude of exoplanets worth studying.
In the past, scientists had focused on identifying planets located in the “Goldilocks zone”, which is at an ideal distance from the planet’s star. In this area, it is more likely to find liquid water on the planet’s surface, and as a result the chances for alien life are much higher.
However, this two-dimensional criterion wasn’t nuanced enough, since it didn’t allow astronomers to differentiate between several planets within that habitable zone.
In contrast, the recently established habitability index provides a much more flexible framework. It is based on a multitude of values, which combine together in order to determine the planet’s likelihood to sustain life.
For example, the metric is dependent on the planet’s rockiness, because according to experts such landscapes are more Earth-like.
Another factor is the “eccentricity-albedo degeneracy”, which measures the planet’s albedo (the energy its surface reflects in space) against the circularity of its orbit (which affects the amount of energy it receives from its star).
If a planet has a high albedo, then it reflects more energy into space, so as a result very little heat remains on its actual surface.
When it comes to orbit circularity, an eccentric rotation leads to much higher levels of energy as the planet passes close to its star.
As researchers point out, a planet located near the warmer inner edge of the Goldilocks zone would require higher albedo, so that the surface would be cooled, as the energy is propagated into space.
In contrast, a planet from the colder outer edge should have a more elliptical orbit, to receive the energy it requires for habitation.
By comparing these characteristics, scientists have already ranked Earth-sized extrasolar planets discovered by the Kepler Space Telescope within habitable areas.
According to them, the most likely candidates that may support life are planets which receive between 60% and 90% of the radiant energy that Earth gets from the Sun.
This research, published in the Astrophysical Journal, was funded by the NASA Astrobiology Institute. As experts explain, the new habitability index will become much more accurate and potent as more data is collected regarding extrasolar planets.
For example, the Transiting Exoplanet Survey Satellite will be launched in 2017, in an effort to identify more habitable worlds.
In addition, the James Webb Space Telescope, scheduled to launch in October 2018, will conduct infrared observation, to measure atmospheric conditions on extrasolar planets, for the first time in history.
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