8,8 billion "Earths": Our galaxy is full of habitable planets.
How many other planets outside the Solar System are habitable? According to the most recent study, there are approximately 8,8 billion in our galaxy, the Milky Way, alone.
By: Oasis Team
The Milky Way is populated with stars of similar mass to the Sun. One in five of these stars could have a planet similar to Earth orbiting it in the so-called "habitable zone." That is, at a distance sufficient to allow for the presence of liquid water on the planet's surface.
Therefore, in our galaxy, there could be billions of other suitable planets capable of supporting life. This is what an important study published a few days ago in the journal Proceedings of the National Academy of Sciences (http://www.pnas.org/content/early/2013/10/31/1319909110) states.
The secret lies in the "quantum" of energy.
The research group, from the University of California, Berkeley, started with a question many people ask when observing the starry sky: How many stars have a planet orbiting them that receives the same amount of energy (in the form of heat) as Earth?
Of the many hundreds of exoplanets discovered so far, most are gas giants. Small, rocky planets similar to Earth are more difficult to identify – but some have been, as demonstrated by the recent discovery of Kepler 78b, a true "clone" of Earth.
Astronomer Erik Petigura, the study's lead author, used data collected by NASA's Kepler telescope before the rupture of its reaction wheels rendered it unusable.
This telescope, nicknamed the "exoplanet hunter," observed the 150 stars in its own field of view to identify the slight fluctuations in brightness indicative of a planet's transit in front of its sun.
Using software created for this purpose, Petigura used data collected by Kepler to formulate a hypothesis about the probability of finding Earth-like planets throughout our galaxy.
40 billion stars similar to the Sun
According to Petigura's estimates, in our galaxy, the Milky Way, there would be about 40 billion stars similar to the Sun. In his hypothetical planetary systems, there would be about 8,8 billion planets similar to Earth and positioned in the habitable zone. Even more generous estimates – again based on Petigura's work – go as far as 50 billion "Suns" and 10 billion "Earths".
The research group specifically monitored the brightness variations of 42 stars similar to the Sun, identifying 603 planets orbiting some of them. Among these, 10 are located in the so-called "habitable zone" and have dimensions similar to, or at most twice the size of, Earth.
"These planets would already be very interesting in themselves, but they are even more interesting for understanding the prevalence of planets with analogous characteristics within the Milky Way," explains Petigura.
It can indeed happen that fluctuations in the star's brightness, not caused by the transit of a planet (called "noise" in technical jargon), confuse astrophysicists, making it impossible to identify a real planet; another problem is that only one in every 100 planets observed by the Kepler telescope orbits its own star with an orientation that the telescope can identify as a "transit".
The software developed by Petigura solves both problems. "By making these two corrections," the astronomer continues, "we conclude that 22%, almost one in every five stars similar to the Sun, hosts an Earth-like planet in the habitable zone."
The next step will be to search for "Earth-like" exoplanets in the star systems closest to us: the closest one, according to scientists, could be just 12 light-years away. Its star, in that case, could even be visible to the naked eye.
Analyses by astronomers at the University of Berkeley and the University of Hawaii show that among stars similar to the Sun, 1 in 5 has a planet similar to Earth and located in the habitable zone.
Video: "Galaxy contains billions of potentially habitable planets". Animation by UC Berkeley/UH Manoa/Illumina Studios, published on 4/11/2013