Astronomers Find that Billions of Stars in the Milky Way Have Planets in the Habitable Zone

Wednesday, 18 March 2015 - 11:56AM
Astrobiology
Wednesday, 18 March 2015 - 11:56AM
Astronomers Find that Billions of Stars in the Milky Way Have Planets in the Habitable Zone
Astronomers have long been predicting that there may be billions of habitable planets in our galaxy. And while this hypothesis has been supported by our discovery of thousands of exoplanets so far, scientists have been unable to say for certain how many potentially habitable planets lie outside our solar system. Now, a new study from the Australian National University and the Niels Bohr Institute in Copenhagen may confirm that the "billions" statistic isn't inflated. They calculated the probability that stars in the Milky Way would have planets orbiting within their habitable zone, and found that billions of stars in our galaxy likely have one to three planets in their "Goldilocks zone." 

The Goldilocks zone, named for the children's story, is a planet that's not too far from the sun and not too close; it's "just right." In other words, it's within the range of distances from the closest star that the temperature wouldn't be so cold that all liquid would freeze or so hot that all liquid would evaporate. The presence of liquid water is usually considered to be paramount to the development of life, so this is one the primary criteria for astrobiologists when looking for habitable exoplanets.

In order to estimate the probability of stars in the Milky Way having planets in their habitable zones, the researchers used a modified version of the Titius-Bode law, which long ago correctly predicted the position of Uranus before it was discovered. According to this law, in each solar system there is a ratio between planetary orbital periods. As a result, the researchers could extrapolate the orbital periods of all the surrounding planets of a star, as long as they knew the orbital period of at least one planet, and can also tell if a planet is "missing" in a chain of known planets orbiting a star.  

"We decided to use this method to calculate the potential planetary positions in 151 planetary systems, where the Kepler satellite had found between 3 and 6 planets. In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets. Using T-B's law we tried to predict where there could be more planets further out in the planetary systems." Steffen Kjær Jacobsen, an astrophysics PhD candidate at the University of Copenhagen, told Phys.org.

So in solar systems that didn't appear to conform to the Titius-Bode law, they predicted more planets orbiting that star, as well as their positions relative to the known planets. Ultimately, they calculated that they had added an average of 1-3 planets per star that were orbiting in the habitable zone.

"In [the] 31 planetary systems that were close to the habitable zone, our calculations showed that there was an average of two planets in the habitable zone. According to the statistics and the indications we have, a good share of the planets in the habitable zone will be solid planets where there might be liquid water and where life could exist," said Jacobsen. 

When they applied their statistics to the rest of the galaxy, they found that the Titius-Bode law predicted billions of exoplanets circling their star within the habitable zone.

This is technically a statistical analysis rather than an actual discovery, but the group focused on planetary systems that can be seen by Kepler for the express purpose of verification: "We then made a priority list with 77 planets in 40 planetary systems to focus on because they have a high probability of making a transit, so you can see them with Kepler. We have encouraged other researchers to look for these. If they are found, it is an indication that the theory stands up," said Jacobsen. 
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