An illustration of an exoplanet like the one Kepler detects by monitoring dips in a star's brightness (which are caused by orbiting planets). (ESO)

For a planet to have liquid water — something necessary to support life as we know it — it has to be within a certain distance of its star. Too close, and the water burns up. Too far away, and it's a frozen wasteland. But according to new research, most stars in the galaxy have so-called "Goldilocks planets" that sit in the habitable zone, where temperatures are just right for life.

The illustration shows the habitable zone for different types of stars. The distance to the habitable zone is dependent on how big and bright the star is. The green area is the habitable zone, where liquid water can exist on a planet's surface. The red area is too hot for liquid water on the planetary surface and the blue area is too cold for liquid water on the planetary surface. (NASA, Kepler)

New calculations in a study published Wednesday in Monthly Notices of the Royal Astronomical Society indicate that billions of the Milky Way's stars have one to three planets in the habitable zone, meaning that they potentially have liquid water as well.

[Two new exoplanets are more Earth-like than any we’ve found before]

The calculations, which were produced by a group of researchers from the Australian National University and the Niels Bohr Institute at the University of Copenhagen, are based on a method called the Titius-Bode law. This law, which was created around 1770, predicts how planets in a solar system will be spaced out. The researchers applied the law to the 1,000 exoplanets (and 3,000 possible exoplanets) found by NASA's Kepler satellite. They looked at 151 planetary systems — ones where Kepler had detected between three and six planets — and found that the Titius-Bode law fit well with the way 124 of them were spaced out.

[After a hardware failure, the Kepler spacecraft finds a second life — and an important new exoplanet]

"Using T-B's law, we tried to predict where there could be more planets further out in the planetary systems. But we only made calculations for planets where there is a good chance that you can see them with the Kepler satellite,"  Steffen Kjær Jacobsen, PhD student in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen, said in a statement. In the planetary systems where ratios were off, they were able to estimate where "missing" planets might be.

Exoplanetary systems where the previously known planets are marked with blue dots, while the red dots show the planets predicted by the Titius-Bode law on the composition of planetary systems. 124 planetary systems in the survey -- based on data from the Kepler satellite, fit with this formula. (Timothy Bovaird, Australian National University)

Once those planets were added, all 151 systems showed one to three planets in their habitable zone. The researchers believe this indicates that most systems do have planets orbiting at the proper distance to hold liquid water.

To help confirm their theory, they've flagged a number of supposed Goldilocks planets that Kepler should be able to see at some point. They hope that other scientists will spot them, adding weight to the "missing planets" they've calculated.

Unfortunately, being in the habitable zone doesn't mean that liquid water is present — and the presence of liquid water doesn't necessarily mean that life ever can, will, or did exist. But here's to hoping.


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