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An Old Astronomical Rule Says There Should Be Way More Habitable Planets

And researchers are applying the theory to findings from NASA's planet-hunting Kepler satellite with interesting results.

​According to a weird and imperfect astronomical rule from the 18th century, there's likely a whole lot more exoplanets out there orbiting stars in their habitable zone—the "Goldilocks distance" that's neither too hot nor too cold, where liquid water could potentially exist—than we've seen so far. In a paper in the journal M​onthly Notices of the Royal Astronomical Society, a group of Danish and Australian researchers explained how they dusted off the old Titius-Bode law, and applied it to findings from NASA's planet-hunting Kepler satellite.

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"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," said Steffen Kjær Jacobsen, PhD student and study co-author. "In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets."

The Titius-Bode Law is a pretty rough rule, developed by the German astronomer Johann Titius in 1766, and formulated mathematically by J.E. Bode in 1778. The rule gives you a series of orbits where planets should exist, like a list of addresses. From there, it's just a matter of spotting the planets themselves, a far easier task when you know where to look.

It only works approximately, and it also sounds pretty arbitrary. As​ described by Space Academy, if you are "starting with the series 0, 3, 6, 12, 24,.. etc and then adding four and dividing by 10, the resultant series was very close to the actual distances of the planets from the Sun when measured in astronomical units."

Weird, yes, but you can't argue with results. Bode used it to predict the orbit of Uranus and what became known as the asteroid belt before we knew that either one existed. Oddly the rule doesn't properly account ​for Neptune, but Pluto fits. And if it helped us know where to look for planets in our own solar system, then dammit, it should help us know where to look in others. That's physics.

For the 124 systems Kepler spotted that fit the Titius-Bode pattern, the researchers added in the planets where the rule said they should be. They predicted an astounding 228 planets in 151 solar systems with, remarkably, one to three planets in each system orbiting in the habitable zone.

Previously known planets are represented by blue dots, and red dots show the planets predicted by the Titius-Bode law. Image: ​University of Copenhagen/ESO

Of course, it's not that the researchers actually found planets there. And for that matter, it's not as if every planet in the habitable zone is going to have water. Mars, for instance, is in our Sun's habitable zone and we have yet to find liquid water there. But the work by the Danish researchers highlights 77 potential planets that Kepler or its heirs could look to in order to find the next batch of Earth-like planets.

"In these 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," said Jacobsen. "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."

Of course, given that the Titius-Bode rule is so far from being universal that it doesn't even cover our solar system, all of this could be for naught. But in the search for life, we only have one example to pull from, and for whatever its worth, the Titus-Bode law at least applies to Earth.