A team of astronomers have dramatically raised the estimate for the number of Earth-like planets in our galaxy that might support life. Their study suggests that stars have an average of two planets at a distance where liquid water can exist, a region commonly known as the "habitable range" or “Goldilocks zone.” The figure is a dramatic increase on past estimates, giving many more chances for life.
The conclusion will face plenty of skepticism, however, because it relies on “Bode's Law,” an observation of great historical significance that is now widely considered discredited.
In 1766, Johann Titius was the first to observe a pattern in the distance of planets from the sun. Titius noted that the distance doubles between each planet's orbit and the next the further from the sun one goes (except for the gap between Mars and Jupiter). For example, Mars to Earth is twice as far as Earth to Venus, and four times the distance from Venus to Mercury.
The pattern was made famous six years later by Johann Bode, and the subsequent discovery of Uranus close to the anticipated location for another planet led to the belief that the “law” was more than a fluke. Astronomers across Europe collaborated to find the missing planet between Mars and Jupiter, and found Ceres—disappointingly small, but perfectly located. Sadly, Neptune was closer than Bode's Law predicted and Pluto didn't fit at all, leading to the widespread view that the pattern was just a coincidence.
However, astronomers at the Australian National University are not so sure. The team studied 68 star systems known to host more than three planets and found that other systems follow a generalized version of what they call the Titius-Bode Relationship. From this, they predicted 141 additional planets. A search for 97 of these found five.
This may seem a poor success rate, but Dr. Charley Lineweaver says, “The relationship does not say the planet has to have a certain mass; some could be too small for Kepler to see. We think 5% is about what you would expect to find.”
Kepler is far more likely to detect planets located close to stars, usually too close to support life. So in the Monthly Notices of the Royal Astronomical Society, Lineweaver and Ph.D. student Tim Bouvaird have extended the patterns observed by Bode's Law out to where the climate should be more congenial, predicting 228 new planets. The solar system appears to be an underperformer, with stars anticipated to average two planets (plus or minus one) within the range where, given suitable atmospheric conditions, liquid water might exist.
Estimations of angular tilting allowed the authors to find 40 planets they predict have a 15% chance of detection. The pattern, Lineweaver says, appears to hold whether applied to sun-like stars, like most in the sample, or red dwarfs.
Lineweaver acknowledges that predictions using Titius-Bode are a hard sell in the astronomical community. “It's taboo,” he says. “So many crazy numerologists have jumped on it that conservative scientists dismiss it, but I say that is throwing the baby out with the bathwater. It is not a universal law, but is a useful rule of thumb.”
Credit: Timothy Bovaird, Australian National University. A sample of stars identified by Kepler to host multiple planets. 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.