Using a 200 year-old statistical technique, a team of Australian astronomers have concluded that virtually every star in the Milky Way hosts at least one to two terrestrial planets capable of fostering life.

Right from the outset, it's important to note that this is not an empirical study per se. The researchers, led by Tim Bovaird and Charley Lineweaver from the Australian National University, reached their conclusions by applying a centuries-old technique called the Titius-Bode relation to predict the positions of hypothetical planets that space-based and terrestrial telescopes cannot detect.

Sometimes referred to as Bode's Law, the Titius-Bode relation hypothesizes that celestial bodies in some orbital systems orbit at semi-major axes in a function of planetary sequence. It was discovered in 1766 by Johann Daniel Titius, who showed that the orbits of planets in the solar system follow a simple arithmetic rule quite closely.


Image: Milan Milanovic.

Inspired by the fact that this relation correctly predicted the orbits of Ceres and Uranus, Bovaird and Lineweaver used it predict how many terrestrial, habitable zone exoplanets may exist in the Milky Way. To perform their calculations, the researchers sampled known multi-exoplanet systems containing at least three transiting planets detected by the Kepler space telescope (dubbed "Kepler multiples").


Then, using the Titius-Bode relation, they predicted the periods of 228 additional planets in 151 of these Kepler multiples. Results showed that, on average, there are about two planets in the habitable zone of each star. A habitable zone is that sweet spot in a solar system where an Earth-like planet is capable of sustaining liquid water at the surface — a crucial prerequisite for life.

Image: Predicted architectures of distant star systems. The solid red rectangles represent predicted exoplanets and their relative uncertainties. Image: Bovaird et al.

"We looked at the sub-set of stars that have multiple planets, not just one or two, and among those we looked for specific pattern called the Titius-Bode relation and we found that these exoplanet systems fit the relation better than our solar system does," noted Lineweaver in an ABC News article. "So based on that we made predictions about where other planets would be if this pattern can be successfully extrapolated beyond what is normally seen."

Here we see the effective temperatures of planets within 31 systems sampled that extend out to the habitable zone (shown in the green band). The sizes of the red hashed squares represent the maximum radius of each predicted planet. Image: Bovaird et al.

What's more, the researchers also produced a short-list of 77 predicted planets in 40 systems with a high probably of transit to our plane of sight.

"Our new prioritized predictions should help on-going planet detection efforts in Kepler multi- planet systems," noted the researchers in their study, which now appears at the Monthly Notices of the Royal Astronomical Society.

Again, it's important to remember that these are just predictions. As an indicator of potential planets, the Titius-Bode relation is far from perfect. It fails, for example, to predict Neptune's orbit in our own solar system. What's more, this analysis cannot be used to predict the occurrence of true habitability and the presence of extraterrestrial life.

All this said, the prediction that our galaxy contains hundreds of billions of potentially habitable terrestrial exoplanets is jaw-dropping and most certainly worthy of discussion.

"The ingredients for life are plentiful, and we now know that habitable environments are plentiful," added Lineweaver in an Australian National University statement. "However, the universe is not teeming with aliens with human-like intelligence that can build radio telescopes and space ships. Otherwise we would have seen or heard from them. It could be that there is some other bottleneck for the emergence of life that we haven't worked out yet. Or intelligent civilizations evolve, but then self-destruct."

Sadly, conclusions like these only make the Great Silence all the more disturbing.

Check out the entire study at the Monthly Notices of the Royal Astronomical Society.

Top image: NASA Ames/JPL-Caltech.