Hundreds of billions of Earth-like planets that support life could be found around most stars in our galaxy, says a study by Australian researchers taking recourse to a 200-year-old theory of planetary prediction.
The standard star in the Milky Way has about two planets in the so-called goldilocks zone, the distance from the star where liquid water, crucial for life, can exist.
"The ingredients for life are plentiful, and we now know that habitable environments are plentiful," said Associate Professor Lineweaver, from the ANU Research School of Astronomy and Astrophysics and the Research School of Earth Sciences.
However, the leap from life to intelligent life that builds telescopes and reaches out is a big one.
The fact that we have not seen or heard from them could suggest bottlenecks for the emergence of intelligent life not considered so far, says the team.
Or that evolving intelligent civilisations end up in self-destruction. This was one of the probabilities suggested by the late Carl Sagan when warning about the power of destructive technologies.
The Kepler space telescope is biased towards seeing planets very close to their stars, that are too hot for liquid water, but the team extrapolated from Kepler's results employing the theory that was used to predict the existence of Uranus.
By applying period calculations from the 200-year-old Titius-Bode law of planetary sequence to the exo-planets discovered by the Kepler space telescope, the study has shortlisted 77 predicted planets in 40 systems.
The period predictions of 228 additional planets in 151 of these Kepler multiples suggest that there are, on average, at least two planets in the habitable zone of each star.
"We used the Titius-Bode relation and Kepler data to predict the positions of planets that Kepler is unable to see," Associate Professor Lineweaver said.
The Titius-Bode law predicts the spacing of planets in a solar system. It relates the mean distances of the planets from the sun to a simple mathematical progression of numbers in the sequence 0, 3, 6, 12, 24,…
The law was first pointed out by Johann Titius in 1766 and was formulated as a mathematical expression by J E Bode. Uranus was discovered as a result of Bode's Law. However, it failed in predicting Neptune.
It led Bode in 1778 to predict the existence of another planet between Mars and Jupiter in the asteroid belt. Ceres fits in that description.
Today the Titius-Bode Law is often debated as being nothing more than a coincidence.