Our spiral galaxy, the Milky Way is very, very big. Measuring 100,000 light-years in diameter (or one quintillion kilometres), our Solar System (including Earth) is located approximately halfway across its radius. To give you a sense of its magnitude, it would take about 25,000 years at the speed of light for us to leave our galaxy from Earth.

Based on our galaxy’s sheer scale, it shouldn’t come as a surprise that scientists have predicted that there are at least 100 billion planets in our galaxy alone (and there are at least 100 billion galaxies in the observable universe). Such astounding discoveries are all thanks to Nasa’s Kepler space telescope.

Launched in 2009, the planet-hunting Kepler space telescope has observed the Milky Way’s incredible and downright bizarre planetary orientations, such as a planet with two suns, a planet which will fall into its own star, and planets almost identical to Earth. Kepler’s seemingly unwavering spirit to meticulously count the heads of those with probable alien life is actually quite prosaic – it doesn’t care either way about what it finds or what it doesn’t find. Its intention is to collect raw data of how many planets other than our own orbit stars, and it does this by looking for telltale dips in starlight when a planet passes in front of its parent star.

It’s a fairly simple method, and measuring the timing of the dip as well as knowing a bit about the star reveals a great deal of detail, such as its size, shape and orbit. The additional step of breaking the spectra of a star into thousands of different colours can reveal the mass of a planet. Luckily for us, a lot of Kepler’s results have been pretty cool. Out of a total of 4,178 current planet candidates, Kepler has currently confirmed 1,019 planets in about 440 stellar planets.

A paper recently published in the journal Monthly Notices of the Royal Astronomical Society described how researchers from the Australian National University and the Niels Bohr Institute used Nasa’s Kepler to analyse a sample of multi-planet systems. They concluded that the billions of stars in the Milky Way (remember, there are at least 100 billion stars) have an average of two (give or take one) planets in the habitable, or "Goldilocks", zone – a region not too hot or not too cold to support liquid water on the surface, and therefore possibly life. That's a huge number of possible homes for alien life.

This discovery sounds awesome, but we shouldn’t get too excited just yet, as it’s based on a new version of the 250-year-old Titius-Bode method, which roughly predicts the spacing of planets in a solar system using a mathematical expression called "Bode’s Law", a way for researchers to estimate how many stars could have planets in their habitable zones based on the planets we know are already there. Steffen Kjær Jacobsen, a PhD fellow in the Astrophysics and Planetary Science Group at the Niels Bohr Institute, said in a statement: "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. In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets."

The researchers predicted 228 additional planets in 151 multi-planet systems which contain at least three transiting planets observed by Kepler. They did this by inserting predictable locations of planets in the multi-planet systems, along with those already seen by Kepler. They also added an extra planet beyond the outermost observed planet.

The habitable zone is relative and its location and differs from star to star. It’s the size as well as the brightness of the star that will determine the “just right” region – the region where there could be liquid water:

Schematic of habitable zone of different planets. Image: Nasa

The Kepler space telescope was designed to survey a patch of sky in the Cygnus-Lyra area. It detected about 150,000 stars in this region for nearly four years until the end of its planet searching days in May 2013 due to mechanical failure. (However, the analysis of Kepler’s planet findings will take years to complete and confirm.) From just 0.28 per cent of sky, Kepler was able to find 51 Earth-like planets. If we include those planets missing from Kepler’s viewpoint due to a slight tilt in the planets orbit, we have about 22,500 potential Earths alone - and extending the telescope’s search by only 3000 light years in distance by extrapolating data, we find that there are 15-30 billion potential Earths in just a fraction of our galaxy.

"We have encouraged other researchers to look for these. If they are found, it is an indication that the theory stands up," Jacobsen concluded.

In 2017, the hunt for alien Earths will be in the hands of the very expensive Transiting Exoplanet Survey Satellite, the next planet-hunting telescope due in orbit, which will search across the skies using Kepler’s transit method. Now Nasa scientists are completely convinced that the habitats suitable for extraterrestrial life are common in the universe, where on Earth (pun intended) are all the oval-headed, cat-eyed, spaceship ramp-loving aliens? Chances are, we’ll have to find them before they find us.