Milky Way probably teeming with life – billions of planets in the Habitable Zone

There are billions of stars in our galaxy with one to three planets in their Habitable Zone orbiting them. This means that the Milky Way should be teeming with life, say researchers from the Australian National University and the Niels Bohr Institute in Copenhagen.

The Habitable Zone, also known as the Circumstellar Habitable Zone, is the region around a star within which planets might have sufficient atmospheric pressure, liquid water and the right temperatures to support life as we know it.

It is also called the Goldilocks Zone, because as in the fairy tale, conditions are ‘just right’.

Many planets in habitable zone

The Milky Way could be teeming with life as we know it. (Image: Mark Garlick NASA)

Scientists have already discovered thousands of exoplanets (planets outside our solar system) in the Milky Way. According to the Kepler satellite, a significant proportion of them are orbiting their host star within the Habitable Zone.

In this latest study, which has been published in the academic journal Monthly Notices of the Royal Astronomical Society (citation below), researchers analyzed these planetary systems and calculated the probability for the number of stars in our galaxy that might have planets orbiting them in the habitable zone.


They estimated that the billions of stars that exist in the Milky Way have between one and three planets in the Goldilocks zone. In other words, our galaxy appears to have several billion planets where there might be life.

Using NASA’s Kepler satellite, scientists have discovered approximately 1,000 planets orbiting stars in the Milky Way, plus another 3,000 potential planets.

From what the Kepler satellite tells us, most of these planetary systems have from two to six stars. However, they could well have more. With our current technology, we are only able to effectively detect the larger planets closest to their stars.

Planets that are close to their stars would be too hot to have life as we know it. Timothy Bovaird, Charles H. Lineweaver and Steffen K. Jacobsen set out to determine how many such planetary systems might also have planets in the habitable zone. They made calculations based on a new version of a 250-year-old method called the Titius-Bode law.

Calculating the positions of planets

German astronomers Johann Daniel Titius (1729-1796) and Johann Elert Bode (1747-1826) formulated what became known as the Titius-Bode law, a hypothesis that correctly predicted the orbits of Ceres and Uranus (before it was even discovered).

According to the hypothesis, there is a certain ratio between the orbital periods of planets in a solar system. In other words, the orbital period between the first and second planet is equal to the ratio between the second and third, etc. Therefore, if you know how long some planets are taking to orbit a star, you can work out how long it takes the others, and can thus calculate their positions in that planetary system.

With the Titius-Bode law you can also calculate whether a planet is ‘missing’ in the sequence.

PhD student in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen, Mr. Jacobsen, said:

“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.”

“Using T-B’s (Titius-Bode’s) law we tried to predict where there could be more planets further out in the planetary systems. But we only made calculations for planets where there is a good chance that you can see them with the Kepler satellite.”

At first glance, in 27 of the 151 planetary systems, the planets that had been observed did not fit the T-B law. They then attempted to place the planets into the pattern where planets should be located.

They added the planets that appeared to be missing between the ones already known to be there, and also added one extra planet beyond the furthest one from its star. In this way, they calculated there were a total of 228 planets in the 151 planetary systems.

Mr. Jacobsen said:

“We then made a priority list with 77 planets in 40 planetary systems to focus on because they have a high probability of making a transit, so you can see them with Kepler. We have encouraged other researchers to look for these. If they are found, it is an indication that the theory stands up.”

Planets in the habitable zone

Habitable zone

The habitable zone is not a fixed area – it depends on how bright and big the star is. The brighter the star, the further away the habitable zone is. In this illustration, the habitable zone is the green area. (Image: Niels Bohr Institute)

Mr. Jacobsen and colleagues evaluated the number of planets in the habitable zone based on the extra planets that were added, according to the T-B law, to the 151 planetary systems. They calculated that each planetary system has 1 to 3 planets in the habitable zone.

They made an additional check on 31 of the 151 planetary systems where they had already found planets in the habitable zone where just one single extra planet was needed to meet the requirements.

Mr. Jacobsen said:

“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.”

“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.”

If you apply the calculations further out into space, it would mean that just in the Milky Way, there should be billions of stars with planets within the habitable zone, where there could be water and the potential for life as we know it.

Citation: Using the inclinations of Kepler systems to prioritize new Titius–Bode-based exoplanet predictions,” Timothy Bovaird, Charles H. Lineweaver and Steffen K. Jacobsen. Monthly Notices of the Royal Astronomical Society. First published online March 17, 2015. DOI: 10.1093/mnras/stv221.