Do habitable planets exist elsewhere in the universe?

Before answering this question, it is important to remind ourselves that life on Earth is the only type of life we know, and therefore it is the only type of life we can detect. To identify a habitable planet would then require detecting signatures of earthly life on that planet.

One may argue that because life had to adapt to many processes during the evolution of Earth, such as Earth’s motion around the sun, its interaction with the moon, its geological and atmospheric evolutions, etc., life is unique to our planet.

In the context of the opening question, this implies that a habitable planet should be completely identical to Earth for life to originate and develop as it did on Earth.

So, do identical Earth twins exist?

The process of planet formation is extremely sensitive to the environment in which planets form. Slight disturbances in that environment will change the outcome drastically. Models of solar system formation indicate that the probability for an environment around a star similar to our sun to produce an exact Earth replica is extremely small. Taken at face value, this seems to imply that the chances for a planet to be an identical Earth twin is close to zero, and argues against the existence of habitable planets outside of our solar system.

Fortunately, that is not the case.

First, although models of planet formation indicate that the probability of the formation of an Earth replica is less than 0.1 percent, when this probability is expressed in the context of our galaxy, which contains billions of stars, it translates into a very large number of Earth-like planets.

This is good news but not the main reason for the existence of other habitable planets in the universe. It is life itself that plays the most important role.

Although we do not know how life originated, we know that soon after life started, it took various chemical and biological evolutionary paths. While many of these paths were unsustainable and ceased, some adapted to the evolution of Earth and continued.

This branching of life is sensitive to the evolution of the planetary environment in which it originates. Slight evolutionary disturbances cause life to take different routes.

The latter has an important implication: The chemical and biological paths taken by life on Earth are not unique, and, depending on the host planet, life can reach the state of earthly life through a combination of other chemical and biological evolutions. In other words, for a planet to be habitable, it does not need to be an Earth twin. It needs to be an Earth analogue.

Physical models governing the geological, geophysical and atmospheric properties of Earth indicate that these processes can develop and sustain in planets up to twice the size of Earth and up to 3.5 times its mass. If such a planet receives from its central star the same amount of radiation Earth receives from the sun, it may potentially be habitable.

There are many such potentially habitable Earth analogues.

Approximately 4,000 planets have been detected around other stars, with almost all being different from planets of our solar system. One interesting difference is the existence of planets with masses slightly larger than Earth to about 10 Earth masses. Dubbed super- Earths, these planets represent a new class of bodies that do not exist in our solar system.

A good number of these super-Earths have masses around 3.5 Earth-masses and smaller. And, among those, some orbit their host stars at distances where they receive the same amount of radiation as Earth receives from the sun. In other words, they could potentially be habitable.

The first two potentially habitable super-Earths were discovered by our team in 2010 and 2012 using the Keck observatory. In 2016 we announced the discovery of 20 more of these objects using the data from the Kep­ler telescope. Many other astronomers have also detected potentially habitable super-Earths.

Back to the opening question: Do habitable planets exist elsewhere? The answer is yes, a fact that is supported by science and so widely accepted that it is now common knowledge.

Nader Haghighipour is an astronomer at the Institute for Astronomy at the University of Hawaii at Manoa. His research expertise is on the formation, detection and characterization of habitable planets. He can be reached at naderh@hawaii.edu.