Exoplanets: Habitable planets beyond the Solar System?


Throughout the last 20 years, one planet after the other has been discovered beyond the Solar System. Over 4,000 exoplanets are known by 2021. This is exciting in itself, but the key question is: what are we looking for on these planets? The answer is almost as old as humanity. We want to discover life beyond Earth, or places where life is possible or that can be colonized. Therefore, astronomers are looking for habitable exoplanets. Fifty exoplanets look promising for life. Scientists are trying to find out what these exoplanets consist of, how big they are, whether there is water or air.

Author: 2021 Kathelijne Bonne 

The age-old question 'are we alone?' drives a lot of space research. With the recent discoveries of exoplanets, it is thought that this question will soon be answered. The universe is not only full of stars, it now seems that it is also teeming with planets. In this article we look at what makes a planet habitable, how we detect it, which are the most promising candidates for life, and the great discovery of scientists from Belgium.

What makes a planet habitable?

Habitable means that life must be able to arise, a process called abiogenesis, and then this early life must be able to thrive over geological periods of time, preferably millions to billions of years, as on Earth.

Water, in the liquid state, is a first prerequisite for life as we know it. A planet lies in the "habitable zone" from a star if surface water can be liquid, that is, at a distance that allows temperatures from 0° to 100° Celsius. However, to be liquid, water needs more than a certain temperature. It also needs pressure. This pressure is exerted by the atmosphere. Without an atmosphere, ice would simply evaporate into water vapor.

This brings us to the second requirement, the atmosphere, the layer of gas surrounding a planet. In addition to exerting pressure, an atmosphere keeps temperature fluctuations in check. Without an atmosphere, the Earth, like the Moon, with its wafer-thin atmosphere, would fluctuate to extremes of -170° at night and a scorching 100° during the day. In addition, the atmosphere protects life from deadly UV and X-ray radiation from a blazing parent star.

The third desirable requirement is an active, dynamic interior of the planet. On Earth, plate tectonics is the result of internal activity, driven by a hot core that causes the hot rock in the Earth's mantle to slowly swirl, like large caterpillars, as described in our article on plate tectonics. As a result, there are volcanoes and other geological phenomena on the Earth's surface. Plate tectonics also causes a planet to be enveloped by a magnetic field. This field acts as a shield against the deadly solar or stellar wind.

In short, currents in oceans, in the air, and the slow 'flowing' of rocks provide a a stable environment protected from violent outer space, and a continuous supply and cycling of elements needed for life, such as carbon, oxygen, hydrogen, nitrogen, phosphorous, etc.

Humanity is looking for planets with these properties. But first we must find the planets. How is it done?

Planets with water vapor and oceans: large exoplanets and Earth (right) (NASA, artistic rendition)
Planets with water vapor and oceans: large exoplanets and Earth (right) (NASA, artistic rendition)

Finding exoplanets: look at the starlight

Astronomers use several techniques to detect exoplanets, including the transit technique. A transit occurs when a planet slides in its orbit in front of its star, relative to the observer (like a solar eclipse). During a transit, the starlight diminishes slightly. If the transit recurs at regular intervals, then a planet is most likely the cause. For example, when Mercury slides in front of the sun with respect to Earth, we hardly notice a change in daylight, but instruments do notice a diminishing in the received light.

The Kepler mission: much more planets than thought

Equipped with transit-detecting instruments, the Kepler telescope was sent into space in 2009. In a narrow sector of our galaxy, the Milky Way, Kepler searched for stars with planets. About 140,000 stars were sampled. Scientists hoped to discover at least a handful of planets, but the mission exceeded all expectations.

About 3,200 of the 140,000 stars were found to be chaperoned by 4,000 planets. The real number is much higher because with the transit technique you only see a fraction of all transits (because you only look in one flat plane). 

Kepler Space Telescope (NASA, artistic rendition).
Kepler Space Telescope (NASA, artistic rendition).
Search space of Kepler in the Milky Way (NASA).
Search space of Kepler in the Milky Way (NASA).

How do we see the atmosphere of an exoplanet?

How do we learn more about the planet if we only see its silhouette? How do we see the distant atmosphere? During a transit, starlight flows through the atmosphere. Depending on the chemical composition of the atmosphere, certain wavelengths of the starlight are absorbed, blocked, or transmitted, and this can be measured.

Water vapor suggests an ocean. Oxygen gas may be indicative for life, because on Earth it is a by-product of living things. CO2 can indicate a greenhouse effect.

These techniques have already been used to characterize a number of exoplanets. In the meantime, scientist have also discovered which size of planet are best suited for life.

Super-Earths: suited for life

The size of the planet turns out to be crucial. Ideally, habitable planets are slightly larger to ten times the size of Earth. These are the so-called super-Earths. 

A super-Earth is large, so there is a greater chance that it has not lost its internal heat (small planets cool quickly). Thanks to the internal heat, the chance of a dynamic interior and therefore plate tectonics is greater. A super-Earth is also heavy. Due to this gravity, the planet holds on well to its atmosphere. On small, light planets, the atmosphere escapes to space easier. Moreover, major climate disasters or meteorite impacts have less impact on a larger than on a smaller planet.

On super-Earths, life has better chances.

Gliese 581 c: a habitable super-Earth?

Gliese 581 c is one of the super-Earths that may be habitable. It weighs five times as much as Earth. It orbits its parent star, Gliese 581, a red dwarf, in the constellation of Libra, 20 light years away. Temperatures there would fluctuate between 0 and 40° C. It is not yet known if liquid water exists. There is still a lot to discover about Gliese 581 c and new space missions will take a closer look at this planet.

So far, about 50 potentially habitable planets are confirmed. Are there more?

How many habitable planets are there in the Milky Way?

Calculations from the Kepler mission and other observations, showed that the universe is actually very abundant in planets: About half of all stars have at least 1 planet with an orbital period of less than 85 days, according to calculations. Why that orbital period? Because the longer the orbital period, the longer a telescope has to look to see the transit. It hence takes longer to detect them and draw conclusions.

How many planets are habitable? According to recent estimates, 1 in 5 planets may be habitable. Based on Kepler mission calculations and an estimate of the total number of stars in the Milky Way, there could be about 60 billion habitable planets in our little corner of the universe!

The closest super-Earth: Proxima Centauri b

Proxima Centauri b (or Proxima b) was discovered in 2016. It orbits the red dwarf Proxima Centauri, the star closest to us. Proxima b is slightly larger than Earth and much closer to its star than Mercury is to the Sun, and it orbits it in 11 days. Yet it is in the habitable zone because the star Proxima Centauri is much fainter than the sun. Despite problems, such as bright x-rays, erratic flares (solar flares), a strong solar wind, and a problematic rotation, the environment of Proxima b could produce life. The average temperature of -40 C and having an atmosphere, may allow water in liquid state at some places of the planet. At best, the planet has a magnetic field that can act as a protective shield to deflect solar wind and flares. There is still much to learn about Proxima b.

Trappist 1: seven earth-like planets

In 2015, the study of an ultracool red dwarf star, 2MASS J23062928-0502285 in the Aquarius constellation, was extensively covered in the media. A Belgian team from the University of Liege revealed a solar system of no less than 7 Earth-like planets. The star was named Trappist-1, since the team from Liege had used the Transit Planets and Planetesimals Small Telescope (TRAPPIST) to study it. The planets are named Trappist 1b to h, of which Trappist 1e, f and g dwell in the habitable zone, which allows water to occur in liquid form.

The ultracool red dwarf Trappist 1 and its seven planets (NASA/JPL Caltech/R. Hurt; artistic rendition).
The ultracool red dwarf Trappist 1 and its seven planets (NASA/JPL Caltech/R. Hurt; artistic rendition).

The planets have very tight orbits, and the star is also much smaller, just slightly larger than Jupiter, and much cooler than the Sun. In fact, this system is more similar in size to Jupiter and its moons than to our solar system. From each planet, the other planets are clearly visible in the sky, and sometimes appear larger than the moon seen from Earth. 

The chances of liquid water are greatest on Trappist-1e, and it is one of the most promising planets, with the highest chances for habitability. It may be a water world, a planet completely covered by an ocean.

The Trappist system is the only known planetary system with seven Earth-like planets so far and is a very exciting discovery indeed.

New Space Missions

Trappist 1e, Gliese 581 c and Proxima b are just some of the exoplanets where life or habitable conditions are possible. These and many other planets will be explored in more detail by new space missions such as PLAnetary Transits and Oscillations of stars (PLATO), to be launched by ESA in 2026, and Transiting Exoplanet Survey Satellite (TESS), launched in 2018 by NASA. 

Read more about space, planets and life on GondwanaTalks:


Super-Earths and Life. An Edx.org online course by Harvard University. References therein. https://www.edx.org/course/super-earths-and-life

Gillon, M., Triaud, A., Demory, BO. et al. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature 542, 456-460 (2017). https://doi.org/10.1038/nature21360

NASA, What is an exoplanet? https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/super-earth/


Kepler Space Telescope.