Exoplanets: Habitable worlds beyond the Solar System?

The age-old question 'are we alone?' drives a lot of space research. With the recent discoveries of thousands of exoplanets, among which about fifty potentially habitable, it is hoped that this question will be answered soon. The universe is not only full of stars, it is also teeming with planets. What makes an exoplanet habitable, how it is detected, which are the most promising candidates for life? And I also like to mention an amazing discovery of scientists from my native Belgium.

Writer: Kathelijne Bonne

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

Prerequisites for life 

Water, in the liquid state, is the main 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 for temperatures from zero to hundred degrees Celsius. However, to be in a liquid state, water also needs pressure, exerted by the atmosphere, otherwise ice simply sublimates into vapor.

The atmosphere also keeps temperature fluctuations in check. Without an atmosphere, temperatures on Earth, like on 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.

An active dynamic interior of the planet is important to for life to last. On Earth, the hot core causes the rock in the Earth's mantle to swirl, like large caterpillars, leading to plate tectonics and volcanism on the surface. The same deep mantle movements generate a magnetic field that acts as a shield against the deadly solar or stellar wind.

Both an active interior and surface, with currents in oceans, in the air, and the slow 'flowing' of rocks, create a stable environment shielded from violent outer space, and a continuous supply, cycling and redistribution of elements needed for life, such as carbon, oxygen, hydrogen, nitrogen, phosphorous, etc.

Space agencies are looking for planets with these properties. But first they must be found.

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). The transit causes 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

Equipped with transit-detecting instruments, the Kepler telescope was launched into space in 2009. In a narrow sector of our galaxy, 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 plane). 

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, which 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.

Habitable super-Earths

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 and holds on well to its atmosphere. On small, light planets, the atmosphere escapes to space easier. 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

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 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:

• A pale black dot: The transit of Mercury.
• The origin of the Moon: a tale of two planets.
• Plate tectonics: the caterpillar tracks of the planet.
• Did lightning provide phosphorous to ancient life?
• The planetary boundaries: A safe place for humans? 
• The origin of oxygen: when did we start breathing?

Sources:

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/

https://www.sci-news.com/astronomy/science-gj-1214b-gj-436b-weather-exoplanets-01657.html

Kepler Space Telescope.

Kathelijne: I am intrigued by how earth, ocean, air and life interact with each other on geological and human timescales.

Why I started GondwanaTalks.

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