We have now discovered over one thousand planets outside the Solar System. And we know that there are about three dozen planets which, to some extent, are similar to Earth – they have a similar size and can orbit their stars at a distance which could allow for the existence of liquid water and, as such, have a habitable or comfort zone. For this reason, it may now be the time to review and define the meaning of this concept.
On Earth, there is biological activity in very diverse environments: from the glaciers on the Antarctic plateau to the stifling humidity of the Tropics to the hot aridity of the Sahara; from the darkness in the deepest abyssal zones to the gleaming snow-capped mountain tops; inside volcanos or in environments as acidic as the Tinto River in Huelva, Spain. The physical and chemical conditions change but there is still a small area where a group of species interacts in more or less complex ways. However, in spite of its variety, our planet only encompasses a narrow range of temperatures and pressures, or radiation levels. The lowest temperature in Antarctica can be around -89.4 degrees Celsius, while the highest measured temperature in the hottest desert is +58 degrees. In other words, a range of about 150 degrees. Temperature oscillations in most of the planet are much narrower.
Habitable zone depending on the distance to the central star and its mass (including labels for the approximate spectral types). For comparison purposes, the image depicts the innermost planets in the Solar System and the four planets which were found to orbit the GJ 581 star.
The Earth’s climate depends on an essential factor, among others: the Sun and the energy received from it. This means that it depends on the energy radiated by our star (about 3.65×1023 kilowatts) and the energy that reaches the Earth (called “solar constant”, i.e. 1366 Watts/m^2), and this depends on the distance between the Earth and the Sun, and the area of the Earth which the Sun actually “sees”. Together with atmospheric pressure, this energy is essential for the water to be in liquid state.
Mars is farther away from the Sun (1.52 astronomical units, i.e. the average distance between the Sun and the Earth) and only receives 43% of the energy that reaches our planet by square meter: it is inversely proportional to the square of the distance. Consequently, water in Mars would mostly be in solid state, since the average temperature on Earth is about +10 degrees Celsius (above the melting point of ice), while Mars’ is around -63 degrees Celsius. In any case, temperatures can vary a lot on a single planet (in Mars, the range is between -140 and +20 degrees Celsius). Also, the distance between the star and the planet is not the only determinant, as it becomes clear from comparing Venus and Mercury. Venus is closer to the Sun than the Earth, and its average temperature is substantially higher – +465 degrees Celsius. On the other hand, Mercury’s surface temperature is lower than Venus’, around +167 degrees Celsius, even though it is closer to the Sun. The reason for this is that the chemical composition of a planet’s atmosphere (Mercury has a very thin atmosphere) and its atmospheric pressure are extremely important.
In any case, our planet has shown biological activity under very diverse conditions: from temperatures below the freezing point of water at normal pressure (down to -20 ºC) to up to 121 ºC.
The Habitable Zone in the Solar System
The habitable zone around a star is the range of orbital distances where a planet can support liquid water. This implies that water is indispensable for life to exist, which is not necessarily correct.
The habitable zone depends mostly on two factors: the star’s mass and its age. As it evolves, a star changes its spectral type (i.e. its color, which is connected with its surface temperature) and luminosity. The lower limit of the habitable zone is estimated from the photodissociation of water. In other words, when the solar radiation is so intense that water breaks down into its basic elements (oxygen and hydrogen), and hydrogen leaves the plant since it cannot be retained by the Earth’s gravitational field.
To a large extent arbitrarily, it is estimated that the required radiation is 1.1 times the solar constant (1.1×1366 Watts/m^2). In the Solar System, this is equivalent to 0.95 astronomical units. The upper limit of the habitable zone is determined by the condensation of carbon dioxide (CO2). A conservative estimate indicates that this happens at 0.53 times the solar constant. Again, in the Solar System, this is equivalent to 1.37 astronomical units.
Stars evolve and their luminosity changes. For this reason, the concept of continued habitable zone (CHZ) has been created. It represents the range of orbital distances for which the solar constant stays within these limits (1.1. to 0.53) during a significant portion of the star’s history. Since the Sun’s luminosity increases slowly, the CHZ in the Solar System is between 0.95 and 1.15 astronomical units. Consequently, liquid water and, as a result, life should be expected within this range of orbital distances. At least, life as we know it.
Nevertheless, it should be noted that the following factors may play a crucial role in the development and continuity of biological activity: greenhouse effect (the Earth’s average temperature would be several degrees below its current value without the impact of this effect caused by the presence of gases such as CO2 and methane in the atmosphere), geological activity (plate teutonics and the subsequent release of gases to the atmosphere), presence or absence of global magnetic fields (they protect us from the burst of high-energy particles coming from the Sun), or albedo (the amount of energy from a star which is reflected back into space).
So far, several super-Earths have been found in orbit around stars that are colder than the Sun. The star Kepler-452 is a solar analog; its surface temperature is almost identical even though it could be a lot older. As for the planet, it is 60% bigger than the Earth. We have no information about its mass, average density or possible composition.
The Habitable Zone in Other Planetary Systems
The habitable zone around other stars is defined in the same way as in the Solar System. To calculate the average distance of this zone, you only need to compare the star’s luminosity with the Sun’s luminosity, as per this formula:
Distance(HZ, star) = [Luminosity(star) / Luminosity(Sun)]0.5, in astronomical units
In order to calculate the minimum and maximum radius of the habitable zone, you only need to multiply Distance(ZH,star) by the factors 0.95 and 1.37, respectively.
Consequently, for an M star (the most common type of star in our galaxy), which is low in mass and luminosity and red in color, the habitable zone is very close to the central star. In fact, the distance is so short that the rotation and revolution periods of a hypothetical planet in this orbit would be the same due to the tidal effect (as it happens between the Moon and the Earth). This factor may or may not affect the planet’s habitability in addition to other orbital elements such as the orbit’s eccentricity or the axial tilt.
The multiplanetary system associated with star Gliese 581 includes a planet which could present these conditions (Gl581c): its mass could be around five times the Earth’s mass and its distance to the central star (M3 spectral type) is 0.073 astronomical units. There is speculation that Gl581c could contain water in liquid state. In any case, a possible satellite orbiting this planet would not be limited by the tidal effect, and could experience day-night cycles. In any event, the Gl581 system is not a unique case, and space missions such as Corot and Kepler have found several other systems such as this one.
So, other systems with planets that meet these conditions have already been detected. And not only around stars colder than the Sun. Recently, the Kepler satellite detected a planet 60% bigger than the Earth which orbits a solar analog in just less than a year. If the planet can be called a “cousin” of the Earth, the central star is effectively a slightly older “twin” of our Sun. And this is another step toward finding the Grail of Earth’s true twin.
David Barrado Navascués
European Space Astronomy Center (ESAC, Madrid)
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