In General, How Does The Size And Location Of A Star's Habitable Zone Depend On The Star's Mass?

We have now discovered over one g planets outside the Solar System. And we know that there are about iii dozen planets which, to some extent, are similar to Earth – they accept a similar size and tin can orbit their stars at a distance which could permit for the beingness of liquid h2o and, as such, have a habitable or comfort zone. For this reason, it may at present exist the time to review and define the meaning of this concept.

On Globe, in that location is biological activity in very diverse environments: from the glaciers on the Antarctic plateau to the stifling humidity of the Torrid zone to the hot aridity of the Sahara; from the darkness in the deepest abyssal zones to the gleaming snow-capped mount tops; inside volcanos or in environments as acidic as the Tinto River in Huelva, Spain. The physical and chemic weather condition alter but there is still a small area where a group of species interacts in more or less complex ways. Even so, in spite of its multifariousness, our planet only encompasses a narrow range of temperatures and pressures, or radiation levels. The lowest temperature in Antarctica can be around -89.four degrees Celsius, while the highest measured temperature in the hottest desert is +58 degrees. In other words, a range of most 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 Organisation and the iv planets which were found to orbit the GJ 581 star.

The Earth'due south climate depends on an essential factor, among others: the Dominicus and the energy received from it. This means that it depends on the energy radiated past our star (about three.65×10²³ kilowatts) and the energy that reaches the Earth (chosen "solar constant", i.e. 1366 Watts/1000^2), and this depends on the distance between the Earth and the Dominicus, and the area of the Globe which the Sun actually "sees". Together with atmospheric pressure, this free energy is essential for the h2o to be in liquid state.

Mars is farther away from the Sun (1.52 astronomical units, i.eastward. the boilerplate distance between the Dominicus and the Earth) and only receives 43% of the energy that reaches our planet by square meter: it is inversely proportional to the foursquare of the altitude. Consequently, h2o in Mars would mostly be in solid country, since the average temperature on Earth is virtually +10 degrees Celsius (to a higher place the melting bespeak of ice), while Mars' is around -63 degrees Celsius. In any instance, 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 information technology becomes articulate from comparison Venus and Mercury. Venus is closer to the Sunday than the Globe, and its average temperature is substantially college – +465 degrees Celsius. On the other hand, Mercury'southward surface temperature is lower than Venus', around +167 degrees Celsius, fifty-fifty though it is closer to the Dominicus. The reason for this is that the chemic limerick of a planet'south atmosphere (Mercury has a very thin atmosphere) and its atmospheric force per unit area are extremely important.

In any case, our planet has shown biological action under very diverse weather: 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 Organization

The habitable zone around a star is the range of orbital distances where a planet tin support liquid water. This implies that water is indispensable for life to be, which is not necessarily correct.

The habitable zone depends mostly on two factors: the star's mass and its age. As information technology evolves, a star changes its spectral type (i.due east. 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 h2o breaks down into its basic elements (oxygen and hydrogen), and hydrogen leaves the plant since information technology cannot be retained by the Earth's gravitational field.

To a large extent arbitrarily, it is estimated that the required radiation is i.1 times the solar abiding (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 (CO_two). A bourgeois estimate indicates that this happens at 0.53 times the solar constant. Again, in the Solar System, this is equivalent to i.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 inside these limits (1.one. to 0.53) during a meaning portion of the star's history. Since the Lord's day'southward luminosity increases slowly, the CHZ in the Solar System is between 0.95 and i.15 astronomical units. Consequently, liquid water and, as a upshot, life should be expected within this range of orbital distances. At to the lowest degree, life as nosotros know information technology.

Nonetheless, it should be noted that the following factors may play a crucial part in the development and continuity of biological activity: greenhouse effect (the Earth's average temperature would exist several degrees beneath its current value without the impact of this outcome caused past the presence of gases such every bit CO₂ and methane in the atmosphere), geological action (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-free energy particles coming from the Sun), or albedo (the amount of free energy from a star which is reflected back into space).

Diagram with several exoplanets orbiting their stars in the habitable or comfort zone, where water (if it exists) could be in liquid state.

So far, several super-Earths have been institute in orbit effectually stars that are colder than the Sun. The star Kepler-452 is a solar analog; its surface temperature is nigh identical even though it could be a lot older. As for the planet, information technology is 60% bigger than the Globe. 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 divers in the same fashion every bit in the Solar Arrangement. To calculate the boilerplate distance of this zone, you simply need to compare the star's luminosity with the Sun'south luminosity, as per this formula:

Distance(HZ, star) = [Luminosity(star) / Luminosity(Sun)]^0.5, in astronomical units

In lodge to calculate the minimum and maximum radius of the habitable zone, yous only demand to multiply Distance(ZH,star) by the factors 0.95 and ane.37, respectively.

Consequently, for an G star (the well-nigh common type of star in our galaxy), which is low in mass and luminosity and red in color, the habitable zone is very shut to the central star. In fact, the distance is and then brusque that the rotation and revolution periods of a hypothetical planet in this orbit would be the same due to the tidal effect (as information technology happens between the Moon and the Earth). This gene may or may not touch on the planet'south habitability in addition to other orbital elements such as the orbit'south eccentricity or the centric tilt.

The multiplanetary organisation associated with star Gliese 581 includes a planet which could present these conditions (Gl581c): its mass could be around 5 times the Earth's mass and its distance to the fundamental star (M3 spectral type) is 0.073 astronomical units. In that location is speculation that Gl581c could incorporate water in liquid state. In any case, a possible satellite orbiting this planet would not be limited by the tidal upshot, and could experience day-night cycles. In any upshot, the Gl581 system is not a unique case, and space missions such every bit Corot and Kepler have found several other systems such every bit this one.

And then, other systems with planets that meet these weather condition have already been detected. And not simply around stars colder than the Sunday. Recently, the Kepler satellite detected a planet 60% bigger than the Globe which orbits a solar analog in just less than a year. If the planet can be called a "cousin" of the Earth, the key star is effectively a slightly older "twin" of our Sunday. And this is another step toward finding the Grail of Earth'south truthful twin.

David Barrado Navascués

CAB, INTA-CSIC
European Space Astronomy Center (ESAC, Madrid)

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