Scientists know a lot of worlds on which life may be present or could exist in theory — simply because the right conditions. Now these remarkable celestial bodies packed in two complementary scientific ranking system.
An international team of astronomers, planetary sciences, biologists and geochemists was two systems assess the suitability of a planet (or moon) for life and, consequently, the likelihood of its presence on a particular celestial body.
The first line is called "similarity index earth» (Earth Similarity Index — ESI), and the second "index habitable planet» (Planetary Habitability Index — PHI).
"The first question is whether we can find on other worlds like Earth conditions, since we know empirically that they are capable of supporting life, — says one of the study authors Dirk Schulze-Makuch (Dirk Schulze-Makuch) University of Washington ( WSU). — The second question is whether conditions exist on exoplanets, to suggest the presence there of any form of life, whether known to us or not? "
ESI index is based on the level of simple physical resemblance of the world and studied Earth: are taken into account the size, mass, density, distance from the star and the planet's temperature.
But PHI allows for more additional and very important factors: a rocky surface of the planet (or moon) or ice if there's atmosphere and magnetic field, how much energy is available for potential organisms (sunlight or tidal friction, warming up the bowels), are there where organic compounds and a liquid solvent and so on.
(Details of the new ranking method described in the article in the journal Astrobiology.)
Calculating PHI broader approach to the possible parameters of habitability, as based on a slightly different biochemistry in theory it is possible the existence of organisms and in hotter and colder conditions than on Earth, and without oxygen, even without water.
It is clear that the index of similarity Earth has the highest rating of the Earth itself — 1. And with the habitability of our world is obvious.
But on other worlds calculations gave the interesting results. Both product lines in descending order of potential habitability (fitness for any form of life) bizarrely mixed objects that are in the solar system and beyond.
- Gliese 581g — 0,89 (recent observations have shown that, most likely, not the planet, the discovery was a mistake, but it is still included in the list);
- Gliese 581d — 0,74;
- Gliese 581c — 0,70;
- Mars — 0.70;
- Mercury — 0.60;
- HD 69830 d — 0,60;
- 55 Cnc c — 0,56;
- Moon — 0.56;
- Gliese 581e — 0,53.
Table raises questions about, say, a very relative similarity with the Earth hot Mercury and the Moon, with its vacuum on the surface and a change in temperature. (Yes, and on that and other objects astronomers have found water ice.)
But exoplanets, trapped in the list above (except the last two), are notable not only decent physical resemblance to our planet. They and are located in the habitable zone, or on its edge. This means that there may be oceans, lakes and rivers, with all the ensuing consequences.
As a second method of ranking takes into consideration a larger number of parameters, even more interesting to look at the following list.
- Titan — 0.64;
- Mars — 0.59;
- Europe — 0.49;
- Gliese 581g — 0,45;
- Gliese 581d — 0,43;
- Gliese 581c — 0,41;
- Jupiter — 0.37;
- Saturn — 0.37;
- Venus — 0.37;
- Enceladus — 0.35;
Here, too, have something to marvel at. For example, a relatively modest net result of Enceladus in relation to Europe (on these moons, apparently, there are large subglacial reservoirs of liquid water).
The high rating of Mars does not cause problems, but the leadership of Titan is not obvious — it is located quite a distance from the Sun.
But on this satellite of Saturn found a large variety of complex organic compounds. Although the orange moon is very cold, scientists suspect that there are chemical reactions that can support a specific type of microbial life.
Not all the points of interest were in the study. Say, for a potentially habitable worlds scientists refer relatively recent discovery — exoplanet HD 85512 b.
Ratings ESI and PHI will still be supplemented and refined. They should help astrobiologists and planetary scientists to focus on the most interesting objects.