Probably everyone at some point wondered if there is life in space. Dominating image of extraterrestial lifeforms usually comprises of small dude with giant head, with scalpel in one hand and microchip in the other. We’re afraid of what we don’t know, and this part of our nature is very hard to change.

Scientists have found a way to bring us a little bit closer to knowing when will the final annihilation of life on Earth occur. They put tons of data and found a formula to calculate Similarity to Earth for every object in space. Is this concept hard to grasp? Let me ease your suffering by providing a simpler explanation:

I hope we’re all on the same page now. This equation above is the product of (evidently sick individuals/) hard work.

It’s called ESI, which stands for Earth Similarity Index. Mean radius, bulk density, escape velocity and Planet surface temperature are main categories to check similarity. Every one of them of course has many inside elements, and in general different level of influence on the final result.

Second criteria is called PHI – Planetary Habitability Index. It is based on “the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent.”.

PHI is ESI’s smarter brother, because it takes into consideration limits of our knowledge, and tries to deliver wider criteria that are capable of sustaining life, not only Earth-like conditions. So both can be called different sides to the same coin. Two sides of one tool. But let’s get back to ESI.

It’s inventors propose a scale of similarity, where Earth is given 1.0 (100% similarity) and everything else is below. So 0.34 would mean 34% similarity, 0.56 – 56% etc. Let’s bring the big guns. Top 5:

1. Gliese 581 g – 0.89 similarity

This is first unwanted child of red dwarf star (no, not an actor from cult TV show) Gliese 581. Location: constellation Libra. Distance: 20.3 light years away from Earth. What’s interesting is that its similarity to anything else than the air is questionable, because HARPS observations deliver strong evidence, that this exoplanet does not exist. (HARPS is a tool that is attached to telescopes for certain type of space observations). Here is its visualization:

2. Gliese 581 d – 0.74 similarity

This one is real. Gliese 581 g’s would be sister was officially confirmed around May, to be the first potentially habitable known planet! Six months later it’s at the top of the list. It seems like ‘d’ is making quite a career for itself in space vacuum. Here’s fresh paparazzi shot:

3. Gliese 581 c – 0.70 similarity

Classified as super-Earth, which means: possessing 1-10 Earth’s masses. Exact mass predicted at: 5.6x. ‘c’ was hyped as the next Earth, before ‘g’ stole the spot.

4. Mars – 0.70 similarity

With temperature reads ranging from -225° F, (-143° C) to +81° F (27° C), practically no atmosphere, this planet is not exactly user-friendly. Because of no atmosphere, cosmic radiation is so heavy that could end you in an instant. Most evidence gently points out to the fact that there was life on Mars once, but it was a different planet then.

5. Mercury – 0.60 similarity

After discarding Pluto as a planet, Mercury is now officially the smallest planet in our Solar System. For a planet that was discovered by ancient Greeks, we know very little about it. Part of the problem is its location (close to the Sun) which makes it impossible for Hubble to look at. Lucky for us, Messenger can still get the job done:

That’s the top 5. Keep in mind, that PHI and ESI are relatively new terms. Two planets of our solar system are in top positions, because we could gather more data on them. Five years from now, this list could look completely different.

And you? What planet do you think aliens will come from to ravage our cities and take our women?