How to search planets human can survive?

There are many many planets in the universe.
The Earth is one of them, and human can survive on the Earth. We have imagined and will imagine planets those we or lives can survive on it. It isn't able to search such planets because there are no method before. But now, we have method to search, it is physics!

We introduce the term "habitable zone". This means "A planet in this zone, water  is possible to exist like liquid". Water is essential to lives. So it is important to know whether extrasolar planet can have it. If it can do, there is possible to lives survive on it.
Then, where is habitable zone? There are near a fixed star and center of galaxy. First called "circumstellar habitable zone(CHZ)" and, second called "galactic habitable zone(GHZ)". CHZ is near a fixed star. It is said CHZ is from 0.97AU to 1.39AU (AU = "astronomical unit" is average distance between the Sun and the Earth, almost 1.5*10^11 m). GHZ has no concrete definition, but it is said it is enough near center of galaxy to contain heavy elements to consturuct a planet like earth, and enough far from center of galaxy to avoid the blackhall of center of galaxy and radioactivity from supernova and other dangers.



Search a planet we or lives can survive!!

For example, the planet called "Gliese 581 d". This is planet of fixed star called "Gliese 581". This planet is in the CHZ, and discovered in 2007. At first, "Gliese 581 d" is not a planet lives can survive, because it is too far from "Gliese 581" and only one side of "Gliese 581 d" faces
to "Gliese 581". But later, Robin Wordsworth, Francois Forget and the others who are CNRS(Centre national de la recherche scientifique)'s scientists showed a possibility of surviving on this planet. The greenhouse effect keeps heat and red light from "Gliese 581" go through clouds of "Gliese 581 d" and warm the surface. It may have something alive on the surface.



How to see far away planets?

Hubble space telescope is one of methods. This was carried into orbit by a Space Shuttle in 1990 and named after the astronomer Edwin Hubble. Hubble space telescope's four main instrument observes near ultraviolet, visible, and near infrared. This is 600km far from the Earth' surface, it makes us to observe extrasolar world without a bad effect from the  atmosphere and weathers.But, it is impossible to use observation about the Sun, because the Sun's light is too strong for the instrument. It  can see very little stars, but when see the Sun it get blinded like a human.

There are another way to see extrasolar world. X-ray telescope is so. JAXA(Japan Aerospace eXploration Agency)'s X-ray astronomy satellite "Suzaku" can see in X-ray. X-ray astronomy is one of frontier of modern science. Techniques to carry budget into orbit makes easier to observe than on the ground. To see in X-ray makes us to discover dark stars or galaxies. Why can "Suzaku" do? "Dark " means in visible light! If you see in X-ray, you can understand the universe is bright.    

Figure: This is a picture of Perseus constellation in X-ray over the picture of visible ray. In this picture, we can see a plasma that we can't see in visible ray. In X-ray, we notice there are high temperature plasma (10000000~100000000 C).


from: http://www.jaxa.jp/press/2009/11/20091102_suzaku_j.html