No series on hunting for alien life sustaining planets can be complete without discussing the advances in our knowledge brought about by the Hubble Space Telescope (HST) and the Kepler missions.

Hubble

Launched in 1990 on board the Space Shuttle Discovery, Hubble remains in a low Earth orbit. Sitting outside of the atmosphere the telescope's four main instruments and 2.4 meter wide mirror have provided the most detailed visual light images of the cosmos, enabling astronomers to view deep into space and - in effect - back in time: if Hubble is focused on a galaxy 100,000 light years away, that light has taken 100,000 years to get here and the telescope sees the galaxy as it was 100,000 years ago.

Although HST is a much lauded success, within it's first few weeks of operations it became apparent that all was not well. The images received, while much sharper than ground based telescopes, were not as sharp as expected. It was discovered that it's huge mirror had been polished into the wrong shape. Luckily Hubble had been designed to be serviceable by astronauts and subsequent Shuttle missions corrected the optics.

It's future was put in doubt though when, in 2003, NASA suffered the catastrophic loss of the Shuttle Columbia and the entire Space Shuttle fleet was grounded: HST was due a fourth servicing mission in 2005. Eventually in 2009, with astronomers convincing NASA that Hubble was important enough to risk human life, the final servicing mission took place, flown by Atlantis. With it's optics and sensors operating properly Hubble has given us breathtaking views of the universe.

Hubble is not designed specifically to hunt for exoplanets (planets that orbit a star outside of our solar system) but it has made significant discoveries of dozens of far off worlds. It also holds the honour of capturing the first visible light of a planet around another star and confirmed that some unknown objects are in fact planets. HST has also been able to see atmospheres on distant worlds such as HD 209458 b. It analyzed it's light spectrum and determined that the atmosphere was comprised of sodium. It also has the capability to detect water, carbon dioxide, oxygen and carbon using this method. Hubble is to be replaced with the James Webb telescope late next year.

Kepler

Kepler launched in 2009 on board a Delta II rocket from Cape Canaveral. It's key objective is to survey our corner of the Milky Way for near Earth-sized planets orbiting within the habitable zone of their own sun.

The habitable zone is also known as the Goldilocks zone: it has to be not too hot, not too cold - just right - like the porridge in the tale of the Three Bears

Searching for exoplanets is not as straightforward as you might think: the light from the stars renders the planets invisible - imagine trying to look at a distant aeroplane or even a bird flying across the sun. In order for Kepler to detect exoplanets it uses what is known as the Transit Method. The level of light measured from a star dims by a fraction, indicating a mass passing in front of it. This mass is measured by how much the light dims, more for a large planet, less for a smaller one indicating it's size relative to the sun. It's orbit is calculated according to how long it takes to orbit the star.

To date the Kepler missions have discovered a staggering 5,016 candidates. Of these 2,483 are confirmed exoplanets. 21 of the confirmed planets are less than twice the size of Earth and lie within the Goldilocks zone. Given the right conditions for life to take hold any one of these could be home to an alien lifeform.

We must bear in mind however that Kepler's field of view is quite limited, able to scan just a fraction of the cosmos around us. It stands to reason that of all of it's discoveries to date, this pales into nothing when contemplating the size of the Milky Way. In the picture to the left you can see where our sun lies within our galaxy and Kepler's very narrow field of view.

The following photograph is called the eXtreme Deep Field (XDF) and is assembled from 10 years of Hubble's photographs from a very small section of the cosmos. This photograph alone contains an estimated 100,000 galaxies.

The current best guess is that there are between 100 - 200 billion galaxies. The Milky Way contains at least 100 thousand million stars. It stands to reason that many other galaxies have as many stars, each with any number of exoplanets orbiting them. Of these any number could lie within the habitable zone.

How many contain life?