For as long as humans have known that
the stars in the sky are other suns,
they have been asking themselves:
Are these suns orbited by other planets?
Is there life out there?
Are we alone in the Universe?
Since the discovery of the first exoplanet
only 25 years ago,
Hubble is among the many instruments
trying to answer these questions.
And astronomers are using it
to hunt for life on other worlds.
Hubble, exoplanets and the hunt for life
25 years have passed since
the first exoplanet was discovered
and eight years since Hubble
made its first direct image of an alien world.
While in the beginning we knew of only a few,
very massive exoplanets
— most often close to their parent star —
today we know of more than 3000.
They are of different sizes
and orbit various types of stars
at different distances.
But one thing hasn’t been found so far:
proof of life.
Despite all the progress made in recent decades,
the hunt for exoplanets is still a challenging one.
They hide in the shadows,
giving off no light of their own.
Any starlight they reflect is swamped
by the overwhelming brilliance of their parent star.
This makes it especially difficult
to find Earth-sized planets
in the so-called habitable zone
the region around the star
where liquid water
can exist on the surface of a planet.
And water is essential to all life as we know it.
Water only remains liquid
within a narrow range of temperatures.
If a planet orbits too close to a star,
the water evaporates.
Too far away
and it will freeze.
The thin band between these extremes,
the habitable zone,
represents the most probable abode of alien life.
So if we know where to look,
how will we know if alien life exists on a faraway planet?
No currently available
or planned telescope
is able to resolve the surface of a planet.
But radio telescopes keep listening
for messages from other civilisations,
in the hope that they are as curious as we are.
We could also be lucky
and find signs of advanced civilisations.
Something like a ringworld,
a giant artificial structure built around a star.
But the chances of such a discovery are rather low.
In their search for life,
optical and infrared telescopes
focus on the analysis of exoplanet atmospheres.
Life is capable of changing the composition
of a planet’s atmosphere on a grand scale.
The oxygen in the Earth’s atmosphere
was released billions of years ago
by microscopic organisms.
If a similar process has occurred on other worlds,
we may be able to detect it in the spectrum of the planet.
From late 2016,
European astronomers will use almost five hundred orbits of Hubble
— corresponding to just over a month of observing time —
to make a detailed study of the atmosphere
of hundreds of already known exoplanets.
Hubble has studied alien atmospheres before,
but this programme offers an unrivalled chance
to learn more about them than ever before.
The data we gather in the next months from Hubble
will be a fundamental database for further studies.
And with the power of the upcoming next generation of telescopes
in space and on the ground,
astronomers may be closer than ever before
to discovering life elsewhere in the Universe.