Is there life elsewhere in the universe? This is the question everyone will come to at some stage in their life, but the answer may lie centuries away. For now, we can only attempt to find the trace of the key ingredients for life by examining exoplanets. One of our best tools for doing this, and for subsequently finding extraterrestrial life, is the Hubble Space telescope. But how does Hubble see into these planet’s atmospheres? How do scientists actually work out the real composition of planets?

Occasionally, a planet’s orbit takes it past its parent star, so that it aligns with Earth. When this happens, the light from the star passes through the planet’s atmosphere. This light can then be put into a spectrograph, where it is split up into its various wavelengths (or colours). Because some of the light has passed through the planet’s atmosphere, some of that light will have not reached us. Instead, it would have been absorbed by the various gases. The result of this is that it leaves what are called "absorption lines," and different elements leave behind different absorption lines, which astronomers can use to decide on the composition of the atmosphere.

Light Curve of a Planet Transiting Its Star via NASA/Ames

Alternatively, we can take the light from the star when the planet is passing behind it, and then the combination of both the planet and the star when the planet is in front of the star. This will leave two graphs which can be compared, and by taking out the features that only appear on combined spectrum, you are left with the chemical traces belonging only to that of the planet!

However, when we search for telltale signs of life, we are only looking for what we know life on Earth needs. This is because we don’t know how many other ways life can evolve. However, we do believe in four fundamental factors needed for the development of life. Basically, a solvent, warmth, a ‘construction material’, and protection.

Here on Earth, our solvent is water. Seemingly abundant, water is used to dissolve molecules and provide a basis for all kinds of biological processes that are key to life being able to survive. We’ve also found that its constituent’s, hydrogen and oxygen, are not rare throughout the universe, and so there is a high chance that life is based upon water elsewhere.

But, a solvent could be any other liquid that is in plentiful supply.

Maybe somewhere out there is a planet cold enough for a small molecule (such as methane) to exist as a liquid. If so, this could act as water does on Earth. In fact, we’ve found what are thought to be oceans of liquid methane and ethane on Saturn’s moon Titan! On cosmological terms, this could mean that life beyond Earth could be literally on our doorstep...

Water only exists as a liquid on Earth because we sit the perfect distance from the Sun– not too far away and not too close so that we receive just the right amount of energy so liquid water can exist (this is known as the ‘Goldilocks’ or 'habitable' zone.) This zone will change from system to system because stars have different sizes, compositions, and ages. A planet orbiting a cooler star might need to be close, whereas a large, hot star will mean the planet must be further away.

Orbiting in the Habitable Zone of Two Suns via NASA/JPL-Caltech/T. Pyle

However, depending on the solvent, a planet could be far away or close. If the freezing point of the molecule is low, the planet could be far away and still support a liquid.

For a liquid to remain on a planet’s surface (even if it’s the right distance away) it needs enough atmospheric pressure to keep it there. Without a thick enough atmosphere, the liquid may vaporise into space. Here on Earth, we are far enough away so that our atmosphere is not blasted away by the solar wind. This means that we are protected from radiation, provided with something to breathe, and the temperature is stable enough for life to exist comfortably. But an alien planet might not need this. It could be covered in thick ice, holding a liquid ocean below.

For example, in our Solar System Europa is believed to be in this state; its crust of ice is thought to be covering a huge body of water below.

Finally, for life to literally build itself, you need something for it to be based upon. For example, here on Earth, life forms are built on carbon. But life somewhere else could choose a different element that is maybe in a greater supply. Perhaps they might prefer a silicon base.

Maybe in the future the James Webb Space telescope will peer deep into space so that one day we do find strong enough evidence to suggest we aren’t alone. Although, it is worth stopping and thinking briefly about our own Solar System. Maybe the other planets are not suitable for life, but as I mentioned above, some of their moons might just hold the right conditions for a completely different form of life. It is a real possibility that Europa’s ocean is an underwater oasis. If we found life on two separate orbital bodies in the Solar System, this place would be something very special indeed!

September 2009 artist conception of the James Webb Space Telescope. Credit: NASA

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