Search for Exoplanets and Life Beyond Earth Begins.

As long as we have gazed up into the night sky, we have wanted to know the answer to one question: Are we alone in the universe?

The question goes back as far as humanity. For centuries, it was a question that remained with philosophers, dreamers, and fictional stories. Now, as a result of incredible advancements in astronomy, it has shifted more from fiction into science.

Exoplanets, planets far away from our solar system that orbit a star (or stars), lie at the forefront of the search for life beyond Earth. With thousands of these types of planets presently discovered, we are beginning to realize that Earth-like planets may not be so rare after all.

The Moment That Changed Everything: The Discovery of 51 Pegasi b

Before 1995, we had not even proven whether the existence of planets outside of our solar system could be possible. Scientists assumed, but could not prove, that planets did exist, but no one had actually observed one. Then came 51 Pegasi b, a massive planet orbiting a star about 50 light years away. This planet was discovered by astronomers Michel Mayor and Didier Queloz and overnight astronomy changed.

This planet was nothing like Earth, it is a massive gas giant, similar to Jupiter. But the discovery demonstrated what we were hopeful was correct – that our solar system was not unique.

Now fast-forward to the present, and space telescopes like NASA’s Kepler mission and the TESS mission have captured over 5,000 exoplanets – with some of those being in habitable zones of their star, else known as the Goldilocks zone – where temperatures could be just right for liquid water, which as far as we know is required for life.

How Do We Spot Planets That Far Away?

It’s easy to lose sight of how challenging this work is. Planets are small compared to the stars, and light years away. Astronomers can’t simply point a telescope and obtain a picture of the exoplanet. They have to use indirect evidence.

One of the most effective means of accomplishing this is the transit method. If a planet passes in front of its star, the light from the star dims a tiny bit. That is why we know about thousands of planets (including hot Jupiters, small rocky planets, and planets with bizarre inclination to their orbits).

Alternatively, you have the radial velocity method, which looks for tiny shifts in the position of the star due to the reaction of the gravity from the orbiting planets. This minuscule movement will cause Doppler shifts of light coming from the star, which can be measured.

Both of these methods allow us to observe star systems that are at once totally, wonderfully different, and sometimes, slightly similar to our own

What Really Makes a Planet Habitable?

The discovery of a planet located within the habitable zone is thrilling, but it is only the beginning of our investigations. Many other factors matter, such as the following:

Is the planet’s atmosphere correct to provide protection and warmth?

Is the planetary mass correct to maintain an atmosphere?

Does the planet have magnetic fields to protect from radiation?

Is there geological activity to recycle vital chemicals?

For example planets like Kepler-186f and TRAPPIS-1e. Both exist in the Goldilocks zone, but without knowing their surfaces and their atmospheres we cannot assert whether they could actually host life. A planet could be perfectly situated in the habitable zone and still be completely uninhabitable.

What Might Life Look Like on Another World?

This is where things get fun and weird. Life on Earth is endlessly adaptable. Microbial life thrives in boiling hot volcanic vents, freezing polar ice, and even radioactive waste. Complex organisms, from whales to ants, have filled every imaginable niche.

So, what about life elsewhere? On tidally locked planets, one side always facing the star with the other locked in complete darkness, any form of life would have to evolve in order to survive wildly varying environments; perhaps creatures with gigantic wings to navigate a thick atmosphere, or creatures with heat-sensitive vision that could detect faint starlight.

And life may not even have to be carbon-based like us! Scientists have admonished against what they term carbon chauvinism or the belief that any life must be “like us”. They speculate about life based on silicon rather than carbon, with life forms that could survive in hotter environments. Or life forms that use either methane or ammonia as a solvent instead of water.

We can only speculate, but there is enough variability to make us rethink just what life can be.

How Are Scientists Actually Searching for Signs of Life?

Currently, the best method that we have to do this, is by investigating the atmospheres of exoplanets. When a planet transits its star, a small lens of that starlight goes through its atmosphere. That light’s spectra can reveal to us a bit about the chemicals present in the atmosphere.

The James Webb Space Telescope is already conducting this work and looking for biosignatures gases such as methane, oxygen, and water vapor. All things that indicate life on Earth.

In the future the European Space Agency’s ARIEL mission and NASA’s Habitable Worlds Observatory mission will take these studies a step further. They really have a very long term simple but ultimately stunning goal—find indications of life on worlds other than our own.