Home Alone? Universe Edition

In 1992, scientists made a groundbreaking discovery. They captured the existence of a planet outside the Solar System, an exoplanet. To determine whether life could exist beyond Earth, research began. Currently, more than 6,000 exoplanets have been documented, most of which are in the Milky Way. Still, astrophysicists are confident there are way more than we can currently see.

Exoplanet Proxima Centauri b (closest exoplanet to Earth):

artist's conception image copyright and credit to ESO/M. Kornmesse

How are exoplanets like

Exoplanets can take up a significant branch of appearances, some similar to Mercury, others to Neptune, and some nothing like any planet in our Solar System. There are four main types: Gas giants, Neptunian planets, terrestrial planets, and super-Earths. 

Gas giants are at least as large as Saturn, though some can be much larger. Neptunian planets are similar in size to Neptune or Uranus and have rocky cores and outer atmospheres composed of hydrogen and helium. Terrestrial planets are Earth-sized or smaller, composed of rock, silicate, water, or carbon. Super-Earths are larger than Earth, but lighter than Neptune, and may or may not have an atmosphere. 

How do we find them

Can you see it with your usual home telescope? No. Exoplanets are far too small to be directly seen. To overcome that issue, scientists use other methods, the most common being transit and radial velocity. Transit is used when a planet passes directly “in front” of the star it orbits (from our perspective). The detection of some of the star's light being blocked is enough of a clue to raise suspicion of the existence of an exoplanet. Radial velocity concerns the planet's effect on the star's light. The planet's gravitational pull affects the star's light spectrum. So, when an exoplanet orbits a star, it changes the color of the light received by astronomers.

Why study exoplanets

The discovery of exoplanets has dramatically expanded our understanding of planets, from their formation to their potential to host life. We haven't yet found signs of life in any of them. Still, that discovery would drastically change our perception of the future and of our past. So, keeping that study alive is essential, even more so nowadays, given the climate conditions we face.

How can we know if a planet has the conditions to hold life? 

First, an exoplanet must be located in a "habitable zone”, or Goldilocks zone: the region where water can remain in its liquid state in the planet's atmosphere. It has all to do with being placed at an adequate distance from its orbiting star. That, however, isn't enough to hold life. Most planets are habitable because of extreme conditions beyond Earth's – temperatures and atmospheres are examples. Not only that, but rotation and radiation from stars also play a substantial role in habitability. In summary, the possibility of raising life on other planets remains a challenge for many scientists due to the stringent requirements.

Related Links:

Nasa: https://science.nasa.gov/exoplanets/

Planetary.org: https://www.planetary.org/worlds/exoplanets

Uchicago: https://news.uchicago.edu/explainer/exoplanets-explained