SCIENCE

The alien world is warmed by the light of a red dwarf star shining fewer than five light-years away. (Nat Geo News)

Use our study guide to learn more about alien worlds, habitable zones, and why liquid water is so important.

Teachers, scroll down for a quick list of key resources in our Teachers Toolkit.

Discussion Ideas

• The Nat Geo News article details the discovery of Proxima b, a newly confirmed exoplanet. What is an exoplanet?
  • An exoplanet is simply a planet outside our solar system, orbiting a star other than the sun. Click here to learn a little about what makes a planet.

• Hundreds of exoplanets have been identified. What makes Proxima b so special?
  • Three major characteristics make the discovery of Proxima b notable.
    • 1. It’s right next door! Proxima b orbits Proxima Centauri, the red dwarf star in the triple-star system known as Alpha Centauri. (The other stars in the system are the big primary, Alpha Centauri A, and its wallflower sister, the slightly less luminous Alpha Centauri B.) The stars in the Alpha Centauri system are the closest stars to the sun, about 4.37 light-years away. (That’s 40 trillion kilometers, or 25 trillion miles. For you astro-nerds out there, it’s about 1.3 parsecs.)
    • 2. It’s about the same size as Earth! Proxima b is likely just a little bigger than Earth in terms of mass (~1.25 times as massive as Earth) and radius (~1.1 Earth radii).
    • 3. It’s in the habitable zone! A habitable zone is a region around a star where conditions can support liquid water on the planet’s surface. Read all about water and habitability in this nice study guide.

• Astronomers caution that even though Proxima b is in a habitable zone, “that doesn’t mean it’s habitable.” Why not?
  • Proxima Centauri is nothing like the sun.
    • It’s less massive (about 12% the sun’s mass) but much more dense (about 40 times as dense as the sun).
    • It has a much stronger magnetic field—600 times stronger than the sun.
    • It emits mostly infrared radiation, but spits out hugely energetic flares, “potentially dealing a hostile blow to any life on an infant planet’s surface” or the planet’s atmosphere itself.
  • Proxima b may be tidally locked with its parent star, meaning the planet may not rotate on its own axis. This would mean the planet “keeps one face pointing toward its star at all times and one face gazing eternally into the cosmic night.” (If the Earth did not rotate, one-half would always be too hot to support life, and the other half would be frozen.) Astrobiologists think habitability on Proxima b may be confined to the region in between the two extreme areas (the terminator line) where the temperatures might allow liquid water to exist.
  • Proxima b is much closer to Proxima Centauri than Earth is to the sun. In fact, at only .05AU, Proxima b is closer to Proxima Centauri than Mercury is to the sun. (Proxima b’s “year”—the time it takes for the planet to revolve around its star—is only 11 days.) This puts Proxima b very close to those energetic flares and increases the likelihood of it being tidally locked.
  • Take heart: “All of these differences don’t prove that life could not have evolved on Proxima b, just that its story would be dramatically different from that of life on Earth.” Which would be awesome.

TEACHERS TOOLKIT

Nat Geo: Potentially Habitable Planet Found Orbiting Star Closest to Sun

Nat Geo: NASA Discovers New Batch of Earthlike Planets

European Southern Observatory: Planet Found in Habitable Zone Around Nearest Star

Nat Geo: What is Earth?

Wikipedia: Proxima Centauri (featured article)