20 years ago, the 8 planets of our solar system were the only ones that were known. Today scientists have discovered hundreds of exoplanets - planets orbiting other stars in our galaxy - and expect to find millions more. Based on what has been found so far scientists predict that there’s on average at least one planet orbiting every star in the Milky Way.

In August 2016, a new exoplanet was discovered around a star called Proxima Centauri. At just 4.25 light years away, this is the closest star to the Sun. Articles have described this planet as potentially Earth-like… but is it really? What does “Earth-like” even mean?  If you had to summarize the most important characteristics of Earth what would you choose?

Perhaps Earth-like can mean a planet that is made of the same materials as Earth, which is mostly rocks and heavy metals like iron. Scientists call these types of planets “terrestrial” worlds. The majority of the exoplanets we have found so far are gaseous, like Jupiter and Saturn, and are unlikely to host life. Looking at our neighbors in the solar system, Mercury, Venus, and Mars are all terrestrial worlds like the Earth but that is around where any similarities end. So there’s more to it than that.

Maybe to be Earth-like an exoplanet needs to have liquid water on its surface. This means that a planet needs to be at a distance from its star where it is not too hot and not too cold but just the right temperature for any water that may exist on there to be liquid. Looking at our own solar system, Earth rests comfortably in this so-called “goldilocks zone” and we certainly have a lot of liquid water here. But Earth’s moon is also in the goldilocks zone, and while scientists have found frozen ice on the poles of the moon there is no liquid water to speak of.

The moon is a much colder place than Earth because it lacks an atmosphere to trap in the heat, so Earth-like could mean worlds that have atmospheres to warm the planet.  Venus has an atmosphere too, but it is choked with greenhouse gases, sending temperatures soaring over 900 degrees F.

In reality, to be Earth-like is to be all these things and more. Change one characteristic of our planet, and it would be a very different place than the one we know so well.

Is the planet orbiting Proxima Centauri similar to Earth?

Even though this is the closest exoplanet to the Earth, it is still so far away that we can’t see it directly with telescopes. With clever techniques, we can learn some important details about this world. We know that this exoplanet is likely 1.3 - 3 times the mass of the Earth. This could mean that it is rocky, but without knowing its radius, its composition remains unknown.

We know that this planet orbits its star at a distance of 4.35 million miles - only 5% the Earth-Sun distance. We know that Proxima Centauri is a much cooler star than our sun meaning it's goldilocks zone is closer in. This planet is technically within the goldilocks zone but it lies so close to its star that it may be tidally locked, with the one side permanently in daytime and the other permanently in darkness. Also, being so close to its star, this planet is potentially bombarded with lethal amounts of radiation.

With these considerations, it seems unlikely that this closest exoplanet would be anything like Earth. Regardless of whether or not we’ve found an Earth-like world, all exoplanet discoveries are significant in that they show us just how common planets really are. It also reminds us to take care of our special planet that has so many things that make it a perfect place for life.

By Haley Sharp.  Visit the Science Factory this fall to explore our new exhibition A View From Space and gain a whole new perspective of our unique planet Earth. Also - don’t miss our featured planetarium show We Are Aliens that highlights the search for life beyond Earth - it’s a wonderful, immersive experience for all ages!