#planetary systems

100 Planetary Nebulae captured by Hubble Space Telescope.

By Cora A. Harris

A candidate short-period sub-Earth orbiting Proxima Centauri

Astronomers have discovered a third planet, about twice the size of Mars, orbiting Earth's nearest neighbor.

Alpha Centauri is approximately 4.37 light years from our Sun. It is a trinary star: a system with three stars orbiting each other. The red dwarf Proxima Centauri is the closest of the three stars, approximately 4.2 light years away from us.

Astronomers have previously discovered two other planets orbiting Proxima Centauri: Proxima b, discovered in 2016, and Proxima c, discovered in 2020. The new planet, Proxima d, was first observed in 2020, but it was confirmed-and-formally-published just last week.

Proxima d has only one-fourth the mass of Earth, or around twice the mass of Mars. It is also much closer to Proxima Centauri than Earth is to Sol, completing an orbit in just over five Earth days. Proxima c is about seven times as massive as Earth. It is much further away from its star, taking around 5¼ Earth years to complete a single orbit.

Planets revolving around other stars is not just science fiction anymore! About 4000 planets have been discovered outside our solar system and the number keeps growing. They are called exoplanets! Here’s some info on the kind of planets we have discovered so far. These are names that astronomers and planetary scientists often use to categorize these planets -

Super-Earth : One of the most common type of exoplanet we see is a giant oversized Earth. This does not mean the planet necessary has life or a means to support life! What that means is the planet’s mass is comparable to Earth but it’s size is much much bigger, about 10 to 15 times bigger! Super-Earth’s can be like our own planet, lots of land, thin atmosphere, maybe liquid water? But they can also look like the gas planets in our solar system, like Jupiter or Neptune, with a thick, thick gaseous envelope around what we think a solid core.

Mini-Neptune : Another common type of planet we see is the mini-Neptune! It’s mass is comparable to Neptune, but it’s size is much smaller. Remember, Neptune is larger than the Earth so even though they are called ‘mini’, they can be pretty big, perhaps twice the size of Earth or more! They look a lot like our own Neptune, a gas planet with a very thick atmosphere around a solid rocky core.

Hot-Jupiter : This is also a popular type of planet we see, perhaps because it is big! As big as the largest planet in our solar system, Jupiter! They are usually very close to their host star and can get very very hot. How did it get so close to it’s star and why is our Jupiter so far away? We don’t exactly know but one thing is for sure, these systems can be very different from our own and there are so many new worlds to see for us!

Sub-Earth : These are small, smaller than the Earth and very very hard to see. That’s why we haven’t detected many of them but they do exist! Smol little balls zooming about their stars. Examples of sub-Earths in our solar system are Mercury and Mars, since both are smaller than Earth is size.

These are categories based on a planet’s mass and radius. If you were to categorize on other physical properties, you can also have terrestrial planets (a rock, thin or no atmosphere), gas planets (thick atmosphere), aquatic planets (planets full of liquid water), frozen planet (too cold for water to stay liquid), and more! Those categories become more important if you were looking at a planet’s habitability.

If any of this intrigues you and you want to chat more, feel free to talk to me! I work on smashing mini-Neptune/super-Earth category planets. Also message me if you have questions or if anything was unclear!

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