#trappist-1f

So, why does it make it to my favorites list?

Pluto is geologically active (like Earth - it has its own geology happening on its surface, like volcanoes, tectonics, and erosion), a reality many did not expect until the New Horizons spacecraft started returning close-up pictures of this distant world. The smoothness of Pluto's signature heart-shaped feature (particularly the left side) is a dead giveaway that geological activity is happening on its surface. Worlds where there's no active geology are covered in craters everywhere, not just in some places, and the more recently it's been active the fewer craters there are per area. So.... Pluto is a 'living world' with enough energy inside it to make its insides move.

Cool.

It might be the close orbit of its moon Charon causing it - the shifting gravity stirring up Pluto's innards (and Charon's!), causing friction, and thus heat.

Either way, there's enough geological activity on Pluto to create these gorgeous mountains:

It has seasons during which the frozen nitrogen at its surface becomes warm enough to create mists - nitrogen fog. Imagine if Earth were cold enough that our very air would turn into blocks of ice that create white vapor - kind of like dry ice does.

Pluto is fascinating: a world that defied our expectations. I love it.

4. Trappist-1f

This artist's rendition shows what exoplanet Trappist-1f might look like if it has liquid surface water. It's too small and too far away to get an actual picture, but the evidence shows this planet exists in the habitable zone of its star.

The Trappist-1 star system lies about 40 light-years from Earth (very nearby in the grand scheme of things). Within it, seven planets orbit a small, cool red dwarf star (that's Trappist-1). The planets don't have 'given' names like Pluto or Earth. They're assigned letters b-h, with b the closest and h the furthest from the star. Planets e, f, and g are all in the star's habitable zone, which means they could have liquid water on their surfaces.

So, why did I specifically pick Trappist-1f? It's the planet most perfectly centered in the habitable zone, and its properties are all quite similar to Earth's: it's radius, mass, density, and surface gravity. In order to be hospitable to life as we know it, it's important for a planet to be able to keep itself warm (not just from the light from its Sun - it needs its own, internal heat source, too, to stay geologically active and produce a protective magnetic field - this keeps the atmosphere stable rather than letting its sun's radiative wind blow lots of it away, and also protects life on its surface from that same radiation that would split our complex organic components apart).

Trappist-1f is extremely close to its star, but the star is very cool, so it's unlikely to be very hot. It only takes the planet 9.2 days to orbit the star once. (Yes, you read that right - Trappist-1f's year is only 9.2 Earth days long! If you lived there, I bet you'd find some other way to celebrate birthdays. You'd be having a party just about every week).

I'm all for finding planets that might have water, and therefore might have life... or might be good for us to live on, some day.

The other cool thing about its place is ALL the planets are quite close to the star, so they'd appear huge in the sky regardless of which of the other planets you were standing on. This is a cool, artistic interpretation of that:

3. Proxima Centauri b

Speaking of places where we might be able to live...

...welcome to Proxima Centauri b - the nearest exoplanet to Earth... and also in its star's habitable zone.

Proxima Centauri b is a small, cool, red dwarf star part of the Alpha Centauri system, a triple-star system consisting of two brighter stars (Alpha Centauri A and Alpha Centauri B, both similar to the Sun) which are close to each other, and the cool red dwarf Proxima Centauri b, still orbiting the others but much farther away.

And around that small cool dwarf star is a planet not too different from Earth.

I can't stress enough how incredible that is.

This is the very nearest star system to our own, only four light-years from here.

And in it, there is already a possible stepping stone for us to move outward into the Cosmos. And not just a stepping stone, but possibly somewhere habitable.

Wow.

We need to learn more about this planet. It may or may not be hospitable. It may or may not have an atmosphere. We just don't know yet. But it's hope. It's one of many signs that the galaxy is absolutely littered with planets, and there may be many worlds like ours.

2. Jupiter

This image taken by the Juno Probe shows a lot of what makes Jupiter awe-inspiring: roiling masses of hydrogen and helium gas with traces of methane, ammonia, silica, and sulfur creating myriad colors in its clouds, storms which could swallow several Earths whole, and of course its huge mass which make it a gravitational well, holding on to a minimum of 80 moons. The huge shadow of its moon Ganymede is visible on the left-hand side in the picture.

First of all, Jupiter is, quite simply, beautiful. Just look at it.

All those colors, the swirls, its absurdly fast rotation (a 10-hour period!) pulling its clouds out into these thin bands. It's so striking. Add to that the shadows of its many moons crossing its surface, and you have a recipe for endless visual fascination.

But that alone isn't enough to make it one of my favorites.

I love Jupiter because it protects us.

Jupiter's gravity is so strong it tends to divert and even capture many objects which hurtle inward toward the Earth and the other inner planets, meaning fewer things actually hit the Earth than they otherwise would. Jupiter is the batter at plate, and we're the catcher. We really don't want any of those strikes thrown to cross the plate.

Thank-you, Jupiter.

I also love Jupiter because it provides a home for other worlds - the moons Europa and Ganymede in particular - which might be places to find life. Europa's the far more likely of the two, but I won't belabor that point. Suffice to say, Jupiter's gravity provides the energy that keeps Europa active, with an underground ocean, potentially a safe haven for organic life. I hope to live long enough to find out.

Mars

My pattern by this point is probably clear. I'm rooting for life and for places we could move to one day. The absolute tippy-top of that list is Mars.

This image shows many features of Mars which show past geological activity and the outline of what once was the shore of a truly vast ocean.

The more missions we send to Mars, the more likely we realize it is that Mars could once have had life on its surface, and could even harbor it right now below ground.

You can see the outlines of several ancient volcanoes in the left-hand side of this picture, and a truly massive TEAR in the planet's surface front-and-center. That thing is as long as the entire continent of North America. There are plenty of craters, which means it hasn't been geologically active ( at least not much) in a long time, but the signs of past activity on its surface are clear. It has dry riverbeds. Dry ocean beds. Dry lake beds. Dry glacial footprints. Ancient volcanoes. The robots we've landed to explore the surface have found clear, smoking-gun type evidence of past running water in these places (clay is a big one, and there's hematite, too), and the basic components for life are there. The one thing we haven't found yet is life itself.

We now understand Mars lost its once-watery exterior thanks to it being too small. Its interior cooled off, slowing and nearly stopping geological activity entirely, and stopping Mars' rotation from creating a magnetic dynamo like Earth has - so Mars lost its protective magnetic bubble stopping the Sun's radiation from striking its atmosphere and surface. The Sun basically blew Mars' air away into space, and irradiated its top soil. So.. the top two inches of the soil are entirely barren.

But RIGHT below that.... we have found water ice. And below that... there are underground rivers. Sinkholes and orbital measurements of density have shown that.

We might be able to send people there and have them survive underground. We'd need some way to deal with Mars' lost protective bubble at some point if we wanted to make the surface habitable. Dome cities might be okay, though those always creep me out. I just picture something puncturing it and causing problems. But.... it's so close. And it might have life, or once had it.