Under Alien Skies

Philip Plait

★★★★★

A fun look at what it would be like to visit other planets or star systems, weaving together sci-fi scenarios, the science behind them, and the history of how those discoveries were made.

It starts with worlds we know the most about – our moon and Mars, where we have plenty of direct measurements and photos from the surface – and works its way out through asteroids, gas giants and their moons, and finally Pluto.

The second half of the book delves into more speculative situations. Types of places we know exist, like star clusters and nebulas and different types of stars. Plait links these to specific locations where possible. We know a system of planets exists around the red dwarf star TRAPPIST-1, for instance, and we have a rough idea of how big, how far, and how fast the planets are that we’ve spotted so far. From there he imagines: if one of the three planets in its habitable zone is enough like Earth to visit, what would it be like to experience a red sun, multiple planets with visible phases, and so forth.

There’s some repetition, but I think part of that is trying to make each chapter stand on its own in case you want to jump ahead to, say, visiting a black hole. (You do not survive the encounter.) And while there’s a lot that I knew already from following space exploration from a distance, there are newer discoveries I’d missed, things I’d known pieces of but never really connected, and a deep dive into topics I’d only skimmed the surface of before.

Also, there’s an insert in the middle of the book with photos. I didn’t notice it until I got there, so I’d been looking up NASA and ESA pictures of the specific comets and asteroids on my phone while reading!

Some fun facts:

1. Martian sunsets are blue!
2. The biggest asteroids are like miniature planets. The smallest are like bags of rocks without the bag. (Or a really unpleasant 3D ball pit.)
3. Saturn’s rings are surprisingly thin. Like, millions of miles across but only 40 feet thick in places.
4. The Orion Nebula is a bubble at the near edge of an even bigger cloud of dust. We can only see it because it broke through on our side!
5. It’s entirely possible for a planet to have a stable orbit around one star of a binary pair – or both! Sunsets on a world like Tatooine would actually look like they do in the movies!
6. I’m still amazed at how much we managed to get from a single high-speed fly-by past Pluto.

Phil Plait writes the Bad Astronomy Newsletter.