The view from everywhere!

 kw: book reviews, nonfiction, astronomy

At its closest approach to Earth, Venus is about 38 million km away. Just before this or just after this (a few weeks), the planet is a bright crescent in the morning or evening sky, and its apparent diameter is just over one minute of arc (1/60th of a degree). Someone with keen eyesight would be able to tell it isn't an unresolved dot, and could see that it looks elongated. That's the closest we can come on Earth to seeing any body in the Solar System besides the Sun or the Moon as anything more than a tiny dot.

If any of the seven known planets of the Trappist-1 system is habitable, the sky would be quite a bit more interesting as the short year (19 days or less) transpired. Some of what we might see is described in the sixth chapter of Under Alien Skies: A Sightseer's Guide to the Universe by Philip Plait, PhD, an astronomer who lives in Colorado. Rather than repeat what he wrote, I analyzed the system a bit to provide this summary:

• The 7 planets are designated by letters from b to h.
• Their sizes are similar, ranging from 0.775 the size of Earth (h) to 1.13 (g).
• Planet d is the most likely to have liquid water on the surface, though it is thought to have little atmosphere, comparable to that on Mars.
• The star is about 12% the size of the Sun, and is a "cool" red dwarf, with a temperature of 2,550K (~2,275°C or ~4,065°F).
• The planets orbit between 1.73 million km and 9.26 million km from the star.
• By contrast, Mercury's orbit ranges from 47 million km to 70 million km from the Sun.

We see both the Sun and the Moon as having a size of 32 arcmin or arc minutes (0.53°) on the sky. Nearly every one of the seven planets in the Trappist system can be seen from any of the others as a disk, any time they are in the sky:

• From planet b, the nearest to the star, planet h, the farthest, appears as large as 4.5 arcmin at closest approach, a tiny but clear disk.
• When planet g is nearly opposite planet h, from g the other appears as small as 2 arcmin, barely discernable as "not a star". All other cases, the sizes appear larger, even quite large.
• From planet d, planet c appears as large as 50 arcmin, or 1.55 times the size of the Moon as seen from Earth.
• From planet d, planet e appears as large as 39 arcmin, or 1.2 times the size of the Moon as seen from Earth.

How does the star look? Its visible brightness is one hundred-thousandth that of the Sun, while it is 45 times closer to planet d, compared to Earth and the Sun. This works out to a visible brightness of 2%, compared to the Sun. This dim light is spread out over a disk more than five times the apparent size of the Sun. Also:

• From planet b, the star's apparent size is just over ten times that of the Sun to us.
• From planet h, the apparent size is about twice that of the Sun.

Finally, what is the color of the sky? As mentioned, so far, planet d is thought to have very little atmosphere (perhaps like Mars), and planets b and c probably have none (like Mercury); the other four planets may have atmospheres, but we don't know. Such a sky would likely be black, or nearly so. The top row of this diagram shows how the Trappist-1 star would appear, firstly from off-planet, and then under a sky as dense as Earth's atmosphere.

Trappist-1 is one of the reddest of red dwarf stars. It is slightly "cooler" than the filament of an incandescent bulb, so it appears orange from space. Under a clear noontime sky with no dust in the air, it would be just a little more reddish because of blue light scattered away to illuminate the sky. The physics of light scattering mean that the sky still would have a slightly blue color. However, if there were even a little dust in the air, the sky would be pinkish, or perhaps salmon-colored as the sky of Mars is due to fine dust that never settles, even in its thin atmosphere.

The other stars I compiled here show the star color from space, its reddened color seen through an atmosphere (this is why the Sun is called a "yellow-white star"; from Earth it seems so), and the color of a clear sky. The relative brightness of sky and star are certain to be quite different from what this diagram shows, depending on the density of the atmosphere. From atop a high mountain on Earth, the sky is much bluer than it could ever be when seen from sea level.

A few words about the four stars other than Trappist-1 and the Sun:

• Proxima Centauri is a red dwarf, known to have at least two planets. It is a little hotter than Trappist-1 and is the closest star to the Sun. Its visible brightness is quite a bit brighter than Trappist-1, but still a tiny fraction of the Sun's brightness. The noontime color temperature is a little more orange than its space appearance, though against a bluish sky it appears closer to bright yellow.
• Sirius is about twice the size of the Sun and is several thousand degrees hotter. A planet would have to be five times as far from it as Earth is from the Sun, to avoid all its water being boiled away.
• Rigel is one of the "knees" of the constellation Orion. It is a blue supergiant about twice as hot as the sun, and so bright that a planet would need to be about 350 times as far away as Earth is from the Sun, to be habitable. But this star won't last long. It will burn out or go supernova in a few million years.
• There are very few of the super-hot "O" type stars. One of four that is naked-eye visible from Earth is Alnitak, the leftmost star in Orion's belt. Such a star doesn't last long. They burn bright and exhaust their fuel quickly. Most of their "light" is ultraviolet and soft X-rays.

OK, that's a bunch of info about planets around other stars. Dr. Plait tells us what it's like to view the sky and space from the Moon, Mars, an asteroid, a comet, a moon of Saturn, and near the Pluto/Charon system. Then he takes us to the Trappist-1 system, and on to a planet orbiting two stars, into a globular cluster (imagine a sky with 100 times as many stars), and near enough to a nebula and a black hole or two to see their fireworks (but from a safe distance!).

I was particularly taken by the description of approaching Saturn. Passing the rings (which would take days at the "sane" speeds of "only" a few miles per second), seeing things like their "spokes" and "propellers", the moonlets and gaps…what a travelogue!

The author writes of the way the absence, or near-absence, of an atmosphere can trick the eye because there is no haziness to distant objects. Distances on the Moon or Mars or an airless asteroid would be very hard to judge. We learn how the different levels of gravity might feel on a different planet or satellite or asteroid…and how landing "on" an asteroid might just take you right inside it (oops!). He writes with compelling grace and subtle humor. I loved the book.