Scientific World Building

In hard science fiction, many authors choose to describe characters who come from (or journey to) worlds surrounding distant stars.  In the early days of science fiction, authors often chose stars whose names were well known:  Sirius[1], Alpha Centauri[2], Arcturus[3], Betelgeuse[4], Altair[5], and the like.  As scientific knowledge has expanded, however, writers have needed to increase the level of detail with which they research stars for their planets.  Fortunately, we have the internet to help us with this research.  Here a a few things to consider (and links you can use) when siting your next habitable planet.

Starting Point

There are billions of stars in our galaxy, but most people only know of a few. How can we learn about some of the others from which we might select? One source of data is Hipparcos, a satellite launched in 1989 by the European Space Agency. During its five years in orbit, Hipparcos collected data on more than one hundred thousand stars. This data is freely availalble online, and some sites provide the ability to search the Hipparcos database for stars in a particular region of the sky.

If you would rather start with known planets, that data is available online, too.  The University of Puerto Rico, Arecibo has an interesting database of known exoplanets, including some information on potentially habitable planets.  The SIMBAD catalog is another useful tool for researching known stars — once you know the name of a star from Hipparcos, you can get its name in other catalogs, to do further research.

Spectral Class

stars by spectral class

The most readily visible attribute of a distant star is its color. Decades ago, this information was used to group stars into spectral classes. Although golden-age SF writers seem to favor the brightest stars in the sky, writers in the 70’s and 80’s more frequently selected yellow (G-class) stars like our own sun. Alpha Centauri A, and Tau Ceti have been popular choices, though both are problematic for other reasons (see below).

For those with a background in chemistry, spectral class raises some interesting questions about the impact on life.  M-class stars produce less ultraviolet (UV) light than our sun does.  How would this effect photosynthesis?  A, B, and O-class stars produce far more energetic X-rays and gamma rays than our sun.  How would this affect mutation rates?  This sort of thing could be fodder for some interesting stories, I think.

Multiple Star Systems

While Alpha Centari A is a nice yellow star, it still has problems due to the proximity of its partner star, Alpha Centauri B. Centauri B is a K1-class star, which makes it somewhat smaller than our sun, and it orbits at a distance between 11 and 30AU from Centauri A. When the stars are close to one another, there is a significant chance that B’s gravity might disrupt the orbit of any planets in the habitable range of A. Even if this didn’t happen, there is also the concern that the periodic approach of B might heat up a planet in the habitable zone, making life unpleasant.

Personally, I try to avoid star systems with multiple stars — the “safe” ones probably have the partner so far away that they look like nothing more than a bright star — but some really great stories have been born from smart people thinking about life on a planet with more than one sun.

Variability

Even if you want to choose a solitary, ordinary yellow (G) or orange (K) star, there are a lot of choices available.  How do you decide?  A good place to start might be HabCat, a list of stars on the PHL site listing the stars they consider most likely to harbor habitable planets. One of the key characteristics in this decision-making process is a star’s variability: our sun is pretty stable, but some stars grow brighter or dimmer on an irregular basis. Planets near a variable star are at high risk of freezing or burning up.  Once again, it could make for an interesting setting for your story.

Metallicity

One assumption behind the HabCat list is that a star needs to have a certain amount of heavy, metallic elements nearby for rocky planets like earth to form.  We can’t necessarily see these planets yet, but we can measure the amount of metals in a star by looking at its spectral lines.  Tau Ceti and another nearby star, Epsilon Eridani, both seem to be metal-deficient.

More recent studies of Epsilon Eridani seem to indicate that it has at least one large rocky planet orbiting there, so the lack of metallicity for dim stars may not be as prohibitive as once was thought.  If you’re designing a world, however, you might want to consider what it would mean for life if (for example) potassium, calcium, or iron were hard to come by.

Proximity

If your story includes space travel to a distant star, you will either need to imagine faster-than-light travel or take into account the massive distances involved.  As it happens, parallax data from the Hipparcos database can be used to calculate how far away a given star might be.  For example, a star like HD-113576, with a parallax of 112.8 milliarcseconds, is 28.9 light years distant.  HD-17511, however, has a parallax of only 10, which places it 326 light years away.  If you’re travelling at “only” 9/10ths the speed of light, that can make a difference.

Takeaway

If there is one thing I learned from browsing Hipparcos, there are a lot of stars out there that might support life. With a little bit of work, we can go beyond the traditional choices and place our planets around real stars where no writer has gone before.

[1] Voltaire, in his early SF tale Micromegas, describes his protagonist as having come from Sirius.  Larry Niven’s planet Jinx is also located there.
[2] Philip K. Dick (Clans of the Alphane Moon), Arthur C. Clarke (The Songs of Distant Earth), and Larry Niven (Wunderland in his Known Space stories) all placed habitable planets around Alpha Centauri.
[3] Arcturus is featured as a planet name in Asimov’s Foundation trilogy, and David Lindsay placed his planet Tormance there in his novel A Voyage to Arcturus.  You may debate whether or not Asimov intended Arcturus the planet to orbit the star of the same name, however.
[4] Much of Planet of the Apes takes place in a planet near Betelgeuse.
[5] SF classic Forbidden Planet takes place on Altair IV, and colonies near Altair feature in several Star Trek episodes.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s