How we conceive of distant worlds is important. After all, we want to be scientifically accurate even as we deal with subjects that fire the public imagination. Thinking about planets in the habitable zones of other suns invariably makes us think of ‘Earth 2.0’ and the prospect of green and blue planets filled with life. But each situation will be different, which is part of the great fascination of this quest. Billions and billions of worlds, each of them sui generis.


Science fiction has offered us glimpses of many worlds tantalizingly like the Earth but in some major respect different. Here, for example, is a prose description of a planet circling the star 82 Eridani, as envisioned by Stephen Baxter in his 2011 novel Ark. We are looking at it from the starship that has taken a band of colonists/refugees from a drowning Earth to what could be their new home:

A big strip of land stretching north to south across the equator was “the Belt,” a kind of elderly Norway with deep-cut fjords incising thousands of kilometers of coastline. The northern half of the Belt was currently ice-free, but its southern half, stretching into the realm of shadow, was icebound, and snow patches reached as far north as the equator. Sprawling across a good portion of the eastern hemisphere was the roughly circular continent they called “the Frisbee,” a mass of rust red broken by the intense blue of lakes and lined by eroded mountains. Its center was dominated by a huge structure, a mountain with a base hundreds of kilometers across, and a fractured caldera at the top. The mount was so like Olympus Mons on Mars that giving it the same name had been unavoidable, and it so dominated the overall profile of the continent, giving it an immense but shallow bulge, that the nickname “Frisbee” was a good fit. Then, to the west of the Belt, an archipelago sprawled, a widespread group of islands, some as large as Britain or New Zealand, that they called “the Scatter.” There was one more continent at the south pole, currently plunged in darkness and buried under hundreds of meters of winter snow, called “the Cap.” The world ocean itself had no name yet; the seas could be named when they were ready to go sailing on them…

Image: An artist’s concept of a habitable zone world, in this case Kepler-69c. This image is, of course, based on an actual Kepler discovery, though like Baxter’s science fictional description, it has to substitute imagination for detailed data. Credit: Ames Research Center/NASA, JPL-Caltech.

Baxter’s world is fascinating, a place the colonists assume is Earth 2.0 until they take a closer look. For one thing, there’s little tectonic activity here, so the kind of geological and biological cycling we take for granted on Earth has been, over the eons, sharply reduced. But the real showstopper is the planet’s obliquity, interesting to note in light of the University of Washington work on axial tilt that we looked at on Monday. This world around 82 Eridani shows an obliquity of ninety degrees — compare that to Earth’s 23.5 degrees. In other words, each part of the planet except for a band along the equator will suffer through months of perpetual darkness, then perpetual light.

Land and colonize such a world or press on for another? I won’t give away that decision, which Baxter handles in a believable and interesting way. But as we saw yesterday, there are models now emerging that show such a planet might make itself habitable by never developing truly global ice. In any case, imagine what life would be like on such a world, and ask yourself whether humans could adapt to it. The guess here is that they could, but the impetus for developing a migratory pattern of development would be profound.

The Kepler-186f Image

Spurring these thoughts was an email from Thomas Barclay, a research scientist working on the Kepler mission at NASA Ames. Tom writes the excellent Planet Hunter blog, which he used several days ago to explain How we designed the Kepler-186f artists concept image that I wrote about on Monday. I seldom post the same image several days running, but today is an exception since I want to relate that image to the entire issue of how we visualize alien planets. Here it is again:


As I mentioned on Monday, this view — created by Tim Pyle and Robert Hurt (JPL/Caltech) — is a splendid piece of work, but you’ll recall that I wondered whether it wasn’t a bit too realistic, given that the public audience contains many who would assume we actually have this level of detailed information about the planet. The flip side of that question is to note how much care went into the image and what decisions were made given that we really know little beyond the size of the planet, the size and temperature of the star, and the distance between planet and star.

What I hadn’t really noticed was the star, Kepler-186, itself. It’s a red dwarf, but as you look at the image, you see that it’s much brighter than we might expect. What we know about Kepler-186 is that its temperature is about 3800 Kelvin. Now if you go to work on the spectrum of various star types and study the response of the human eye — check What color are the stars? for more — you’ll find that in the absence of any atmosphere, Kepler-186 would be yellow/orange in color. Tom writes that the team chose to make it a bit more orange in this image that it would actually appear to the eye, to get across the fact that the star is not truly like our G-class Sun.

Now look at the planet itself, which shows continents that are yellow and oceans in blue/grey. The ice caps as well as the clouds have an orange hue. Why these choices? Let me quote Tom on this:

This star emits very little blue light which we represented by making the sea a dull grey/blue color. Ice and clouds Mie scatter light [see this Wikipedia entry on Mie scattering] which is fairly uniform across all wavelengths hence clouds and ice appear the same color as the star. Then we come to the color of the continents – we had fun with this one. When we were designing the image Elisa Quintana found an article by Nancy Kiang titled The Color of Plants on Other Worlds. Nancy is a scientist based at NASA Ames (she moved to Ames from GISS the week after we talked to her, small world heh!) who works with the Virtual Planetary Laboratory. We called her up and chatted about what colors plants might be on planets orbiting cool stars. While this is a very complex issue involving evolution of photosynthesis, she recommended a dark yellow/green color as a potential color for alien planet life on this world.

And reminding us how little we know about this planet, Tom goes on to note that the artists chose to depict the planet as a bit colder than Earth, realizing that we have no knowledge about its atmosphere, which will have a great deal to say about its temperature. This was an educated guess to show a planet with prominent ice caps and plant life in the equatorial regions. It’s based on the understandable analogy with the Earth, which has the water, continents and clouds we see in this image translated to a hypothetical planetary body around another star.

So am I being too fussy in talking about people getting the wrong idea from such images? Maybe so, in the sense that along with the excellent artwork, we have to be careful to get the message out about what we actually know about the world. I think Tom gets it right when he says “Hopefully this image provides a nice tool to explain what might be the same and what might be different between this planet and Earth.” Making those explanations is a job for those of us who try to communicate the findings of our exoplanet hunters to the general public, and it’s something we need to do well to separate the genuine excitement of the work from the frequent media hype.