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Envisioning Alien Worlds

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.


Comments on this entry are closed.

  • Alexander Whitaker April 23, 2014, 10:24

    The number of worlds in the galaxy that are exactly like Earth are probably a few hundred at most.

  • RobFlores April 23, 2014, 11:02

    While we’re envisioning Terrestrial Worlds, It’s a good idea
    to keep in mind that continental size, arrangement, and physiography will have a huge impact on the climate of alien planets.

    For descriptions it helps to have earth references for alien landscapes.
    but what about worlds that are in their present conditions offer spectacular
    climate habitability. Island continents larger than Madagascar straddling
    the Temperate areas, creating a Mediterranean analog, but extending
    along a belt 360 deg, on alien worlds.
    Or for those of a more exotic nature, A dense belt of large islands extending from pole to pole at several Longitudinal positions, a plethora
    of shorelines. think Indonesia to New Guinea rotated 90 Deg, But with several smaller islands instead of Borneo, and New Guinea.
    On the other hand it might be a good idea to model the continental drift
    of a potential colony planet. Maybe 100 million years forward the continental arrangements might cause a severe upset of sea currents causing
    a large extinctions.

  • Alex Tolley April 23, 2014, 11:02

    The climate regimes on a world with 90 degrees axial tilt are going to be very different, as the Armstrong paper in the earlier thread indicates. At the equinoxes, insolation pattern will be similar to Earth’s, but at the solstices, the poles will either be frozen in a 24 hr dark, or have blazing sunlight overhead all day. The equator will have a very low sun or twilight,for 24 hrs, rather like our Arctic circle. As a result, temperatures will be highly variable, almost everywhere.

    While large, terrestrial homeothermic animals may migrate, large poikilotherms cannot (no crocodiles in the Arctic or Antarctic) , and plants and small animals will probably go through active and dormant states. Marine life will probably be migratory, but there will be a problem for organisms that are sessile or unable to migrate, and they may need to go dormant for parts of their annual cycle unless the primary producers are motile, e.g. single celled algae.

    Interestingly, Antarctica during the Eocene could have been subtropical near the coastline , despite the winter darkness. It would be interesting to consider what adaptations would have been needed for the flora and fauna living in that extreme lighting condition. Photosynthesis would shut down in the darkness and the plants would need to metabolize stored starches due to the warmth. This might allow herbivores to continue to survive, although predators might find the light conditions problematic for hunting. Or maybe the plants were highly seasonal and only swimming animals and birds could come and graze on the plants, migrating away during the winter.

    I was reading that the new telescope being constructed in Chile might be able to image Earth analog exo planets. We might start getting clues to some of the biosphere questions within a few decades.

  • ljk April 23, 2014, 13:16

    No, Paul, you are not being “too fussy” about how artists’ impressions of aliens worlds we barely know exist let alone can see in any detail affect and influence the general public and media.

    Look what happens every time a new Earthlike exoworld is announced, including this latest one: The media makes it sound like the planet is a copy of ours (Earth 2.0 is one favorite moniker I see, though they stick in a question mark as if that will make it all better) and the public starts clamoring for starship flights to migrate there, while I bet most of them do little or nothing to really support the very institutions that could make such a thing possible.

    Someone has to keep the media honest or at least make the attempt. Otherwise this decline in journalism which I see every day will continue. Especially when it comes to science.

    By the way, I am certainly all for encouragement when it comes to getting the public excited about space. But when the media hypes up something like an exoplanet and makes the public think it is a new world to colonize, they are only setting them up for disappointment and that leads to the masses not supporting NASA and its kind.

    Look at the last starship conferences: There was more media focus on hyperdrives and warp drives even though anyone who reads this site knows we are so utterly far away from such concepts becoming reality, if they can ever really happen at all. People want Star Trek and I co-blame both Hollywood and the real science community for not making real space more palatable and exciting to garner support.

  • Michael April 23, 2014, 14:38

    Sorry Paul could you use this one, I noticed the year is important

    If you wanted to stay in permanent daylight on these axially perpendicular earth sized and in the same 365 day orbit you would only need to walk/swim/fly at around 4-5 km/hr at the equator, less as you go to higher/lower latitudes. Walking in permanent dusk or sunrise might give an advantage to animals in there been a thicker optical depth of atmosphere to protect from UV, it would be considerable in factors of ten or more. Personally I like the images they give a real feel of home, but more than likely completely different, in another word ‘Alien’

    @Alexander Whitaker April 23, 2014 at 10:24

    ‘The number of worlds in the galaxy that are exactly like Earth are probably a few hundred at most.’

    I agree, I feel earth has been very ‘lucky’ and they are few and far between.

  • Paul W April 23, 2014, 14:43

    I see both sides of the argument. This image was not made without considering the real object, and wow is it beautiful. Good points about the host star’s color, which I hadn’t really imagined.
    I also agree that it’s biting off a big chunk to render an unknown world in such detail. Hard to see how some wishful thinking doesn’t work it’s way in.
    Of course, unless we revert to digital versions of ourselves in the future (and hopefully, even if we do) we NEED an Earth 2 (Too). An egg has to come out of the basket, and we are so evolved exactly for earth that not much else short of what we have would work very well. Well, it might, I don’t really know.
    There are people waking up every day who will be able to figure out LOTS of details about these far off worlds without actually going there. However, that’s the rub, at some point we NEED to actually go there. Send a probe to check it out? Of course! Well, let’s see, this one is 500 light years away. Over a thousand years to return data from just one place. Sigh.
    Unless something much much closer can be located, places like this Kepler planet will be footnotes in a computer base (Very important footnotes)
    Without FTL, humanity will likely have to pursue the Singularity.

  • Paul W April 23, 2014, 15:31

    “I was reading that the new telescope being constructed in Chile might be able to image Earth analog exo planets. We might start getting clues to some of the biosphere questions within a few decades.”

    I hope so, anyway I sure agree with the sentiment. As much as possible should be done with instruments located where the humans can fix them.
    However, a NASA lady said that Kepler and other such telescopes used to image exos need to be far enough from Earth to escape its reflected light and that our atmosphere interferes with light signatures from the exos.

  • Mauro Cavalcanti April 23, 2014, 16:38

    One of the most detailed and fascinating exercises in depiction of extrasolar and ‘alternate’ planets is the Planetocopia website: http://www.worlddreambank.org/P/PLANETS.HTM. Many sci-fi authors could benefit from the worldbuilding presented there.

    As of Baxter (a good author who, in my humble opinion, is somewhat defeated by his own exaggerated plots), his ‘Terra 2.0’ described in the “Ark” novel could hardly exist around Epsilon Eridani (https://en.wikipedia.org/wiki/Epsilon_Eridani).

  • andy April 23, 2014, 18:03

    As regards the choice of yellowish light for the Kepler-186: it is hotter than the typical filament lightbulb or even burning magnesium. Sure if you have your camera settings set up for Earth at midday it will come out yellowish but if you were there it would be a pretty decent white light, no noticeable tint.

    @Mauro Cavalcanti: Epsilon Eridani ≠ 82 Eridani. (IIRC the hot super-Earths in the 82 Eri system had not been discovered when the novel came out)

  • Brett April 24, 2014, 0:48

    So the Terra 2.0 would experience the full range of day and darkness everywhere except the equator? I thought it would be less than the full six months of darkness/day away from the poles.

    I still think you might be able to make a go at it with a combination of living near the cold equator and possibly with migration as well. It wouldn’t be anything resembling comfortable, but Arctic living isn’t that today or in the past.

  • RobFlores April 24, 2014, 14:10

    I guess the Imaging/Designing team assumed Kep 186f has plate tectonics.
    Is this really a given? The models suggest it is a fine line that separates Venus From Earth.

    Rare Earth Hypothesis Redux: Without recycling of surface elements from Plate tectonics either on the seas or dry land, can life be sustained for long or even arise.?

    Would a world w/o lighter continents really be water world,
    or would the hot mantel create plumes that break through to the surface creating large islands.

  • Andrew Palfreyman April 24, 2014, 20:38

    Joe Ritter (who spoke at Icarus Interstellar) talks about a new generation of telescopes with ~10^-8 arcsecond (5*10^-14 rad) resolution. Claudio Maccone speaks of 2*10^-10 arcsecond (10^-15 rad) resolution with the gravscope. The near future of the field of exoplanet imaging looks bright and diverse.

  • ljk June 18, 2014, 11:13

    Painting Space: Then and Now

    Posted By Andrew Fazekas on Friday, 13 June 2014. Posted in AstroArts Blog


    Astronomical artists are sometimes divided into First Generation and Second Generation (and I suppose we now have even more). The first group were pioneers; they were the first in their field and had nothing to work with but their own knowledge, observations, intuition, and talent. They include James Nasmyth, who in 1874 made models of the Moon’s surface and photographed them against a black, starry background. Scriven Bolton later used a similar technique for The Illustrated London News. By far the most accurate artist was French astronomer Lucien Rudaux who, because he was an observer, knew what the lunar mountains really look like in profile. His 1937 book Sur les autres mondes (On Other Worlds) is a classic.

    But the best-known space artist from the 1940s and 50s was the American Chesley Bonestell (see my first blog: http://astronomerswithoutborders.org/component/zoo/item/moon-by-earthlight.html )

    Full article here: