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Slime Worlds
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Alien Life
Posted:   02/02/09

Summary: At the most recent NASA Astrobiology Science Conference, a panel of scientists discussed different types of planets where alien life might be found. In part one of this series, Seth Shostak reviews the search for extrasolar planets, and Tori Hoehler describes a place of "colorful microbial goo" called Slime World.
Seth Shostak at the Arecibo Observatory.
Photo credit:

Seth Shostak: Tonight’s public event will discuss worlds that might have life. They’re not all “M-class” planets, as you’ll learn. This event has been organized by Victoria Meadows of the University of Washington, and she has selected four distinguished panelists who are designed to dazzle and programmed to provoke.

After my soporific introductory remarks, each of the panelists will talk about a certain category of worlds that might hold life. After each of them has spoken, there may be a little bit of back and forth among them. Then we open it up to you, the car-buying public, to ask the kind of questions you know they’re deliberately avoiding.

Arthur C. Clarke said, among many other things, that he hoped we would find direct evidence of extraterrestrial life before he died. Unfortunately, he didn’t live long enough to realize this dream. However, he gave much sage advice to those engaged in the search for life and understanding life, including this possibly prescient statement from his 1957 book, The Exploration of Space. He wrote: “If we have learned one thing from the history of invention and discovery, it is that in the long run, and often in the short run, the most daring prophecies seem laughably conservative.” So perhaps the prophecies we’re going to hear tonight will at some point seem laughably conservative.

Artist’s concept of a Hot Jupiter
Credit: David A. Aguilar (CfA)

When I was much younger, it was routinely assumed that planets were plentiful. In the 1970s, a small group of Canadian astronomers thought they had a method for finding planets around other stars. Their intention was not to see the planets directly, but to measure the planets’ effect on their host stars; the wobble that their host stars would have as a consequence of the planets existence. The Canadians built the requisite technology, a very sophisticated spectrometer, and looked at a half dozen stars. They didn’t find any planets. However, had they not given up so quickly, had they had a certain amount of persistence and looked at more candidate stellar systems, they would have been the first to find planets orbiting other stars. As it happens, that honor went to a couple of Swiss astronomers in 1995. It’s been 13 years since the first planet around an ordinary star was found. 51Peg was that planet, and it caused a sensation. Since then there have been over 300 planets found. That’s a lot of planet pleasure.

As those of you who are familiar with this field are aware, many of these planets are very large and very close to their suns. They are so-called hot Jupiters. They have other unfortunate properties that suggest that while there are a lot of worlds out there, many of them hardly seem like the kind of worlds with a chance for life. But that situation is changing as the data continue to come in.

There is a strong tendency for planets we find to be very close to their stars, but that’s the result of an obvious selection effect. The planets that are close to their stars are the easiest to detect.

Tori Hoehler dives for slime.
Photo credit: NASA

Some of the planets we’ve found have the mass of Jupiter or more. While Jupiter is an interesting planet, it’s not where ET would want to call home. It has an atmosphere of methane and ammonia and things that you use to clean the bathroom. You probably wouldn’t want to live there.

As our planet-hunting techniques are refined, we’re finding smaller and smaller worlds. The number of planets discovered each year is also increasing very rapidly. When the Kepler mission goes into orbit there may be a veritable shower of newly discovered planets.

For many years Mars was our only hope for finding life nearby. That’s changed. Today we think life might even be found on worlds that weren't really expected to harbor life. Many of the moons of the outer solar system, which only appeared as little dots in earlier photos, have been looked at much more closely. Jupiter’s moon Europa is covered with ice, but underneath that ice may be more water than the Atlantic and Pacific put together, and there might even be life similar to what we see in those terrestrial oceans. We also will hear about worlds that might be covered in slime, as well as worlds that might, like this one, be covered with intelligent beings.

Our first speaker is Tori Hoehler. Tori is a research scientist at NASA Ames. He specializes in microbiology, microbial mats and the like - work that takes him to places like Baja California, Yellowstone, the bottom of the oceans and the top of the world, looking for life that’s on the edge.

Tori Hoehler: I get to talk to you about Slime World, a planet inhabited by nothing but micro-organisms. For those of you who are thinking “yuck,” consider that our own world is mostly a Slime World. For at least three-quarters of the history of our planet, we had nothing but microbial life here. Looking at that life gives us some ideas about what a Slime World might look like if we encountered it elsewhere.

Colorful microbial mats live in Octopus Spring in Yellowstone National Park.
Credit: David Strong, Penn State

We can learn about Slime World by looking at microbes on Earth, such as those that live in Sulfur Caldron in Yellowstone National Park. It is essentially a lake full of boiling sulfuric acid. If you’re a 10-year-old boy, this is the coolest place on Earth. If you’re a 37-year-old boy, it’s the coolest place on Earth. The microbes here are really tough. We define a clement environment in our specific human terms, and we’re not very tough. But microbes are perfectly capable of living in a pond full of boiling sulfuric acid. We find that they can tolerate extreme temperatures, extremes of acid and base. They live inside nuclear reactors. At home in your hot water heater, there are gangs of bacteria. Anywhere we find water, for the most part, we find micro-organisms. So we expect there are quite a broad range of planets and environmental conditions where microbial life might be possible.

How might a Slime World look? Micro-organisms love surfaces, first of all. If you want to do an experiment about this, don’t brush your teeth for a couple of days. You will develop a Slime World for yourself. The only reason we don’t see thick mats of microbes all over the place now is simply because the modern world is filled with “toothbrushes” -- bigger organisms that eat, out-compete, or otherwise hinder the accumulation of their microscopic counterparts.

Micro-organisms do the same kind of things that we do - "we" meaning our macroscopic world. So, for example, there are microbial counterparts to plants, photosynthetic micro-organisms that have the same colors associated with plants. Some that grow in a Yellowstone hot spring form beautiful mats, with interesting textures and colors. We could imagine that a Slime World would have vast carpets and reefs of colorful microbial goo. That’s exactly what our planet looked like for about 3 billion years of its history.

A view of Earth from Voyager 1, at a distance of more than 4 billion miles. Earth is the pale blue dot in the middle of the bright streak.
Credit: NASA/JPL

How will we find a Slime World? Within our own solar system, there is only one place where we know water exists on the surface, and that’s here on Earth. So if we’re going to find Slime World somewhere else, we going to have to look beyond our own solar system, and that means we won’t be able to physically touch it. All we will be able to do is look with telescopes, which means we’ll only see a tiny little bit of fuzzy light.

When the Voyager 1 spacecraft was heading outside the solar system, it took a picture of Earth from 4 billion miles away. Our planet looked like a fuzzy dot of light. That’s the sort of picture we’ll get on a great day if we looked at planets around another star -- tiny pinpoints of light juxtaposed against an incredibly bright star.

So how will we find Slime Worlds? Microbes are small, right? They’re all around us, but we don’t see them. But there are lots of them, and there’s strength in numbers. There’s a quote from Louis Pasteur that applies to this notion: “The power of the infinitesimal can be infinite.” In Yellowstone National Park, a place called Roaring Mountain is literally being dissolved by the organisms that live there. If we looked inside we’d see the same little guys that we saw in Sulfur Cauldron, only now those little guys are producing the sulfuric acid that they live in. They are dissolving the mountain around them. When they are there in abundance, the impact of these tiny little micro-organisms is very large.

We can use that to our advantage to find Slime World elsewhere. Every single living thing changes the environment around it. We humans change the environment simply by breathing gas in and out. Individually, we change the environment just a little bit, but all of the breaths of all of the people who ever lived added together lead to a significant change in our atmosphere.

Earth's biosignatures include methane, liquid water, and ozone.
Image Credit: NASA PlanetQuest

It turns out that what we humans do chemically is not a good way to find life. But that’s not true for all organisms. The atmosphere we breathe is about 20 percent oxygen, and that oxygen is the product of the combined activities of a lot of photosynthetic micro-organisms through time. Our world had no oxygen at all for about the first half of its history. It is micro-organisms that gave us the atmosphere that we breathe now.

That’s how we’ll find life on other planets, because we have telescopes that are able to see chemical information. These telescopes will be able to look, for example, at the carbon dioxide gas that I breathe out. For organisms that breathe out gas that is particularly indicative of life, if there were enough of those organisms working over enough period of time, we might be able to see something in the atmosphere.

These telescopes will collect nothing but light from that tiny little pale blue dot, billions of miles away, and yet still can have something to say about who’s there and who’s breathing. And if we see the right sort of thing, we will know that we’ve found a Slime World.

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