A Diversity of Worlds
We face this problem when we search for life in other solar systems. As yet, we have no pictures of extrasolar planets; the evidence for their existence comes from the gravitational and spectral effects they exert on their host star. Over the next decade, however, space telescopes may begin to search for and provide images of Earth-sized planets orbiting distant stars. These telescopes include the European Space Agency’s COROT and Darwin missions, and NASA’s Space Interferometry Mission (SIM), Kepler, and Terrestrial Planet Finders. These missions may be able to tell us about the geology, chemistry, and atmosphere of terrestrial worlds in alien solar systems. Such information could help determine if planets are rich with life like the Earth, or dead, barren worlds where life never took hold.
In May 2007, Victoria Meadows, Principal Investigator for the Virtual Planetary Laboratory at the California Institute of Technology’s Spitzer Science Center, presented a lecture at NASA’s Jet Propulsion Laboratory. In part four of this six-part edited series, she explains how different types of worlds, even ones not like the Earth, can still be potential havens for life.
A Diversity of Worlds
A lecture by Vikki Meadows
So these models help us generate environments of planets that could be physically and chemistry reasonable. Then they also let us generate a spectrum of what that planet would look like if I was looking at it with a telescope.
When we use our planet formation models, the simulation is a bit like bumper cars. You put a lot of rocks around a star, and then you let those rocks crash together and eventually form planets. Some planets are small, some are large, some end up water-rich, and some have almost no water at all. Using this model, we do produce Earth-like planets. However, a whole bunch of weird planets of different sizes and different water masses also get formed. So we don’t expect all the terrestrial planets out there to be just like our solar system.
And also, you can have planetary diversity in time. The planet you’re standing on right now is just one example of the Earth. The Earth has changed its face quite dramatically over the last 4.6 billion years. The atmospheric pressure of oxygen, the amount of oxygen in our atmosphere, has changed drastically over time. Modern day oxygen levels are relatively high, but back in the Proterozoic they were pretty low, and back in the Archaean they were non-existent. About 2.3 billion years ago, there was a rapid rise of oxygen in our atmosphere, and that allowed multi-cellular life to develop. The rise of oxygen in the atmosphere probably was due to bacteria working away at photosynthesis.
The results from the M star were interesting. M stars have always been considered the low rent district of the galaxy. They were not thought to be good places for life, because the planet has to be right up against the star in order to get enough radiation to stay warm, and then it gets tidally locked, plus it’s more susceptible to flaring from the star. However, these stars are very plentiful. So if we want to increase our chances of finding life, it would be great if we could detect life there.
In our M star experiment, we put an Earth in orbit around a star called AD Leo. AD Leo is a very young M star; it’s very active; it flares a lot. We saw a drop off in the amount of ozone on the planet, but we also saw an increase in the amount of methane. That was exciting because methane in the presence of oxygen (or ozone, O3) is a good biomarker. So it turns out that for planets around M stars, many of the biosignature gases survive for longer in their atmospheres and are much easier to see.
On the Earth, there’s a compound called methyl chloride that is a product of biomass burning. It’s also produced by plankton in the ocean. So it is a biosignature, but on the Earth there’s so little of it it’s very difficult to detect in our spectrum. On an M star planet, it builds up. So this is another potential biomarker that we hadn’t been considering before that we might be able to see.
Using our models, we went back in time and had a look at the early Earth. We looked back to the Proterozoic, when we had one-tenth of the oxygen we currently have now, and yet the ozone was just as detectable. The Earth back then also had methane from methanogens, types of bacteria that generate methane. Methanogens were around even in the Archaean, and there probably would have been a fairly high level of methane that was pumped out into the atmosphere as a result.
It turned out that it was easier to detect life 2.3 billion years ago then it is right now. If you go back into the steamy mists of time, way back into the Archaean period when we didn’t have a lot of oxygen in our atmosphere but we had a lot more CO2, you can see strong effects from carbon dioxide, strong effects from methane, but no ozone. That means the planet didn’t have any oxygen on it. Is it less habitable? No. It’s still just as habitable. It still has life on it, it just doesn’t have life that has produced enough oxygen to be able to see it.
That massive atmosphere scatters radiation much more effectively than the less massive atmosphere, and that Rayleigh scattering tells us we’re looking at a more massive atmosphere. So that’s very good, because remember that we need to know the mass and the greenhouse gases in order to understand the surface temperature.
Looking at the high carbon dioxide early Earth in the mid-infrared was the weirdest thing we’d ever seen. Mid-infrared is the heat radiation coming off the planet. We’re used to seeing planets that have a nice sloping infrared curve; even Venus and Mars have that. But our high carbon dioxide early Earth had an intensely spiky spectral line because all the carbon dioxide was just eating away at that spectrum.
We could see from spectra that our model planet had a hotter planetary layer than the Earth. This planet had a surface temperature that was about 30 degrees hotter than the Earth’s surface temperature. So those are examples of the types of planets we can model and the sort of things we can learn by just looking at these squiggly spectral lines.”