Questioning Habitable Planets


Vikki Meadows, Principal Investigator for the Virtual Planetary Laboratory.

The Voyager 1 spacecraft, after traveling about 4 billion miles into space, turned around and looked back home. From such a distance, the Earth appeared as a pale blue dot, a single point of light suspended in the vast blackness of space. If aliens from much more distant worlds were to look at our solar system, the Earth, if it could be seen at all, would seem even more tiny and faint. How could they know that dot of light represents a world teeming with life?

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 the final part of this six-part edited series, she answers audience questions about habitable worlds.

Questioning Habitable Planets

A lecture by Vikki Meadows

Q: Wouldn’t a magnetic field be necessary for a habitable planet?

The Earth’s magnetic field shields the planet from high energy solar particles. Image Credit: NASA

Victoria Meadows (VM) : Yes. I didn’t mention that because it’s hard to model in spectra. But that’s another thing you need to have for habitability, to protect life from particles. A magnetic field is related to the planetary mass, because if a planet has enough mass to have a liquid iron core, then you can get a dynamo effect generated. Tidally-locked planets that are too close to their star may not be able to rotate fast enough to generate a magnetic field, so that’s another mark against them.

Q: Is there a historical record that shows that photosynthesis in the past was spectrally shifted from what it is today?

VM: No. As far as I know there is no record of where the photo pigments were previously. We can only model it and take a good educated guess.

Q: What about the effect of atmospheric composition on peak photon number?

VM: That’s what we were doing when we were modeling planets around different stars. We changed the atmospheric composition quite a bit — we had less ozone, for example, in some places and more ozone in the others. If the composition had been completely different, say mostly carbon dioxide, that also would affect where that pigment peak was. But that’s what we were trying to model for. We were trying to show that the star and the planet composition and the plant all have to interact with each other to finally come to an optimal position.

The familiar continents of Earth are embedded in tectonic plates on the planet’s surface that slowly collide with each over time. Click image for larger view. Image Credit: USGS


Q: You mentioned that plate tectonics was important for life. Can you explain a bit more why?

VM: Plate tectonics helps you control the concentration of gases in your atmosphere over time. Carbon dioxide is vented into our atmosphere by volcanic reactions, for example. And there is something called the carbonate-silicate cycle, where carbon dioxide is captured in rocks on the surface of the planet and then buried in the ocean sediment and then subducted down through the plate tectonics and then comes back up again. But that process takes a long time, and it helps to control the amount of carbon dioxide in the atmosphere. So without plate tectonics but with active volcanism, you just continuously build up carbon dioxide and there’s no way to bury it back into the planet. That’s why it’s important. A huge amount of carbon dioxide in the atmosphere creates a runaway greenhouse, which is what happened to Venus. That’s why Venus is so incredibly hot today.

Q: In your graph of time variation of methane on Earth, it looked like there was a whole lot more in the northern hemisphere.

The moon, our closest planetary companion. The moon helps the Earth maintain a fairly stable axial tilt. Click image for larger view. Image Credit: ESA


VM: Yes, that’s because we have a whole lot more landmass in the Northern Hemisphere. What’s producing the methane signature is life, and there’s just more life on the landmass.

Q: Do you think the fact that Earth has one and only one satellite plays any role in life?

VM: Some people think that the Earth’s moon has been an enormous stabilizing force for us, because it stops our pole from wondering too far. Mars doesn’t have a massive satellite and its pole has gone all over the place with time. If you were on the equator and liked being on the equator, being at the pole the next day would be a bit disruptive. So having a moon as a stabilizing force to prevent these wild excursions is another habitability parameter. However, when we look at planets around other stars, detecting whether or not there’s a moon is going to be quite difficult. We might be able to see it if we get a transit and the moon is large enough, but we’d have to be phenomenally lucky to have the alignment of the moon and the planet and the star be just right to see that. So that’s not something we’ll look for, but it’s certainly something that’s desirable.

Q: How supportive are government agencies in this science, or is the interest solely from the scientific communities?

Artist’s concept of a planet 3.2 times more massive than Earth. Image credit: NASA/JPL

VM: If you count NASA as a governmental agency, then it has been very supportive in the past. In fact I am the lead of a team of people who work at the NASA Astrobiology Institute, which NASA originally founded and funded. So it raised astrobiology up from its infancy. We recently received a budget cut in NASA, and astrobiology was cut especially hard. But this is just a phase, and this too shall pass. There are probably about 1400 astrobiologists in the US today, and there are more in Europe and Australia and various other places where they’ve started this science. The NAI itself comprises about 700 astrobiologists. NASA’s influence in this particular field has been in part to support missions like this and the Mars Rovers. Many of my colleagues work on looking for signs of water and life on Mars. I work in the Navigator Program, which includes things like SIM and TPF. And beyond TPF we have Life Finder. But that’s what we call a Vision Mission. It’s so “out there” that we haven’t even started drafting what it is yet. But the hope is that Life Finder will have very good spectral resolution, and we’ll be able to break that up very finely and look for more subtle signatures in the atmospheres of planets than we’ll be capable of with TPF.

Q: Earth has incredible bimodal altitude distribution, with ocean floor and continental material. We’ve been told the continental material has been increasing over billions of years toward higher land mass. I wonder how planets with plate tectonics and the change in the amount of continental rock fit into your model.

VM: Our models are not yet sophisticated enough to take that level of detail into account, but we’ve just been given another 5 years of funding by the NASA Astrobiology Institute to do this. So we will be working with more sophisticated models that will be able to take those kinds of special variations into account. But it’s certainly true that Earth has bimodal distribution but Venus does not, for example, and it’s not a habitable planet.

Q: How far away are you going to be able to detect adequate spectra resolution of extrasolar planets?

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

VM: That depends on the instrument you fly. The nominal plan for the Terrestrial Planet Finder was to see out to 45 light years away. That’s still within our solar neighborhood, but it’s far. It’s not just Alpha Centauri, it’s stars beyond that as well.

Q: How many prospects do you have within that volume of space?

VM: I can’t remember the exact numbers, but the lists we’ve been putting together have at least 100. I believe the Darwin mission was looking at a target list of about 500 or 600. I can’t remember if their range was a little bit longer than ours based on their design. So you’re looking at a fair number of stars.

Q: Can you have a hospitable planet without a large Jupiter-like planet in the solar system?

VM: Having a large Jupiter-like planet really does help. We call it the vacuum of the solar system, because it stops small objects from coming in and bombarding us all the time. But it is possible to form planets around stars where there is no Jupiter. Whether or not they could cope with all these rude interruptions from asteroid and comet impacts is another thing. We’re very solar system centric; we tend to think having a Jupiter as a good thing because we have one.

Q: How far away is the recently discovered terrestrial-like planet?

Montage of Jupiter and the comet fragments of Shoemaker-Levy 9. Having such a comet-attracting planet in a solar system could be beneficial for life on smaller, terrestrial planets. Image credit: NASA/ESA

VM: Gliese 581c is 20.5 light years away. But it’s around a very small M star of only 0.3 solar masses. A star that small has a habitable zone very close to the star, and that means it’s extremely observationally challenging for any telescope to be able to separate the planet and star. So that star may not be studiable with Terrestrial Planet Finder as currently envisaged. However, I hope that, if we have time to design this, we would make observing that planet a top priority since it seems to be the best candidate of a habitable planet that we have so far.

Q: Do you think instruments can be developed that will allow us to probe much deeper into the galaxy?

VM: Yes, with time I believe we can do anything. Human beings are amazing – if we can imagine it, we’ll get there. JPL has been a pioneer in developing many of these techniques. I think recently there was a press release showing that for the nominal TPF design, they had gotten down to the detection limit in the laboratory that was required to detect an Earth-like planet around another star. So we’re nearly there. We’re very close. But as I said, that M star planet is going to be more challenging then the F, G, and K star planets that we’ll be looking at.

Q:Do binary star systems have any effect on the habitable zone?

VM: Yes, they have their own sets of rules for habitable zones, and it depends on the separation of the binary. If the binary is really close, then there’s a habitable zone that goes around the two stars. So it’s like Tatooine in Star Wars — a system like that would be perfectly feasible. But if the two stars are very far apart, you have separate habitable zones around each star.

Q: Has anyone considered looking for life that’s not Earth-like, or is that too expensive?

Binary and multiple-star systems are twice as abundant as single-star systems in our galaxy. Image credit: NASA/JPL/Caltech

VM: Well, there’s Earth-like and there’s Earth-like. When we look for life elsewhere, we assume that it requires water. That’s not such a crazy assumption because you do need a fluid in which to perform the chemical reactions of life, and water is a very good fluid for that. Even on our own planet there is such a huge diversity of life that is water-based. I don’t know if you’ve been watching that Planet Earth series on TV, but there are some really weird life forms out there. So needing water doesn’t limit you very much.

However, do we require that they have oxygenic photosynthesis? No. In the history of our planet, we’ve had anoxic photosynthesis, we have microbes that put out sulfur and methane and all sorts of weird and wonderful things. There are many different metabolisms, different ways of using energy, and we’re open to looking for all of those. We can imagine a planet dominated by sulfur bacteria, for example. The signatures of the atmospheres would be very different than what you would expect from a photosynthesis, oxygen-based world.

My colleague Steve Benner makes weird life in the lab, and I think has even come up with a 12 base pair DNA. But since we don’t fully understand how those would work yet, we stick with what we know, which is this basic DNA-based, water-based life.