April 12, 2005
Surfing the Wavelengths in Search of Earth-like Planets
Astrobiology Magazine -- Maggie Turnbull, an astronomer with the Carnegie Institution, has spent many years thinking about what kind of stars could harbor Earth-like planets. Her database of potentially habitable star systems could be used as a target list for NASA's upcoming Terrestrial Planet Finder (TPF) mission.
Turnbull presented a talk, "Remote Sensing of Life and Habitable Worlds: Habstars, Earthshine and TPF," at a NASA Forum for Astrobiology Research on March 14, 2005.
This edited transcript of the lecture is part one of a four-part series.
As soon as we ask that question, a million more questions pop up: "What is life?" "How does life originate?" "Could life be originating on Earth now, and if not, why not?" "Where is life found?" "Can life spread between planets or even between stellar systems?" "If life can travel between stellar systems, do we have relatives among the stars?" "Are there other technological civilizations out there?"
Those are too many questions for me to try to answer right now, but I will address the question, "Are there habitable terrestrial planets orbiting nearby stars?"
That immediately leads us to ask, "What is a habitable planet?" All life on Earth depends on the availability of liquid water, so I'll just say that a habitable planet is one that has liquid water on its surface.
So the habitable zone will be that location around a star where an Earth-like planet will be at the right temperature so that it will have liquid water on its surface. Around our sun, the habitable zone extends from about .7 AU out to about 1.5 AU. (1 AU is the distance between the Earth and the sun.) For other stars, we'll just scale that as the square root of the luminosity of the star.
The first mission objective of the Terrestrial Planet Finder is to hone in on the zone where a terrestrial planet could have liquid water on its surface, and directly image any terrestrial planets in that zone. We want to see these planets with our own eyes. We want to take a picture, and see a little dot.
The goal is to image planets in a habitable zone that have at least half Earth's surface area. We want to be able to image planets somewhere between the size of Mars and Earth, or larger.
The second goal of TPF is to characterize the atmospheres of any planets we find for indicators of life. The third goal is to do comparative planetology, so that when we have a database of planetary systems in the solar neighborhood, we can ask questions like, "How common are terrestrial planets?" "How diverse are they?" "How common is it to have a habitable terrestrial planet?" "Is liquid water a common thing, or do most planets look like Venus or Mars?" "Are terrestrial planets common at all, or is it more common to have massive eccentric Jupiters?"
The Terrestrial Planet Finder originally was envisioned as a mid-infrared mission, because it was thought that mid-infrared wavelengths would be the best way to search for extrasolar Earths around nearby stars. In the mid-infrared, planets emit their own light, while the light from the star is tailing off, so the contrast shown for the mid-infrared is better than in the optical.
At the shorter optical wavelengths, the Earth's spectrum mirrors the sun - it's just reflecting sunlight. As we get into the mid-infrared, the Earth starts emitting its own light, because it has a temperature of 300 degrees Kelvin (80 F). That heat translates into light in the mid-infrared.
So a planet's mid-infrared light can give us a handle on the temperature of the planet, and tell us if that temperature is right for liquid water at the surface. Also in the mid-infrared, we can see some exciting signatures, such as carbon dioxide, water, and ozone. Since ozone is a proxy for molecular oxygen, it's an indicator of life.
Spectra in the infrared taken by the Galileo spacecraft of Mars, Earth, and Venus showed that the three planets look fairly similar. But Earth had two indicators of habitability and life, namely, water and ozone.
The Earth is one-ten-billionth as bright as the sun in the optical. In the mid-infrared, it's only a factor of one million, so it's not quite as bad. But still, if you take the Earth-sun system, and put it at 10 parsecs distance - about 30 light years away- there will be an angular separation between the two of 100 milliarcseconds. That's very small. So even looking in the mid-infrared -- with the planet being one millionth as bright as the sun at 100 milliarcseconds at 10 parsecs -- that's not easy.
In order to do high resolution imaging in the mid-infrared, we need to have a very long baseline, which means we need to either have a huge telescope, or we need to somehow fly an interferometer in formation. That technology is more advanced than what we'll have over the next ten years.
So the first mission will be TPF-C, a coronagraph that is slated for launch in 2014, and it will operate at optical wavelengths. The key issue here is that we need to suppress the light of the star so that we can see the light of the planet - planets are very much fainter than the star.
For the optical, we don't have observations that are analogous to the Galileo infrared observations. We've got lots of satellite observations of Earth in the optical wavelengths, but the low-flying satellites only see a small footprint of ground at a time. We don't have satellite observations that have the whole spectrum of the visible Earth all summed up in one pixel.
So to get those spectra, we have to observe them from the ground. Luckily, we can do that by looking at the moon, by pointing our telescopes at where "Earthshine" lights up the dark portion of the thin crescent moon. The way this works is, the sun shines on the Earth, the Earth shines on the moon, and that light reflects off the moon and goes back to the Earth and into our telescope. The dark portion of the moon shows the spectrum of the whole Earth, all summed up together.
Sunlight reflecting off the bright crescent of the moon also goes into our telescope as we observe it on the ground. We just take our dark moon spectrum and divide it by our bright moon spectrum, and what we have left over is the spectrum of the Earth.
In the Earth's optical spectrum, we see Rayleigh scattering in the blue part of the spectra - we're seeing the blue sky of our planet. We also have signs of oxygen, ozone, and water. We may even be able to see signs of vegetation in the optical.
The interesting thing about observing the Earth in the optical is that you can see all the way through Earth's atmosphere to the ground. The light that reflects back contains the spectrum of whatever is on the ground of that planet, whether it's oceans, or soil, or plants. And plants happen to have a very distinctive spectrum that could be observable even across stellar distances.
All land plants have pretty much the same spectral signature -- they're very dark in the optical, where 10 percent of the light that strikes them gets reflected. For the most part, in the optical, they absorb just about all of the photons that fall on them, which makes them good photosynthesizers. They become strongly reflective in the near-infrared, where they reflect 70 percent of the light that falls on them.
This "vegetation edge" is a distinctive spectral feature that allows satellites to map the health, density and even different species of plants on Earth.
Now, you could ask how relevant this is to the search for life elsewhere. Do we really expect Earth-like plants to be on other terrestrial planets? It's not reasonable to expect that, but life does have a way of exploiting whatever energy source is available. So life on the surface of a planet should be expected to be photosynthetic. It's going to absorb very strongly in some wavelength range where the atmosphere of the planet is transparent and where the star is emitting a lot of energy.
Is it just a coincidence that plants on Earth become strongly reflecting in the infrared? Or is that useful to the plant biologically, for instance, for cooling purposes? If it is biologically advantageous to reject photons that are not energetically useful rather than absorbing them, and to strongly absorb those wavelength ranges where you can do something with the energy, then we should not be surprised if photosynthetic life on other planets also has a strong spectral "edge" feature.
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