Are We Alone? Where Are Our Nearest Neighbors?August 06, 2001 / Posted by: Shige Abe
Excerpts from the written testimony submitted by Edward Weiler, Associate Administrator for Space Science, National Aeronautics and Space Administration, to the “Life in the Universe” hearings held by the House Subcommiteee on Space and Aeronautics on July 12, 2001.
“There are countless suns and countless Earths all rotating around their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse and no less inhabited than our Earth.”
These words, written in 1584 by Giordano Bruno, lay out the major challenge of NASA’s Origins program – namely, to use 21st century science to discover whether Earth-like planets exist beyond our Solar System and whether any of those planets are habitable, or even inhabited, by primitive life. The public and the scientific response to NASA’s search for habitable planets and life has been considerably more enthusiastic than that of Bruno’s contemporaries, who had him burned at the stake in 1600.
So how do we determine whether or not a planet has life? When the Galileo spacecraft flew by Earth on its way to Jupiter, the spacecraft turned its instruments toward Earth to look for signs of life. Other than the radio signals and the lights being on at night, the signs of life from Earth were surprisingly subtle. There was a complex green color on the continents (which we know are plants) and chemicals like carbon dioxide, oxygen, methane, and nitrites coexisting in the atmosphere – a chemical impossibility unless maintained by something like life.
But Earth did not always have this kind of atmosphere. Early Earth hosted a high-temperature, non-photosynthetic biosphere that was rich in carbon dioxide and poor in oxygen. Life on Earth was microbial and acquired energy-consuming hydrogen and sulfide, resulting in a broad array of reduced carbon and sulfur gases. What chemicals would be identifiable signs of life in the early Earth’s atmosphere?
The challenge to astrobiologists is to determine what biosignatures can be expected on any living planet. To this end, astrobiologists are studying microbial ecosystems in extreme environments here on Earth as microcosms of what might have been on early Earth and what may be possible on extrasolar planets.
Techniques Currently Used in the Search for Extrasolar Planets
One of the most successful means of discovering extrasolar worlds is the Doppler technique. The small tug of a planet on its parent star causes a small (only a few miles an hour) variation in the velocity of the star. This variation can be detected by measuring the Doppler shift — the change in frequencies of the light when the star is moving toward us versus moving away from us.
To date, we have found almost 75 stars showing significant Doppler variations. From these we have learned that approximately 7 percent of stars like the Sun have large planets located within a few Astronomical Units (the Earth-Sun distance, or AU). These large planets range from 0.2 Jupiter masses to approximately 15 Jupiter masses.
Although masses measured with the Doppler technique suffer from an ambiguity related to the orientation of the orbital plane to the line of sight, the vast majority of objects detected to date are certainly much less massive than stars – most are gas giant planets similar to Jupiter or Saturn. The recent measurement of one object that happens to pass directly in front of its star (as seen from Earth) has shown definitively that this object is a planet with a mass slightly smaller than Jupiter’s and with the density of a light, gas giant planet like Saturn.
More than half of the stars under study may have additional planets on more distant, longer period orbits. The data strongly suggest the existence of a large number of objects that are just below the present limits of detection. While multiple systems eventually may prove to be common, as yet we know of no similar counterpart to our own solar system. Furthermore, the broad range of eccentricities and small orbital radii of the known giant planets may be inconsistent with the stable conditions needed for the formation and survival of habitable, terrestrial planets.
Some have argued that these results mean solar systems like our own are rare. However, most scientists would respond that this is because the Doppler technique is fundamentally limited to finding massive planets on short-period orbits. Before being discouraged about the prospects for finding other Earths, we should note that we do not yet have the observational capability to find solar systems like our own!
The Promise of Astrometry
A second indirect planet-search technique looks for the positional (astrometric) wobble of a star induced by the presence of a planet. NASA has two complementary astrometric experiments aimed at planet detection: the Space Interferometer Mission (SIM) and the Keck Interferometer (Keck-I). SIM will have the exquisite sensitivity needed to detect planets of just a few Earth masses in 1 to 5 AU orbits around stars as far away as 30 light years. SIM will push the detectable mass limits for planets around the nearest stars into the range predicted for the “rocky” as opposed to “gas giant” planets. Keck-I will be less sensitive than SIM, but because it will operate for up to 25 years, Keck-I will be able to find planets as massive as Uranus on long-period orbits. Together SIM and Keck-I will provide a complete and unbiased census of thousands of nearby stars to determine whether systems more similar to our own are the exception or the rule.
The Challenge of Direct Detection and the Terrestrial Planet Finder
While indirect techniques are very powerful at finding planets, the search for habitability and for life requires that we directly detect the planets and use spectroscopic analysis to learn about their physical and atmospheric conditions. Thus, the goal of the Terrestrial Planet Finder (TPF) is to find and characterize any Earth-like planets orbiting 250 of the closest stars. This search will focus on the habitable zone, which is defined by the range of temperatures where liquid water, and thus the conditions for the formation of life, might be present. TPF will make detailed observations of the atmospheres of the most promising candidates to search for the spectral signatures of habitability and of life.
Understanding the conditions needed for life and identifying promising bio-signatures requires a close and continuing collaboration with biologists, atmospheric chemists, and geologists. NASA’s astrobiology scientists have been intimately involved in setting the observing requirements for TPF.
While the launch of TPF is more than a decade away, we are not standing still in terms of expanding our scientific knowledge. The results of other projects will help us to understand better the difficulty of the TPF challenge by finding out, for example, the distance to the nearest systems likely to harbor Earths. We are also beginning to think about the next steps beyond TPF, including a “Planet Imager” to provide more detailed images and/or spectroscopy of any planets found by TPF.
What will be the legacy of NASA’s Origins program as seen from 20 years in the future? We will have a complete census of the planets orbiting thousands of stars over a wide range of periods (from days to decades), planetary masses (from Jupiter’s to Earth’s), and distances (a few to a few hundred light years). We will have correlated these facts with the properties of the parent stars to develop a deeper understanding of the physical processes controlling the formation and evolution of planetary systems. We will have identified what nearby stars, if any, harbor analogs to our solar system with its stable habitable zone. From this information we will understand whether our Solar System and our Earth are common or rare. And, if we are lucky, we will have found one or more places where the complex physical and chemical processes we call life were able to develop. Through the NASA’s Origins program, we are beginning to answer one of the longest standing questions in the history of the human intellect: Are we alone?
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