Statement of Dr. Edward J. Weiler, Associate Administrator for Space Science, National Aeronautics and Space Administration
before the Committee on Science House of Representatives
Mr. Chairman and Members of the Committee:
I am pleased to be here today to present to you "The Search for Earths Beyond Our Solar System".
The Questions Inspiring NASA's Origins Program: Are We Alone? Where are our Nearest Neighbors?
"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.
Most recently, the National Academy of Sciences Decadal Review of Astronomy and Astrophysics endorsed these Origins goals, noting:
"Key problems that are particularly ripe for advances in the coming decade [include] studying the formation of stars and their planetary systems, and the birth and evolution of giant and terrestrial planets."
"Search for Life outside of earth and, if it is found, determine its nature and its distribution in the galaxy [This] is so challenging and of such importance that it could occupy astronomers for the foreseeable future."
I will first summarize what we know about planetary systems based on present observational techniques and then lay out what we will learn in the coming decade as new NASA missions bring us a deeper knowledge about those most basic human questions: Are we alone? Where are our closest stellar neighbors?
Results from Indirect Measurement Techniques
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 using a technique that measures 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% of stars like the Sun have large planets located within a few Astronomical Units (the Earth-Sun distance, or AU) of their parent star. These large planets range in size from 0.2 Jupiter masses (MJ) to ~15 Jupiters. The data, much of it obtained using NASA-funded observing time on the Keck telescope, strongly suggest the existence of a larger number of still smaller objects just below the present limits of detection. 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 smaller than stars; most are gas giant planets similar to our own Jupiter or Saturn. The recent measurement of one object, detected by the Doppler technique, 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.
While only a handful of systems with multiple planets are known, the Doppler data for more than half of the stars under study are highly suggestive of additional planets on more distant, longer period orbits. While multiple systems may eventually prove be common, as yet we know of no counterpart to our own "grand design" solar system with gas giants on circular orbits located beyond the water-ice condensation line and rocky planets nestled within the habitable zone. 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 unexpected results mean that 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 systems like our own! Learning about the planets that may orbit the remaining 93% of solar type stars for which the Doppler technique has provided only upper limits is one of the Origins program's long term goals.
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 1-5 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 large 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 detect photons directly from planets and use spectroscopic analysis to learn about 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 (within 50 light years) stars. This search will focus on the habitable zone (1 AU around a star like our Sun), 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.
The technology to detect planets directly is within our grasp. The challenges include faint signals, the enormous contrast ratio between the star and the planet, the close proximity of the planet to the star, and the presence of dust emission in our own and in the target solar systems. The nature of these problems differs in the two wavelength regimes: reflected light in the near-IR and visible and thermal emission in the mid-infrared.
Visible Light Systems
An observatory that can detect the light of a parent star reflected off of a distant Earth requires a large, visible light telescope (roughly the same 6-8 m size of the Next Generation Space Telescope, but with much better optical performance), equipped with an advanced coronagraph to block the glare of the star. The advantages of such a telescope include operation in a traditional imaging mode on a single spacecraft. The chief disadvantage arises from the extreme star-to-planet contrast ratio of more than one billion to one, which implies the need for exquisite control of the scattered and diffracted light. Spectral features of water and molecular oxygen could exist and be used to characterize a planet's atmosphere.
An observatory that can detect the emitted thermal radiation from an Earth requires a nulling infrared interferometer consisting of four small telescopes, each 2-3 meters in diameter. The telescopes would operate on separated spacecraft over baselines of ~100 meters to achieve the angular resolution required to separate the planet's light from the star's. The chief advantages of this system include the relatively favorable star-to-planet contrast ratio (only a million to one!) and the presence of very broad, deep bands of carbon dioxide, water and ozone that can be used to characterize the planetary atmosphere and serve as signposts of life. The disadvantages include the need for multiple spacecraft and cryogenic operation.
All of these options for direct planet finding (visible coronagraphs or infrared interferometers) are being investigated in the US by four NASA-sponsored studies involving 16 industrial concerns, 30 universities and 75 scientists. These groups are investigating the broadest possible range of architectures for TPF. From a list of more than 20 concepts, NASA and its contractors have chosen 4 of the most promising for study. At the end of this year, NASA will select 2 architectures for intensive technology development over the next four years in preparation for a final choice of architecture and a new start around 2008, and a launch around 2014.
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 some chemicals 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 20% oxygen atmosphere. Early Earth hosted a high-temperature non-photosynthetic biosphere, rich in carbon dioxide and poor in oxygen. Life on Earth was microbial and acquired energy consuming hydrogen and sulfide, producing a broad array of reduced carbon and sulfur gases. What chemicals would be identifiable signs of life in the early Earths 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.
A Step-by-Step Approach to NASA's Origins Goals
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 earlier 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 and the amount of exo-zodiacal dust to expect:
In parallel with improving our scientific understanding in advance of TPF, NASA is working to ensure that the technology for TPF will be ready as well.
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. While we are still in the preliminary phases of conceptualizing such a mission, we must ensure that the Origins program as currently structured enables us to build toward this ultimate goal.
The Legacy of the Origins Program
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 Jupiters to Earths), 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. From this information we will understand whether our Solar System and our Earth are common or rare. We will have identified what nearby stars, if any, harbor analogs to our solar system with its stable habitable zone. 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. Assuming these successes, we also expect to be preparing for missions that will go beyond TPF. 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?