The Search for Terrestrial Planets Around Other Suns

"Innumerable suns exist; innumerable earths revolve around these suns in a manner similar to the way the seven planets revolve around our sun.  Living beings inhabit these worlds."
-- Giordano Bruno (1548-1600), Italian philosopher, On the Infinite Universe and Worlds (1584)

The above quote was a bold declaration in the 16th century -- before the invention of the telescope and at a time when belief said there was only one world inhabited by people.  Now, over 400-years later, Bruno's words are not just bold but those of a visionary: in the last decade, numerous planets have been detected around other stars.  The count should surpass 100 as we enter mid-decade.  Such is the pace of discovery that the topic is blasé to the press and no longer "evening news" worthy on television.

Planet transiting its sun.  NASA

All the extrasolar planets are thought to be gas giants.  Hundreds or thousands of times the mass of Earth, their composition is thought to be mostly hydrogen and helium.  Some of these bodies may turn out to be closer to "failed" suns in size -- the debate on "what defines a planet" is ongoing.  These planets are not like those envisioned by Bruno or imagined on popular science fiction shows like Star Trek.  No Vulcans, Klingons, or other fantastic civilization can inhabit them -- according to our current understanding of life -- no future starship could "land" on one -- the existence of a solid core in these worlds is debatable, and the temperature and pressure on such a surface would be enormous.

It's not that smaller planets don't exist: ground-based detection systems can only detect large planets.  Planet seekers monitor a star's position, brightness, and spectra through an atmosphere that constantly imparts refraction wobbles on the star's light.  Periodic variation in these observations can only be attributed to large planets, since the effects of smaller bodies are miniscule compared to the data "noise" imparted by our atmosphere.  This is demonstrated by the "twinkling" phenomena of stars, especially after sunset, as temperature differences cause layers of air to rise and fall through the path of starlight.

To detect a range of planets, we will need to take the atmosphere out of the picture -- literally.  This will happen very soon, as we are on the verge of the next "big event" in extrasolar planet discovery: employing space-based detectors to locate small, rocky planets, which you and I could stand on.  These will be terrestrial-sized planets, similar to Venus, Earth, and Mars in size.  While the first batch of space detectors, lofted at decades end, will identify planets by their impact on a star's light, the generation of detectors launched in the next decade will image the actual planets.  This later group may even achieve the "holy grail" of extrasolar planet research -- important enough to make it back on the nightly news -- the detection of habitable, terrestrial planets with indications of biologic activity -- life!

Habitable Zone and Life Signatures

The habitable zone around a star is the region where liquid water could exist on the surface of a terrestrial planet.  Gas giants -- the type of planet detected so far -- couldn't exist for long in this zone, because their gasses would boil off.  The habitable zone for our Sun starts beyond Venus and ends before Mars -- Earth is right in the "sweet spot" for life to exist.

Once a planet is detected, determination of its orbital period and knowledge of a star's properties (such as the spectral type) will indicate if it exists within that system's habitable zone.  Orbiting in the zone doesn't automatically indicate that it is inhabited.  To understand how we can examine an extrasolar planet for life signs, scientists have been taking a close look at how the Earth would appear from distant space. 

As our world turns on its axis, the varied weather and biomes of Earth -- water, ice, clouds, forests, fields, deserts, etc. -- reflect varying amounts of light out into space.  In the course of a day, the light variation could be 150-percent.  Other planets in our system have light-curves that vary by a fifteenth as much per rotation.  An extrasolar observer looking at the planets of our system may notice Earth's "winking" act.  Taking into account Earth's location in the "sweet spot" of the habitable zone around our star, the alien observer may conclude that Earth is a candidate for harboring life. 

Earth inhabits the habitable zone of our system. Note the lack of water vapor in the spectrum of Venus and Mars.  NASA

If the light from a world can be detected, spectral analysis is possible.  The shape of the spectral curve would confirm the temperature of the planet, and absorption lines would tell us the contents of the atmosphere.  Earth's spectrum is characterized by water vapor absorption -- a sign that our planet inhabits a habitable zone around our star -- and ozone -- a form of oxygen that indicates possible biologic activity.  The presence of hydrocarbons, like methane, on a small planet is also indicative of life processes.

If we detect terrestrial planets with all the indicators of life, would it be possible to determine if intelligent life existed?  Given a list of these planets, scientists involved in the Search for Extraterrestrial Intelligence (SETI) could turn their radio and laser detection telescopes to the target planets in search for signals.  Barring that, future detection missions may be able to search other parts of the planetary spectrum for signs of air pollution or other industrial activity.  For pre-industrial civilizations, like the one that Giordano Bruno lived in, detection of intelligent life would be difficult. 

Perhaps the question of what "intelligent life" is must first be resolved.  As for Giordano Bruno, his unorthodox foresight branded him as a heretic, and he was burned at the stake in 1600 -- just six years after his insightful quote.

Below is a roundup of technologies to be employed on NASA and European Space Agency (ESA) space-based detectors that will engage in the search for terrestrial planets.

"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."

-- Giordano Bruno, On the Infinite Universe and Worlds

Transit Timing

COROT COROT -- ESA COROT (COnvection ROtation and planetary Transits) is a French Space Agency mission with ESA support.  It will employ a 30-cm space telescope to search nearby stars for ever-so-slight dimming: the sign that a possible planet has passed in front of the star.  COROT will detect planets down to a few Earth-diameters. The ability to detect minute brightness changes in a star will allow COROT to conduct asteroseismology: the study of sound waves resonating within a star.  First discovered on our Sun, these "sun-quakes" cause brightness fluctuations that COROT can detect. Launch is scheduled for 2005 on a Russia launcher. More information: European Space Agency; Feb 28, 2002; Press Release: "European Astronomers Get Their First Chance to Detect Rocky Planets Around Other Worlds"

This technique has netted dozens of extra-solar planets from ground-based telescopes: the light from a star is monitored for fluctuations caused by planets passing in front of the star.  Our own planet imparts a variability of 80-parts-per-million in the brightness of our Sun, when Earth transits between it and a distant observer.  On the ground, the natural fluctuation of light as it passes through our atmosphere limits this system to detection of Jupiter-sized planets.  Space telescope detectors escape the atmospheric interference -- thus lowering the size of planet that could be detected -- and allows the telescope to continue observations without regard to "day" and "night."  This is an important advantage over ground scopes, because transits will dim the starlight for hours or days. 

The type of stars studied must also be considered when analyzing data from planetary transits.  Our Sun varies in brightness by 10-parts-per-million during the half-day our planet takes to cross its face.  It is hoped that stars similar to our Sun also have small deviations, so as not to mask the detection of Earth-sized planets.  For NASA's Kepler mission, the planet detection criteria are for three transits of consistent period, brightness drop, and duration. 

An estimate of the orbital period of the planet can be made from the duration of the dimming.  This will determine if the planet orbits in the life-friendly hospitable zone around the star.

Kepler Kepler -- NASA Scheduled for launch in 2006, NASA's Kepler Mission will be the first able to detect Earth-size and smaller planets.  The heart of the system will be a 0.95-meter diameter telescope that dumps light onto an array of 42 CCDs.  The field of view will be huge: 12-degrees in diameter -- enough to put 24-full Moons across.  It will use this wide field to maintain a constant vigil on a group of 100,000 stars for four years, looking for any periodic brightness changes that are the signature of planetary transits across the face of the stars. The Kepler team believes "many hundreds of terrestrial-type planets" will be detected in the course of the mission. Originally designed to monitor the stars from L2 orbit (see Darwin), cost reductions will force Kepler into an Earth-trailing orbit around the Sun. More information: NASA/Ames; Dec. 21, 2001; Press Release: "NASA Ames’ Kepler Mission Selected for Discovery Program"

Eddington Eddington -- ESA Eddington is an ESA "reserve" mission -- meaning a spacecraft will be constructed only if feasible and the resources are available.  A transit spotting telescope more capable than COROT but less so than Kepler, it will monitor half-a-million stars in a nearby cluster.  It will be sensitive enough to detect planets half the size of Earth. The possible launch date is sometime after 2007. More information: Nature.com; Jan. 15, 2002; News: "Planet Hunters Pledge Eddington Support"

Interferometry

Space Interferometry Mission SIM  -- NASA NASA's Space Interferometry Mission (SIM) will determine the distances and positions of stars, with unprecedented accuracy, by doing interferometry with mirrors on a 10-meter boom.  Stars displaying subtle position wobbles are the sign that planets are swinging around them.  SIM should be accurate enough to detect planets of a few Earth-masses among the nearest stars, out to 16-light years.  Larger planets to 5-Earth-masses should be found, out to 33-light years. SIM will be the first small planet detector to employ the nulling technique to dim the light of a star -- by combing some of its light out of phase -- allowing direct detection of dim planets in nearby orbit.  Launch to an Earth-trailing solar orbit is currently proposed for 2009. More information: JPL.NASA.gov, Space Interferometry Mission home page

Interferometry developed from the study of the wave nature of light in the 19th century.  Interferometers can measure the position and even diameter of a star with great precision, without necessarily creating an image of the object.  These instruments are very sensitive to the angle between adjacent light sources.  To give you the idea of the power of interferometry, consider the case of light from a source (the wave-front) arriving at two mirrors, separated by a distance.  The mirrors reflect this light some distance before combining (interfering) the two beams at a detector: the result forms a pattern of light and dark bands.  The center band is light (constructive interference) if the two beams traveled the same distance to the point of interference.  But, if one of the beams is delayed, by additional mirrors, to travel just a half-wavelength of light longer, destructive interference occurs, and a dark band forms at the center of the pattern.  Because a small distance of a half-wavelength of light can cause a detectable difference in result, this is a powerful tool for measuring angles between objects, if the alignments of the mirrors are known.

Another interferometry technique called "nulling," proposed for planet spotting missions, will create images of planets.  In this system, light from multiple telescope mirrors are combined a half-wavelength out of phase, so destructive interference blots out the image of the star.  The technique is so sensitive that a nearby planet, as slight as 1/3,600-degree from the star, will not be "nulled."  Hence a dim planet can be detected in the vicinity of a bright star.

Planet Imagers

Darwin Possible Darwin configuration -- Alcatel Space Industries Still in the planning stages, ESA's Darwin mission is an attempt to detect Earth-sized and Earth-like planets around nearby stars, within 30-light years of Earth.  Darwin is envisioned as a constellation of satellites flying in an exacting formation: 4 to 6 space telescopes, with 1.5-meter diameter mirrors, will feed light to a central hub spacecraft.  Each telescope must maintain its distance from the hub to an accuracy of 20-billionth of a meter -- made possible by a system based on GPS techniques.  In orbit behind this group will be a communications satellite to relay instructions and data between Earth and the hub.  This will be among the first detectors to actually image planets.  The orbit will be at the Sun/Earth Lagrangian-2 (L2) point -- 1.5-million km from Earth in the direction opposite the Sun.  Darwin's special orbit is far beyond where it can be serviced in case of trouble.  Normally, an Earth-launched spacecraft placed into a roughly circular solar-orbit further from the Sun will "fall behind" the Earth over time.  At L2 satellites still orbit the Sun, but the gravitational tug of the nearby Earth will give them an orbital period the same as that of the Earth.  As a result, the constellation will have the Earth, Moon and Sun "behind" them -- simplifying communications and keeping these bright objects "out of the picture".  During the course of a year, orbital motion will allow Darwin to image most of the sky. All the satellites will be launched at the same time in an Ariane-5 rocket.  So they all must fit, stacked in two tiers, within the nosecone fairing; this restricts mirror diameter to 1.5-meters.  At present, the mission is expected to launch some time around 2014 More information: ESA.int; Science; Darwin home page

These are the most exciting of the proposed terrestrial planet detecting systems.  They are designed to actually image a planet and conduct analysis of its light and possible atmosphere.  To accomplish this task, they employ a unique design: a constellation of telescopes flying together that feed light to another satellite, where it is combined to form an image.  This design is driven by the need to "tease out" the image of a planet as a distinct object, separate from the star it orbits.  

Resolution of closely spaced objects is partly a function of the light-collecting system diameter.  While the world's largest telescope has a segmented mirror of 10-meters diameter, a telescope many times that size would be needed to fulfill a planet imager's resolution requirements.  Dreams of building a telescope of that size on the ground exist, but not in space.  In fact, the Next Generation Space Telescope (NGST) -- the planned follow-up to the Hubble Space Telescope at decades end -- will be only an 8-meter or smaller telescope. 

The resolution of a giant telescope, albeit with less light-gathering area, can be accomplished by combining the light from small telescopes some distance apart.  But, this must be done with high precision: each telescope must maintain its distance from the light integrating satellite to an accuracy measured in billionths of a meter -- you read that right!  The widely spaced telescopes that form the planet imagers should best the resolution of the NGST by an order of magnitude.

Another factor in resolving a planet near its star is the brightness difference: even with good resolution a brighter object can "drown out" a dimmer object in an image.  Terrestrial planets at visual wavelengths, shining by reflected light, are expected to be a billion times dimmer than their sun.  But, at infrared wavelengths the ratio becomes "just" a million times dimmer.  This is the wavelength the planet imagers will operate at.  Still, in order to overcome this brightness difference, the imagers will employ the "nulling" technique (see Interferometry) to blot out stars and search for planets.  NASA's TPF mission will be able to null the light of a star by a factor of more than one hundred thousand.  Large sunshields will protect each satellite from heating up and emitting infrared radiation that would "fog" the imaging system. 

Infrared light is also the wavelength where water vapor and other gasses indicative of life display their unique absorption signature, for analysis by the onboard spectrometer.

Terrestrial Planet Finder TPF -- NASA NASA's answer to Darwin is the Terrestrial Planet Finder (TPF), now under study.  It will employ a constellation of four telescopes of 3.5-meters in diameter each.  The baseline distance between telescopes can vary from 75- to 1,000-meters.  TPF's goal is to search for Earth-sized planets out to 50-light years.  The mission could detect Moon-sized objects in systems nearby. Like Darwin, it will operate in the infrared and use nulling techniques to detect planets and spectral analysis to characterize their atmospheres. The proposed mission launch of 2011 would beat Darwin to orbit. More information: JPL.NASA.gov, Terrestrial Planet Finder home page