The primary objective of the Terrestrial Planet Finder missions is to detect directly and characterize Earth-like planets around nearby stars. The measurement requirements of TPF-I for the detection and characterization of the constituents of extrasolar planetary systems are given in Table 1.

Types of Stars

On astrophysical grounds, Earth-like planets are likely to be found around stars that are roughly similar to the Sun. Therefore, TPF target stars will include main sequence F, G, and K stars. However, M stars may also harbor habitable planets, and the nearest of these could be investigated using the high angular resolution of the interferometer.

Terrestrial Planets

Considering the radii and albedos or effective temperatures of Solar System planets, TPF-I must be able to detect terrestrial planets, down to a minimum terrestrial planet defined as having 1/2 Earth surface area and Earth albedo. In the infrared, the minimum detectable planet would be one with an infrared emission corresponding to the surface area and optical albedo, positioned in the orbital phase space stipulated below.

• Habitable Zone: The TPF missions should search the most likely range as well as the complete range of temperatures within which life may be possible on a terrestrial-type planet. In the Solar System, the most likely zone is near the present Earth, and the full zone is the range between Venus and Mars. The habitable zone (HZ) is here defined as the range of semi-major axes from 0.7 to 1.5 AU scaled by the square root of stellar luminosity. The minimum terrestrial planet must be detectable at the outer edge of the HZ.

• Orbital Phase Space: The distribution of orbital elements of terrestrial type planets is presently unknown, but observations suggest that giant-planet orbits are distributed roughly equally in semi-major axis, and in eccentricity up to those of the Solar System planets and larger. Therefore, TPF-I must be designed to search for planets drawn from uniform probability distributions in semi-major axis over the range 0.7 to 1.5 AU and in eccentricity over the range 0 to 0.35, with the orbit pole uniformly distributed over the celestial sphere with random orbit phase.

Giant Planets

The occurrence and properties of giant planets may determine the environments of terrestrial planets. The TPF-I field of view and sensitivity must be sufficient to detect a giant planet with the radius and geometric albedo or effective temperature of Jupiter at 5 AU (scaled by the square root of stellar luminosity) around at least 50% of its target stars. A signal-to-noise ratio of at least 5 is required.

Exozodiacal Dust

Emission from exozodiacal dust is both a source of noise and a legitimate target of scientific interest. TPF-I must be able to detect planets in the presence of zodiacal clouds at levels up to a maximum of 10 times the brightness of the zodiacal cloud in the Solar System. Although the average amount of exo-zodiacal emission in the "habitable zone" is not yet known, we adopt an expected level of zodiacal emission around target stars of 3 times the level in our own Solar System with the same fractional clumpiness as our Solar System's cloud. From a science standpoint, determining and understanding the properties of the zodiacal cloud is essential to understanding the formation, evolution, and habitability of planetary systems. Thus, TPF-I should be able to determine the spatial and spectral distribution of zodiacal clouds with at least 0.1 times the brightness of the Solar System's zodiacal cloud.

Spectral Range

The required spectral range of the TPF-I mission for characterization of extrasolar planets will emphasize the characterization of Earth-like planets and is therefore set to 6.5 to 18 µm in the infrared. The minimum range is 6.5 to 15 µm.

Spectrum

The TPF-I mission will use the spectrum of a planet to characterize its surface and atmosphere. The spectrum of the present Earth, scaled for semi-major axis and star luminosity, is used as a reference and suggests a minimum spectral resolution is 25 with a goal of 50. TPF-I must measure water vapor and ozone with 20% accuracy in the equivalent width of the spectral feature. Additionally it is highly desirable that TPF-I also be able to measure carbon dioxide as well as methane (if the latter is present in high quantities predicted in some models of pre-biotic, or anoxic planets).

Number of Stars to be Searched

To satisfy its scientific goals, TPF-I should detect and characterize a statistically significant sample of terrestrial planets orbiting F, G, and K stars. Although at this time, the fractional occurrence of terrestrial exoplanets in the Habitable Zone is not known, a sample of 150 stars within 30 pc (including a small number of nearby M stars) should suffice based on our present understanding.

Extended Number of Stars

It is desired to search as many stars as possible. We anticipate that any mission capable of satisfying these objectives will also be capable of searching many more stars if the overall requirements on completeness are relaxed. It is desired that TPF-I be capable of searching an extended group of stars defined as those systems of any type in which all or part of the continuously habitable zone (see below) can be searched.

Search Completeness

Search completeness is defined as that fraction of planets in the orbital phase space that could be found within instrumental and mission constraints. We require each of the 150 stars to be searched at the 90% completeness level. For other targets in addition to the 150 stars, the available habitable zone will be searched as to limits in planet's orbital characteristics.

Characterization completeness

While it will be difficult to obtain spectra of the fainter or less well positioned planets, we require that TPF-I be capable of measuring spectra of at least 10 (~50%) of the detected planets.

Visitations

Multiple visits per star will be required to achieve required completeness, to distinguish it from background objects, to determine its orbit, and to study a planet along its orbit. TPF-I must be capable of making at least 3 visits to each star to meet the completeness and other requirements.

Multiple Planets

After the completion of the required number of visitations defined above, TPF-I should be able to characterize a planetary system as complex as our own with three terrestrial-sized planets assuming each planet is individually bright enough to be detected.

Orbit Determination

After the completion of the required number of visitations defined above, TPF-I shall be able to localize the position of a planet orbiting in the habitable zone with an accuracy of 10% of the semi-major axis of the planet's orbit. This accuracy may degrade to 25% in the presence of multiple planets.

Imaging

The interferometer will be capable of imaging in the nulling mode and in the classical mode of constructive interference. This is to meet major science goals in the areas of:

• Star and planet formation and early evolution.
• Stellar and planetary death and cosmic recycling.
• The formation, evolution, and growth of black holes.
• Galaxy formation and evolution over cosmic time.

Nulling

• Sensitivity: A point source of 1 µJy should be detectable with a signal divided by noise (S/N) of 5 in one hour's integration time
• Baseline Lengths: Maximum 200 meters

Classical interferometry

• Same as above
• Operate over the wavelength range of 3-15 µm
• A synthesized field of view (FOV) of 100-1000 resolution elements in two orthogonal coordinates
• A dynamic range of 100:1
• 1000 spectral elements

Performance Requirement

As a minimum, TPF-I must be able to detect planets with half the area of the Earth, and the Earth's geometric albedo, searching the entire HZ of the core-group stars with 90% completeness per star. Flux ratios must be measured in three broad wavelength bands, to 10% accuracy, for at least 50% of the detected terrestrial planets. The spectrum must be measured for at least 50% of the detected terrestrial planets - to give the equivalent widths of water vapor, and ozone to an accuracy of 20%.

Performance Goal

As a goal, TPF-I must be able to detect planets with half the area of the Earth, with Earth's geometric albedo, searching the entire HZ of the 150 stars with 90% completeness. The flux ratio must be measured in three broad wavelength bands to 10% accuracy for at least 50% of the detected terrestrial planets. The spectrum must be measured-for at least 50% of the detected terrestrial planets-to give the equivalent widths of water vapor, and ozone in the infrared to an accuracy of 20%. Further, we desire that the mission search an extended group of stars defined as those systems of any type in which all or part of the HZ can be searched.