Using large space-based telescopes (>10-meter) Astronomers could search for terrestrial planets with atmospheres suitable for life as we know it. Spectroscopy could be used to detect the presence of Ozone, an indicator of oxygen in the atmosphere as well as water bands. Methane produced as a result of biogenic activity could be searched for using the same methods.
Using telescopes in Earth orbit planets could be searched for using direct detection methods. Stellar coronagraphs can be used to supress the light from the planet's parent star making detection easier. There is a higher probability of detecting planetary companions around nearby stars in the infrared portion of the electromagnetic spectrum. This is due to the fact that the star to planet flux ratio is less in this region of the spectrum than in the visible.
To separate a planetary companion from its primary stellar halo, one must use a telescope (or array of telescopes) with an aperture (or baseline) B such that r/D is greater than or equal to the wavelength at which the observation is being carried out divided by the aperture (or baseline) of the telescope (or array of telescopes).
Habitable Zones About Main Sequence Stars
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Mass(Ms) Type Continuously Habitable Zone
r(inner) r(outer) width
1.20 F7 1.616 1.668 0.054
1.15 F8 1.420 1.481 0.061
1.10 F9 1.240 1.310 0.069
1.05 G0 1.086 1.150 0.064
1.00 G2 0.958 1.004 0.046
0.95 G5 0.837 0.867 0.030
0.90 G8 0.728 0.743 0.015
0.85 K0 0.628 0.629 0.001
0.835 K1 0.598 0.598 --
Distance and widths are given in AUs
Our galaxy is one of the most massive galaxies in the nearby universe, which makes it unique. Our solar system seems to have some unique qualities which makes our star capable of supporting a habitable zone and thus liquid water necessary for life as we know it. Some of these unique qualities include:
The metallicity of our Sun appears to be just right to support terrestrial planet formation. Old stars located near the center region of our galaxy are metal-poor because they formed from just hydrogen, helium, and lithium. Some of the more massive stars complete their nuclear fusion cycles and explode as supernovas. The heavy elements produced from successive cycles of nuclear burning are dispersed into the ISM. Second generation stars are created out of these heavier elements, and each stellar explosion lead to a greater abundance of available metals. A star of high metallicity, therefore, has material that originated from many previous generations of stars.
Our sun has an unusually high metallicity compared to other solar-like stars, and the reason for this is not yet understood. It is possible that our star formed in a part of the Milky Way galaxy that had a high abundance of metals (elements heavier than helium), and then migrated to its present location.
Recent statistical studies on the metallicities of stars with extrasolar planetary candidates around them indicate that metal-rich stars are more likely to have planets orbiting them. The most likely explanation for this is that a certain minimum threshold of metals is required to form rocky terrestrial planets and the cores of giant gaseous planets. Therefore, a star that forms from an interstellar cloud high in heavy elements is more likely to form planets than a cloud deficient in metals.
The Sun's circular orbit about the galactic center (with an orbital period of about 250 million years) manages to avoid the hazardous spiral arms of the Milky Way galaxy.
Keeping away from spiral arms, where new stars are being formed, prevents our solar system from being gravitationally disrupted. Fortunately, our star orbits through the galaxy at nearly the same rate as the spiral-arm rotation. This synchronization protects our solar system from crossing a hazardous spiral arm to often. Avoiding the spiral arms prevents our planet from encountering supernovae and giant molecular clouds, which can perturb the cometary bodies out of the Oort Cloud leading to a high number of cometary showers in the inner solar system.
The low eccentricity of our star's orbit (near zero) makes it unique from most other solar-type stars which have higher eccentricities. The Sun's low eccentricity decreases the probability that it will cross a spiral arm.
Our solar system is located at a safe distance (about 8.5 kpc) from the Galactic Center, which provides a safe haven from disruptive gravitational effects and harmful radiation produced from clouds of ionized gas rapidly moving around a supermassive black hole.
If our solar system were located close to the Galactic Center, the combined perturbing effects from all the stars would cause a high flux of comets to rain in from the Oort Comet Cloud. The high number of cometary impacts would create global extinction events that would prevent complex life from evolving on the inner terrestrial planets.
If our solar system were located near the inner region of the Galaxy, the increased exposure to gamma radiation and x-rays, in addition to cosmic rays, would be lethal to any life trying to evolve on a terrestrial planet. The effects of radiation would damage the protective ozone layers of such planets. In addition, secondary particle cascades created in the planet's atmosphere would be produced by high-energy particles. This would, in turn, increase radiation levels at the surface of the planet.
When combined together, these three conditions produce what is known as a Galactic Habitable Zone. This existence of life on our planet, from simple microorganisms to human beings, is a result of these unique conditions.
It has been suggested that approximately 95 percent of all stars in the Galaxy wouldn't be able to support complex life from evolving. This is due to the fact that their rotational rate around the Galactic Center is not synchronized with the rotational rate of the hazardous spiral arms. When additional conditions required for keeping a solar system habitable are factored in, the probability of finding another solar system with the Galactic Habitable Zone, is extremely low.
The radial velocity technique used to detect exoplanets involves measuring the amount of wobble of the host star due to the gravitational tug from planets orbiting the star. Multiple planets will produce complex radial velocity curves with multiple frequencies superimposed on the curve. The method uses very high-resolution spectra of the star to look for subtle doppler shifts in spectral lines that result from the wobbling motion of the host star due to gravitational perturbations from any planets orbiting the star. With present-day technology the radial velocity method can achieve measurement accuracies of approximately 1 meter/sec.
The astrometric method involves measuring a star's position on the sky plane and observing how the position varies over time. If the star has a planet (or planets) then it will move in tiny circular or elliptical paths around the center-of-mass of the star system as a result of gravitational perturbation from the planet (or planets). The astrometric method is most sensitive to planets with large orbital distances, which makes it complementary to other methods that are sensitive to planets with close-in orbits around their host star. Planets in orbits with large semi-major axes take many years, if not decades to complete one orbit. Thus, the astrometric method requires decades of observations to accurately determine the orbit of such planets. Differential astrometic measurements of the host star against a set of reference stars located in close proximity (typically within a degree) of the star will allow measurement of the star's reflex motion with an astrometric accuracy of about 1 microarcsecond in a single measurement. The path of a host star orbiting the star-planet center-of-mass appears projected on the plane of the sky as an ellipse with angular semi-major axis. Therefore, the astrometric method is proportional to the planet's mass and the radius of the planet's orbit, and is inversely proportional to the distance to the star system from Earth. The astrometric signature (or amplitude) resulting from a Jupiter mass planet orbiting a solar type star at a distance of 10 pc would be 500 microarcseconds. For an Earth mass planet at 10 pc located 1 AU from a solar type star, the astrometric amplitude would be 0.3 microarcseconds. A star with multiple planetary companions would exhibit a complex motion about the solar system's center-of-mass due to the combined effect from all of the planetary gravitational perturbations. One significant advantage of the astrometric method is the capability to deduce the mass of the planetary companion directly, provided that the semi-major axis is known from spectroscopic measurements, the distance of the planet from the star's parallax motion, and the mass of the host star from its spectral type or from evolutionary models.
Shown below are spectra for Venus, Earth, and Mars. The presence of ozone band shows plentiful oxygen, probably produced by life. The detection of water indicates a planet with an ocean. The presence of carbon dioxide indicates that a terrestrial planet has an atmosphere. The detection of ozone coexisting with a reduced gas such as nitrous oxide or methane could be evidence of a planet that is inhabited. However, the presence of such gases will not tell us how complex the life inhabiting the planet is. It could be microorganisms or a highly advanced civilization. It is also possible that planets without oxygen could sustain life. It is possible that photosynthesis could occur with another element, such as sulfur, with sulfur playing the role of oxygen.
The TPF will allow astronomers to search for atmospheric gases such as carbon dioxide, water vapor, and ozone. Together with the temperature and radius of the detected planets, this information will allow astronomers to determine which planets are habitable, or even whether they may be inhabited by rudimentary forms of life.
The question as to whether comets could have delivered amino acids and water to the Earth during its long 4.5 billion year history is still in debate. The main concern is whether or not the heat and explosive forces of the planetary impacts would destroy any amino acids that existed in the cometary bodies, thus vaporizing any possible biological "seeds".
Interplanetary Dust Particles (IDPs) are thought to contain a significant amount of Polycyclic Aeromatic Hydrocarbons (PAHs), in addition to more complex aromatic networks. These PAHs are formed in the outflows of carbon-rich stars or by shock fragmentation of interstellar grains. Recent observations indicate that PAHs may be the most abundant free carbon molecules in space comprising up to 15% of the cosmic carbon. In addition to comets, asteroids and micrometeorites, PAHs may have been a source for delivering a rich variety of organic compounds to the primative Earth. This primative prebiotic soup played a key role in the origin of terrestrial biochemistry.
A Dyson sphere is an artifical shell or ring surrounding a central star. The sphere or ring is the size of a planetary orbit and would theoretically allow an advanced civilization to utilize all of the energy radiated by the central star. A Dyson habitat would have an enormous surface area, in fact, a habitat with a radius of one AU would have a surface area of at least 2.7E17 square kilometers or about 600 million times the surface area of the Earth.
Gauss' law states that the integral of the force across an arbitrary closed surface is proportional to the amount of mass inside it. If the surface is a sphere surrounding the Dyson sphere, there is an inward force on the surface of the sphere since there is mass inside it. A rigid Dyson sphere would not be stable, since there is no net attraction between a spherical shell and a point mass inside.
Earth, do you read me? OSETI detectors are designed to intercept brief pulses of laser light that might indicate extraterrestrials are trying to communicate with us. To test their detector, the Harvard team took time exposure oscilloscope readings of successive flashes from a light- emitting diode (LED). The small bumps, lower left, are single-photon flashes, and the large bumps, lower right, are multiple-photon flashes. Light from astrophysical sources like stars arrives with the photons spread out in time and does not produce double pulses. Signals from a large laser on another planet would stand out because of artificial characteristics: large amplitude in a short time and successive pulsing.
