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Planets are scattered throughout the universe, but intense light emanating from their parent stars hinders observation. Such obfuscation has dogged astronomers, and scientists are working to develop telescopes that can look directly at planets outside our own solar system, so-called exoplanets.
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The first exoplanet discovered |
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Discovery is the essence of astronomy. Using powerful telescopes capable of looking into deep space, astronomers are hoping to locate an exoplanet that resembles the Earth.
Dr. Motohide Tamura, an associate professor at the NAOJ's Exoplanet Project Office, notes that all known exoplanets "were found through indirect methods of observation, not direct telescopic viewing. The light reflected by exoplanets is concealed by the intense brightness of their parent stars. Therefore, they are very difficult to detect."
The first exoplanet, 51 Pegasi b, was discovered by Swiss researchers in 1995 using the radial velocity method. This measures changes in a star's spectra caused by the small gravitational effect an unseen body (in this case 51 Pegasi b) has on the star. If the star "wobbles," it is indirect proof there's a planet nearby. Approximately 50 light-years away, 51 Pegasi b is a massive, Jupiter-like planet with surface temperatures of around 1,000°C (1,800°F), that is closer to its sun than Mercury is to ours, circling it in only about four Earth days compared with Mercury's 88-day orbit.
The excitement over 51 Pegasi b sparked a search for other exoplanets. More telescope time was devoted to radial velocity searches and more than 200 exoplanets have been discovered.
High-contrast telescope adopting coronagraph
The JTPF project is hoping to launch a satellite housing a telescope equipped with a coronagraph. The 3.5-meter (137-inch) diameter device will be used for direct observation of exoplanets, unhindered by atmospheric fluctuations.
(Drawings: courtesy of NAOJ)
Image captured using a coronagraph
DH Tau b, a brown dwarf* orbiting around the star DH Tauri, 460 light-years from the Earth. The coronagraph allowed observation of DH Tau b by screening otherwise blinding light from DH Tauri.
(Courtesy of NOAJ)
*A brown dwarf is an object sometimes called a "failed star," because it does not have enough mass to start and sustain nuclear fusion like a normal star. Planets have even lower masses.
Dr. Tamura wants to move beyond indirect (radial velocity) observation to direct (telescopic) observation. He's working with the Coronagraphic Imager With Adaptive Optics (CIAO), which suppresses light from parent stars, aiming for direct exoplanet observation. CIAO is slated to be replaced this summer by an even more powerful, high-contrast system called HiCIAO, which should increase observational capabilities tenfold.
Finding an Earth-like exoplanet presents many challenges, even to equipment as sophisticated as HiCIAO. "Earth-sized planets are just too small to detect in the vastness of space," Dr. Tamura says. "Using the Subaru Telescope, we may be able to discover Jupiter-sized planets at best."
Dr. Tamura and his team are now focused on the Japanese Terrestrial Planet Finder, or JTPF, a space-based, 3.5-meter (137-inch) optical telescope with technology similar to HiCIAO. "We'd like to launch JTPF by 2018 or 2020 at the latest, after similar European and American projects. Our ultimate goal is to discover a planet with life on it, a ‘second Earth.’ This is our dream."*
*While this issue of Nikon Horizons was in production, using the radial velocity method, astronomers discovered the most Earth-like exoplanet to date. Named Gliese 581 c, it orbits Gliese 581, a red dwarf star 20.5 light-years (about 194 trillion km/120 trillion miles) away from Earth. Scientists are excited about this discovery for many reasons, as Gliese 581 is one of the 100 closest stars to Earth and Gliese 581 c might have water and surface conditions suitable to support life, even if very different from our Earth.
Believe it or not, there are telescopes without eyepieces or lenses. A radio telescope, usually a large parabolic dish antenna (or a system of linked antennas), "sees" objects by detecting radio waves in their electromagnetic spectrums.
One such telescope is the Atacama Large Millimeter/submillimeter Array (ALMA), scheduled for completion in 2012 on the Chajnantor plain 5,000 meters (16,400 feet) up in Chile's Atacama Desert, famous for its abundance of clear, dry nights. A partnership of Europe, Japan and North America in cooperation with the Republic of Chile, ALMA will comprise 80 high-precision antennas capable of observing optically invisible parts of the universe that shine brightly in the millimeter and submillimeter wavelengths.
"It is very difficult to catch and analyze radio waves from space," explains Dr. Masato Ishiguro, professor of NAOJ and Director of the Japanese ALMA project office. "For example, if you were to put a mobile phone on the moon, its signals could be interpreted as a star as strong as the brightest radio objects in the universe. In other words, the electromagnetic waves from astronomical objects traveling through space are extremely weak and difficult to detect. This is why ALMA must be so powerful."
ALMA
This huge system of linked radio telescopes (interferometer array) uses a computer to transform radio signals into images. The antennas, mounted on transporters to facilitate changes to the array configuration, obtain the highest resolution (0.01 arcsecond*) when they're spread out over 18.5km (11.5 miles). Such performance is 10 times more powerful than the Subaru Telescope.
(©European Southern Observatory; Courtesy of NAOJ)
*A subdivision of an arcminute. An arcminute, used to measure arc, is 1/60 of one degree; an arcsecond is 1/60 of one arcminute, or 1/3,600 of one degree.
Spectra of Electromagnetic Waves
It's possible to receive radio, visible light and infrared and ultraviolet light with wavelengths closer to visible light on the Earth's surface. In order to fully examine distant objects and phenomena, astronomers must be able to view and measure plural or even entire range(s) of spectra.
Astronomers have high expectations for ALMA. "It has a resolving power 100 times greater than conventional millimeter arrays," Ishiguro explains. "This will enable us to detect the birth of very distant stars and planets, and determine how galaxies are formed, how they age and how they die."
ALMA will literally be looking into the past at stars and galaxies formed at the dawn of the universe. This will help us understand how the universe has evolved and, perhaps, help us in our search for exoplanets and extraterrestrial life.
