Astronomers earlier this week announced they believe they have discovered nine new planets, bringing the total number of known "exoplanets" -- those beyond our solar system -- to 50.
But none of the exoplanets resembles the Earth. The uncovered alien worlds do not have rocky surfaces or breathable atmospheres. They are similar to Jupiter, which makes them inhospitable to life.
The search for Earth-like planets is restricted to only a fraction of the Milky Way with current technology, and unless Earth-like worlds are extremely common, finding any around the nearest stars is a long shot, says astronomy Professor Neville J. Woolf of the University of Arizona.
Woolf is optimistic that Earth-like planets ARE common. Woolf, Roger Angel and others at UA Steward Observatory have been developing ground-based and space-based telescopes and technologies for exoplanet detection for the past 14 years. Woolf is talking on that research today at the 24th general assembly of International Astronomical Union meeting in Manchester, England.
"Every third or fourth star that we look at could have such a planet, and, if so, there is a decent chance that we will find signs of life after a reasonable effort," he says.
"Current planet discoveries are like the tip of an iceberg. For every one of those planets discovered so far, there are likely to be perhaps a dozen lower-mass objects. We have not yet found them because we do not have instruments that are sensitive enough."
The Steward Observatory is developing adaptive optics for the UA/Smithsonian Institution 6.5-meter MMTO telescope on Mount Hopkins, Ariz., and for the twin-8.4-meter-mirrored Large Binocular Telescope being built on Mount Graham, Ariz. They also are developing optics for the Next Generation Space Telescope and the NASA Jet Propulsion Laboratory=B9s Terrestrial Planet Finder, a space observatory that NASA plans to launch in 2012 as part of its Origins Program. All are to be used in future exoplanet exploration.
***See the full story by writer Agnieszka Przychodzen of the UA Lunar and Planetary Lab on the UA News Services Science & Research web site.
INTERNATIONAL ASTRONOMICAL UNION
XXIVth GENERAL ASSEMBLY, MANCHESTER, UK
7 - 18 August 2000
10 AUGUST 2000
"From space it looks quite possible but we haven't built the devices to do it yet, " says Neville J. Woolf, professor of astronomy at the University of Arizona's Steward Observatory in Tucson. Woolf and his team have been investigating various ways to observe extra-solar planets from Earth and space for the past 14 years, looking to the day when Earth-like planets around other stars are finally discovered.
The search for Earth-like planets is restricted to only a fraction of the Milky Way with current technology. Astronomers are able to look in detail only at the sun's closest neighbors.
"Suppose we send out an instrument to look for Earth-like planets and we give it 70 years of observing time. It will take it about a day to observe a particular star system to find whether it has an Earth-like planet. At this rate we might survey perhaps 25,000 stars, a tiny bit of the Milky Way Galaxy, which contains about 10 billion stars. After 70 years, only 2.5 millionth of a percent of our galaxy would have been examined -- and the Milky Way is only one of about 100 billion galaxies," says Woolf. There could be 'billions and billions and billions' of Earth-like planets out there, and we would have missed them all. Our only hope is that Earths are extremely common. If so, then looking at the planets around the nearest stars, we will find some."
In the past, astronomers assumed that planets similar in size to Earth were extremely common and Jupiter-like objects were rare. But starting in 1995, discoveries revealed planetary systems very different from the solar system, with giant planets in close-in orbits unlike any body in the solar system. Astronomers began to wonder how rare planetary systems like ours were. The giant planets in orbits larger than Mercury's had very eccentric orbits so that the Earth-like planets of those systems would have been ejected early on. Woolf however notes that of the four planets with largest orbits (still less than half the size of Jupiter's orbit) two have modest eccentricity. This may be a sign that systems with giant planets in longer period orbits are more like the solar system.
Woolf considers the search for other Earths worthwhile though: "I believe that they are quite common. Every third or fourth star that we look at could have such a planet, and if so, there is a decent chance that we will find signs of life after a reasonable effort." He adds, "Current planet discoveries are like the tip of an iceberg. For every one of those planets that we have discovered so far there are likely to be perhaps a dozen lower-mass objects. We have not yet found them because we do not yet have instruments that are sensitive enough."
Woolf and Roger Angel, UA Regents' Professor of astronomy and director of the UA Mirror Laboratory, are involved in the Jet Propulsion Laboratory's "Terrestrial Planet Finder" (TPF) project, a space observatory that NASA plans to launch in 2012 as part of Origins Program.
According to Woolf and Angel, most can be learned about the planets by observing in the middle infrared part of the spectrum, the radiation emitted by any object at approximately room temperature. Planets are easier to detect in the infrared because stars are not nearly as bright as they are in visible light.
"We are still finding that the easiest method to see the planets involves the process of 'nulling'," Woolf says. He and his collaborators have been testing this innovative technique for about 5 years. "The star is usually about 10 million times brighter than its planet. But we can cause the light waves from the star to interfere with themselves, making the star nearly invisible while radiation from the planet comes through. It is possible either to make direct images of a planetary system with this technique or to reconstruct an image by mathematical data processing, Woolf says.
Another imaging technique appropriate for shorter wavelengths is called coronography. This technique requires that the telescope mirror surfaces be extraordinarily smooth to reduce the scattered starlight. Second, the bright rings around a star image must be made fainter by a process called apodization. Third, an Earth-like planet is itself so faint that a very large telescope is needed to analyze its spectrum.
"As a result, telescope systems necessary to observe the visible (wavelengths) from planets are not much different in size from nulling telescopes, and we learn less fundamental information about a planet from its visible spectrum. Studying the planet's infrared glow will allow us to measure its size and how warm it is as well as test the planet's atmosphere for the presence of oxygen in the form of ozone and water," he says.
Woolf and his collaborators have already begun the first explorations in the infrared with the Multi-Mirror Telescope situated on Mt. Hopkins, (before the telescope was converted to a 6.5-meter telescope). In 2004 they will start using the twin-8.4-meter-mirrored Large Binocular Telescope being built on Mount Graham. Both observatories are located near Tucson, Ariz. These instruments will be able to image the dust around stars as well as Jupiter-like planets but will require the use of 'adaptive optics' technique to correct for blurring effects of the Earth's atmosphere.
"There is a good chance to find Earth-like planets and even find life on these planets if it's there. The main reason for thinking that life is not difficult to get started on an Earth-like planet is, that on Earth life began so soon after the period of catastrophic collisions 4 billion years ago. It is hard to imagine that there was any really difficult process on the way. I think that there is continuity from chemicals cyclically combining and breaking up in volcanically warmed places of early Earth, to the formation of self-replicating chemicals, to the development of life and ourselves," Woolf says.
INTERNATIONAL ASTRONOMICAL UNION
XXIVth GENERAL ASSEMBLY, MANCHESTER, UK
7 - 18 August 2000
7 AUGUST 2000
"Dust blocks our view and makes it hard to definitively detect planets," said lead author Dr. Nick Gorkavyi, a National Academy of Sciences National Research Council Senior Research Associate at NASA's Goddard Space Flight Center, Greenbelt, Md. "We turned this problem into an advantage -- by reading the patterns imprinted on the dust disk, we can determine whether a planet is hiding there. The planet is still hidden, but it writes its signature in the dust."
The scientists applied their method to observations of dust disks around three nearby stars: Beta Pictoris, Epsilon Eridani, and Vega. The researchers estimate Beta Pictoris has a planet 10 times the mass of Earth orbiting about 6.5 billion miles from the star, while Epsilon Eridani has a 0.2 Jupiter-mass planet about 5.5 billion miles away, and Vega has a planet twice the mass of Jupiter in an orbit about 5 billion miles away. These distances from the parent star are larger than any of the planets in our solar system.
The research will be presented August 7 during the International Astronomical Union General Assembly meeting at the University of Manchester, Manchester, UK, and was published in the July 10 issue of the Astrophysical Journal Letters (Vega and Epsilon Eridani models only).
Astronomers believe a solar system is born when a cloud of gas and dust in interstellar space collapses. The densest region at the center of the cloud becomes a new star, while the outer regions form a surrounding disk of material, called a circumstellar disk. The disk is unstable, and portions collapse further under their own gravity, forming planets, asteroids, and comets.
According to the new method, a planet's gravitational influence redistributes dust in the disk, forming beautiful features including swirls, arcs, voids, warps, and clumps. Because the pattern depends on the planet's mass and orbital characteristics, determining the kind of pattern reveals this information about the planet.
"These patterns persist for a long time, because the dust becomes trapped in these special orbital patterns by the planet's gravity," said Dr. Gorkavyi. "If there were no planets present, radiation and particle emissions from the star would slow the orbital velocity of the surrounding dust, which would spiral into the star relatively quickly, causing the disk to vanish. So, we believe these special patterns are the signature of a planet."
If confirmed by further observations, the new method could be applied to analyze circumstellar dust disks and identify planets where it is difficult or impossible using other methods. For example, a common planet detection method is to use the wobble produced in a star's motion by the gravitational pull of unseen massive planets orbiting it. The wobble causes light emitted by the star to change color very slightly. (The change is too small to be noticed by the human eye.) By analyzing this change with a special instrument called a spectrograph, astronomers can deduce the unseen planet's mass and orbit. This method has a few significant limitations, however.
First, the star's wobble must have some component that is directed towards the Earth, or the color change will not be seen. Thus, alien worlds whose orbital plane happens to be tilted perpendicularly to Earth's orbit will remain unseen, because that kind of orbit does not pull the star toward or away from the Earth. Also, planets whose orbits are remote from the star will not be seen because their pull is too slight to produce a noticeable color change in the star's light. Even if a remote planet is sufficiently massive to exert a detectable pull, it will be many years before they are identified -- the color change will occur very slowly because it takes centuries for them to complete their huge orbits.
The new method overcomes these limitations; however, the star must be close enough for the pattern in its disk to be identified. With current telescopes, the scientists estimate their method is good for stars within approximately one hundred light years from Earth. Although the patterns are long-lived, the dust still disperses over time, so the method works best for relatively young stars, which have more circumstellar dust. The researchers include Leonid M. Ozernoy of George Mason University, Fairfax, Va., Nick N. Gorkavyi, John C. Mather, and Sara R. Heap of NASA's Goddard Space Flight Center, and Tanya A. Taidakova of Crimean Astrophysical Observatory, Ukraine.
SPACE.com Newsletter for Monday 07 August 2000
Astronomers Discover Bundle of Extrasolar Planets
Astronomers Find a 'Vulcan' Planet
the U N I V E R S E T O D A Y
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August 7, 2000 - Issue #286
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August 4, 2000 - Issue #285
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