planet formation和planet migrate的阅读jj背景资料

ted810

On the likely effect of disk self-gravity on low mass planet migration

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Since the discovery of Jupiter-like planets around nearby stars, it is commonly believed that planets form far away from their host star and migrate inwards, as a consequence of gravitational torques exerted by a gaseous disk. Spiral density waves are excited in the disk at Lindblad resonances, leading to angular momentum exchange and making the planet drift in a time much shorter than the planet formation timescale.
Two researchers (among whom A. Pierens from Paris Observatory) have for the first time determined analytically the possible effet of the disk gravity on the orbital motion of the planet, and conclude that that migration should be accelerated ! Will very high resolution numerical simulations confirm this issue ?

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The first extrasolar planet was discovered ten years ago at the Observatoire de Haute Provence (Mayor et Queloz 1995,Nature 378, 355). Today, more than a hundred of planets are known (Schneider 2005, The Extrasolar Planets Encyclopaedia), and many of them rotate on a tight orbit (their semi-major axis lies between 0.01 and 1 AU). So, in comparison with planets in the Solar System, extrasolar planets have very different orbital properties. These observations are commonly explained thanks to planetary migration. In this scenario, planets and planetesimals form in the outer regions of the circumstellar disk and then migrate inwards because of the gravitational interaction with the disk (see figure 1). In the framework of the standard theory (Ward 1997, Icarus, 126, 261), migration is caused by a slight imbalance of the gravity field created by the spiral density waves. When the gravitational force exerted by the outer disk is greater than the one created by the inner disk, the planet looses angular momentum and drifts inwards. However, the migration timescale is very short and that is the reason why people seek for a mechanism able to slow down (or even stop) the migration. Several mechanisms have been proposed like the effect of the magnetic field, tri-dimensional effects, torques at corotation (where the planet rotates at the same speed as the disk particles), eccentricity effets, interaction between several planets, and so on.

第二篇背景资料

By Robert Sanders, UC Berkeley; Don Savage, NASA headquarters; and Jane Platt, Jet Propulsion Laboratory

Washington, D.C. - After 15 years of observation and lots of patience, the world's premier planet-hunting team has finally found a planetary system that reminds them of our own solar system.

At a press conference today (Thursday, June 13) at National Aeronautics and Space Administration (NASA) headquarters, University of California, Berkeley, astronomer Geoffrey W. Marcy and Carnegie Institution of Washington astronomer Paul Butler announced the discovery of a Jupiter-like planet orbiting a sun-like star at nearly the same distance as Jupiter orbits our sun.

"This is the first near analog to our Jupiter," said Marcy, professor of astronomy and director of UC Berkeley's Center for Integrative Planetary Science. "All other extrasolar planets discovered up to now orbit closer to the parent star, and most of them have had elongated, eccentric orbits. This new planet orbits as far from its star as our own Jupiter orbits the sun.''

The star, 55 Cancri in the constellation Cancer, was already known to have one planet, which was announced by Butler and Marcy in 1996. That planet is a gas giant slightly smaller than the mass of Jupiter, and whips around the star in 14.6 days at a distance only one tenth that from the Earth to the sun.

Using the 93-million-mile Earth-sun distance as a yardstick, called an astronomical unit or AU, the newly found planet orbits at 5.9 AU, comparable to Jupiter's 5.2 AU distance from the sun. With a mass between 3.5 and 5 times that of Jupiter, the planet has a slightly elongated orbit that carries it around the star in about 13 years (5,360 days), comparable to Jupiter's orbital period of 11.86 years. The team also reported today a third planet around 55 Cancri, a gas giant with an orbital radius of 0.24 AU and a mass of about 0.21 Jupiter masses. That planet orbits in 44.28 days.

"We haven't yet found an exact solar system analog, with a planet in a circular orbit and a mass closer to that of Jupiter," Butler said. "But this shows we are getting close, we are at the point of finding planets at distances greater than 4 AU from the host star. And we found this planet among the 107 stars we first targeted when we started looking for planets at Lick Observatory in 1987, so I think we will be finding more of them among the 1,200 stars we are now monitoring."

The planet-hunting team, funded by grants from the National Science Foundation and NASA, announced a total of 15 new planets today, including the smallest ever detected: a planet circling the star HD49674 in the constellation Auriga at a distance of 0.05 AU - one-twentieth the distance from the Earth to the sun. Its mass is about 15 percent that of Jupiter, or nearly half that of Saturn, and 40 times the mass of the Earth. This brings the total number of known planets outside the solar system to more than 90.

The team of astronomers passed their data on 55 Cancri along to theoretical astronomer Greg Laughlin, assistant professor of astronomy and astrophysics at UC Santa Cruz, who conducted dynamical calculations that show an Earth-sized planet could survive in a stable orbit between the two inner gas giants and the outer planet.

"We tried a hypothetical configuration of a terrestrial planet in the habitable zone around one AU from the central star and found it very stable," said Laughlin, who also is associated with Lick Observatory. "Just as the other planets in our solar system tug on the Earth and produce a chaotic but bounded orbit, so the planets around 55 Cancri would push and pull an Earthlike planet in a manner that would not cause any collisions or wild orbital variations."

For the foreseeable future, any such planet in the habitable zone around 55 Cancri will remain speculative.

"Nevertheless, this planetary system will be the best candidate for direct pictures when the Terrestrial Planet Finder is launched later this decade," said UC Berkeley astronomer Debra A. Fischer, referring to NASA's planned space-borne imaging telescope designed to take pictures of Earth-sized planets.

The star 55 Cnc is 12.5 parsecs (41 light years) distant and a middle-aged, 4-7 billion-year-old G8 star rich in heavy elements like carbon, iron, silicon and sulfur, Fischer said. The sun is about 5 billion years old, with half that amount of heavy metals.

Laughlin speculated that the large, inner planets probably formed farther from the parent star, where ice could form and rocks accrete to form a solid core, and only migrated inward after they had scooped up a shroud of gas. This inward migration is a characteristic of giant planets in a disk of gas and dust that is typical of forming planetary systems, he said. They create a spiral wake that actually tugs on the planet, slowing it down and sending it spiraling inward toward the star.

"To me, the question is why they stopped before crashing into the star," Laughlin said. Numerous giant extrasolar planets have been found in very short-period orbits - 3 to 3.5 days - when, by all rights, they should have spiraled to a flaming death.

Marcy and Butler originated a sensitive technique for measuring the slight Doppler shift in starlight caused by a wobble in the position of a star, a periodic shift due to a planet yanking on the star as it orbits. From measurements over a period of years, they are able to infer the period, its approximate mass and the size of its orbit. Uncertainties arise because there is no way to determine the orientation of the orbit - whether we are seeing it edge on, or tilted to face toward us.

Discovery of a planet orbiting 55 Cnc at the distance of Jupiter is the culmination of 15 years of observations using the 3-meter telescope at Lick Observatory, which is owned and operated by the University of California. Four of the 15 newly found planets were discovered at the 3.9-meter Anglo-Australian Telescope in New South Wales, Australia.

In addition to the 300 stars the team monitors with the Lick telescope, the astronomers are following another 650 with the 10-meter Keck Telescope in Hawaii and another 250 southern hemisphere stars with the 3.9-meter AAT. Within a couple of years, they hope to use the 6.5-meter Magellan telescopes at Las Campanas Observatory in Chile to ramp up to 2,000 stars, all within 50 parsecs (150 light years) of Earth.

"This will cover all the good candidates out to 50 parsecs, so we will know where to look when we have the Terrestrial Planet Finder and the Space Interferometry Mission, which will do the first reconnaissance to identify Earth-like planets," Butler said.

In addition to Marcy, Butler, Fischer and Laughlin, collaborators on the project include Steve Vogt, professor of astronomy and astrophysics at UC Santa Cruz; Greg Henry of the Center of Excellence in Information Systems at Tennessee State University, Nashville; Dimitri Pourbaix of the Institut d'Astronomie et d'Astrophysique, Universite' Libre de Bruxelles; Hugh Jones of the Astrophysics Research Institute at Liverpool John Moores University in the United Kingdom; Chris Tinney of the Anglo-Australian Telescope; Chris McCarthy of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington; Brad Carter of the University of Southern Queensland, Australia; and Alan Penny of the Rutherford Appleton Laboratory in the United Kingdom.

[此贴子已经被作者于2008-5-20 12:42:12编辑过]

ted810

第三篇背景资料：

An accepted assumption in astrophysics holds that it takes more than 1 million years for gas giant planets such as Jupiter and Saturn to form from the cosmic debris circling a young star. But new research suggests such planets form in a dramatically shorter period, as little as a few hundred years.

The forming planets have to be able to survive the effects of nearby stars burning brightly, heating and dispersing the gases that accumulate around the giant planets. If the process takes too long, the gases will be dissipated by the radiation from those stars, said University of Washington astrophysicist Thomas R. Quinn.

"If a gas giant planet can't form quickly, it probably won't form at all," he said.

The standard model of planet formation holds that the spinning disk of matter, called a protoplanetary disk, that surrounds a young star gradually congeals into masses that form the cores of planets. That process was thought to take a million years or so, and then the giants gradually accumulate their large gaseous envelopes over perhaps another 1 million to 10 million years.

But the new research, culled from a much-refined mathematical model, suggests that the protoplanetary disk begins to fragment after just a few spins around its star. As the disk fragments, clusters of matter begin to form quickly and immediately start to draw in the gases that form vapor shrouds around gas giants.

"If these planets can't form quickly, then they should be a relatively rare phenomenon, whereas if they form according to this mechanism they should be a relatively common phenomenon," said Quinn, a UW research assistant astronomy professor.

The existence of gas giant planets, it turns out, seems to be fairly common. Since the mid-1990s, researchers have discovered more than 100 planets, generally from the mass of Jupiter to 10 times that size, orbiting stars outside the solar system. Those planets were deduced by their gravitational effect on their parent stars, and their discovery lends credence to the new research, Quinn said.

Lucio Mayer, a former UW post-doctoral researcher who recently joined the University of Zurich, is lead author of a paper detailing the work, published in the Nov. 29 edition of Science. Besides Quinn, co-authors are James Wadsley of McMaster University, Hamilton, Ontario, Canada, and Joachim Stadel at the University of Victoria, British Columbia, Canada. Their work is supported by grants from the National Science Foundation and the National Aeronautics and Space Administration's Astrobiology Institute.

Since the early 1950s, some scientists have entertained the notion that gas giant planets were formed quickly. However, the model, using a specialized fluid dynamics simulation, had never been refined enough to show what it does now. The Mayer-Quinn team spent the better part of two years refining calculations and plugging them into the model to show what would happen to a protoplanetary disk over a longer time.

"The main criticism people had of this model was that it wasn't quite ready yet," Quinn said. "Nobody was making any predictions out of it, but here we are making predictions out of it."

The new model explains why two other giant planets in our system, Uranus and Neptune, don't have gas envelopes like Jupiter and Saturn, Quinn said. At the time those planets were being formed, the solar system was part of a star cluster. The outer planets of Uranus and Neptune were too close to a nearby star ?one that has since migrated away ?and therefore lost whatever gas envelopes they might have accumulated.

Neither the new model nor the standard model accounts for why most of the gas giant planets found outside the solar system are much nearer their suns than are Jupiter and Saturn, Quinn said. The most common belief currently is that the planets formed farther away from their stars and then migrated inward to the positions where they have been discovered.

The new model also doesn't account for the formation of terrestrial planets, like Earth and Mars, near our sun. But Quinn suspects that perhaps the smaller terrestrial planets were formed over longer periods by processes described by the standard planet-formation model, while the new model explains how the larger gas giants came to be.

"That's my bet at the moment," he said.

[此贴子已经被作者于2008-5-20 12:42:47编辑过]

re395cyf

Thanks!

ted810

我都看了一遍，大致内容说一下，也不知和考试有没有相关：

第三篇文章：说原来以为行星是需要很长时间形成的，先由disk上的物质冷冻为一个核，然后在吸引disk上的气体及其他物体形成行星。但现在研究发现，行星的形成是很快的，因为the protoplanetary disk 很快就begins to fragment，如果短时间内行星没有形成，那他就不可能形成了。原来的理论是错的。

文章最后说，新理论和老理论都不能明确解释，为啥太阳系之外的行星都离母恒星那么近呢？有一种流行的解释是行星migrate理论。

第一篇文章：恰好是解释行星migrate。说行星不是在离母恒星很近的位置形成的，而是在离母恒星很远的地方形成的，然后migrate过来的。

[此贴子已经被作者于2008-5-20 12:55:40编辑过]

kenzopeace

thanks!