[meteorite-list] Planet Formation Confirmed

[Pete Pete]

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=40113



Hubble observations confirm that planets form from disks around stars 
[heic0613]
10 Oct 2006

The NASA/ESA Hubble Space Telescope, in collaboration with ground-based 
observatories, has at last confirmed what philosopher Emmanuel Kant and 
scientists have long predicted: that planets form from debris disks around 
stars.


More than 200 years ago, the philosopher Emmanuel Kant first proposed that 
planets are born from disks of dust and gas that swirl around their home 
stars. Though astronomers have detected more than 200 extrasolar planets and 
have seen many debris disks around young stars, they have yet to observe a 
planet and a debris disk around the same star. Now, the NASA/ESA Hubble 
Space Telescope, in collaboration with ground-based observatories, has at 
last confirmed what Kant and scientists have long predicted: that planets 
form from debris disks around stars.


Orbiting Epsilon Eridani

The Hubble observations by an international team of astronomers led by G. 
Fritz Benedict and Barbara E. McArthur of the University of Texas, Austin, 
USA, show for the first time that a planet is aligned with its star's 
circumstellar disk of dust and gas. The planet, detected in 2000, orbits the 
nearby Sun-like star Epsilon Eridani, located 10.5 light-years from Earth in 
the constellation Eridanus. The planet's orbit is inclined 30° to Earth, the 
same angle at which the star's disk is tilted. The results will appear in 
the November issue of the Astronomical Journal.

The planets in our Solar System share a common alignment, evidence that they 
were created at the same time in the Sun's disk. But the Sun is a 
middle-aged star – 4500 million years old – and its debris disk dissipated 
long ago. Epsilon Eridani, however, still retains its disk because it is 
young, only 800 million years old.

The Hubble observations also helped Benedict's team determine the planet's 
true mass, which they calculate as 1.5 times Jupiter's mass. Previous 
estimates measured only the lower limit, at 0.7 times the mass of Jupiter. 
The planet, called Epsilon Eridani b, is the nearest extrasolar planet to 
Earth. It orbits its star every 6.9 years.

"Because of Hubble, we know for sure that it is a planet and not a failed 
star," McArthur explained. Some astronomers have argued that a few of the 
known extrasolar planets could be brown dwarfs because their precise masses 
are not known. If an object is less than 10 Jupiter masses, it is a planet, 
not a brown dwarf.

McArthur was part of an earlier team at the University of Texas at Austin's 
McDonald Observatory who discovered Epsilon Eridani b. They detected the 
planet using the radial-velocity method, which measures a star's subtle 
motion toward and away from Earth to find unseen companions.

Epsilon Eridani is a young and active star, so some astronomers claimed that 
what appeared as a planet-induced wobble of the star could have been the 
actions of the star itself. Turbulence in the atmosphere may have produced 
apparent velocity changes that were intrinsic to the star and not due to a 
planet's influence.

Determining the Orbit Inclination
The current Benedict-McArthur team calculated the planet's mass and its 
orbit by making extremely precise measurements of the star's location as it 
wobbled on the sky, a technique called astrometry. The slight wobbles are 
caused by the gravitational tug of the unseen planet, like a small dog 
pulling its master on a leash. The team studied over a thousand astrometric 
observations from Hubble collected over three years. The astronomers 
combined these data with other astrometric observations made at the 
University of Pittsburgh's Allegheny Observatory. They then added those 
measurements to hundreds of ground-based radial-velocity measurements made 
over the past 25 years at European Southern Observatory in Chile, McDonald 
Observatory at the University of Texas, Lick Observatory at the University 
of California Observatories, and the Canada-France-Hawaii Telescope in 
Hawaii. This combination allowed them to accurately determine the planet's 
mass by deducing the tilt of its orbit.

If astronomers don't know how a planet's orbit is tilted with respect to 
Earth, they can only estimate a minimum mass for the planet. If only the 
radial velocity wobble along the line of sight is known, the planet's mass 
could be significantly larger if the orbit were tilted to a nearly face-on 
orientation to Earth. The star would only move toward and away from Earth 
slightly, even though it had a massive companion.

"You can't see the wobble induced by the planet with the naked eye," 
Benedict said. "But Hubble's fine guidance sensors are so precise that they 
can measure the wobble. We basically watched three years of a nearly 
seven-year-long dance of the star and its invisible partner, the planet, 
around their orbits. The fine guidance sensors measured a tiny change in the 
star's position, equivalent to the width of a Euro coin 1200 km away."

Epsilon Eridani has long captivated the attention of science fiction 
writers, as well as astronomers. In 1960, years before the first extrasolar 
planet was detected, astronomer Frank Drake listened for radio transmissions 
from inhabitants of any possible planets around Epsilon Eridani as part of 
Project Ozma's search for intelligent extraterrestrial life. In the 
fictional Star Trek universe, Epsilon Eridani is considered by some fans to 
be the parent star for the planet Vulcan, Mr. Spock's home.

No Vulcan or any other alien could live on this gas giant planet. If moons 
circled the planet, they would spend part of their orbit close enough to 
Epsilon Eridani to have surface temperatures like that of Earth, and 
possibly liquid water. However, the planet's highly elliptical orbit also 
would carry the moons far away from the star, a distance equal to Jupiter's 
800-million-kilometre separation from the Sun, where oceans would freeze. If 
a moon were massive enough, like Saturn's giant moon Titan, it could have a 
sufficiently dense atmosphere that would retain heat. Such an atmosphere 
would suppress wide swings in surface temperatures. This could make such a 
moon potentially habitable for life as we know it, Benedict said.

Although Hubble and other telescopes cannot image the gas giant planet now, 
they may be able to snap pictures of it in 2007, when its orbit is closest 
to Epsilon Eridani. The planet may be bright enough in reflected sunlight to 
be imaged by Hubble, other space-based cameras, and large ground-based 
telescopes.

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