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Hunting Planets Along the Milky Way
We wonder...
NASA concept of an extrasolar terrestrial planet
Is Earth a lonely island where human beings cling tenaciously to life, or is the Universe teeming with planets where myriad forms of life have been nurtured?
From the earliest times, humans have pondered the night sky, wondering first what it was, then what was out there, then how to go out there, and eventually how to live out there.
We came to understand that we're on a speck of dust bobbling in a vast ocean of mostly nothingness and that there are eight other major specks flying around the brilliant lightbulb that is our Sun, our star.
We see other stars. Could there be unseen dust specks orbiting those distant suns? Could the Universe be like a droplet of water alive with microorganisms? Or is there no one else out there?
Recently, science and technology have led us to believe the answer will be found. For instance, astronomers have discovered numerous planets — as many as 230 so far — circling around stars far beyond our own Sun. That has given the researchers great confidence that our Solar System is not unique. Especially since such extrasolar planets seem to be common in our neighborhood of the galaxy.
Because even our most powerful telescopes still are relatively weak, the extrasolar planets we've been able to discover so far are very big. They are giants similar to Jupiter and Saturn in our Solar System. Scientists think that such huge planets are not likely to be able to support life. At least, life as we know it.
On the other hand, the planetary systems which are home to the giants might also contain smaller, more Earth-like planets. If they exist, the smaller bodies might be terrestrial planets like Venus, Earth or Mars.
Over the next 15 years, the U.S. space agency has set a course to learn more about those faraway worlds and find more of them. NASA plans a series of missions using the most sensitive instruments ever built.
In the beginning...
A disk of dust swirls around a new-born star. Over a very long period of time, the mutual gravity of the specks of dust pulls them together and condenses them into larger and larger rocks. Large rocks are called planetesimals. The countless planetesimals forming in the dust disk smash into each other -- like a cosmic demolition derby -- shattering and re-condensing until they are hot enough to melt together into a few very large planet-sized globules. As the infant star ignites in nuclear burning, its brisk wind of radiation blows away most of the remaining dust in the disk, leaving the planets alone to cool and solidify.
Even with the most modern astronomy equipment, detection of distant planets beyond our Solar System is very difficult. Stars are millions of times bigger and brighter than the tiny globes that circle them, washing out our view of any planets which might be there.
False alarms in the 1980s and early 1990s soured the reputation of planet hunting. However, technological advances in the 1990s changed the picture. There has been a renaissance in the search for planets as astronomers have acquired both better telescopes and better understanding of how planets behave as they circle the mighty powerhouses that we call stars.
Exoplanets...
Astronomers coined the terms "extra-solar planet" and "exoplanet" to distinguish those planets found circling distant stars from the nine planets that circle our star, the Sun.
Many have been found...
Astronomers could just feel it in their bones. There had to be other stars, beyond our own Sun, with planets in orbit around them. But, they couldn't find any until 1995.
That year, in October, astronomers at Switzerland's Geneva Observatory, Michel Mayor and Didier Queloz, found an extra-solar planet orbiting the distant star 51 Pegasi. Mayor and Queloz knew it was orbiting out there because they watched it over a long period of time and observed that the star was moving slightly toward and away from Earth every four days. That told them the star was affected by the gravitational tug of an unseen planet orbiting around it.
Today's telescope technology does not allow astronomers to see distant planets directly. Rather, they find small shifts in the movements of stars that reveal the presence of unseen planets. As a star tugs on a planet and holds it in orbit, the planet tugs back on the star, affecting the star's orbit. Of course the star far outweighs the planet, so the star barely changes its position.
A vast sweep of the sky in February 2004 by the Hubble Space Telescope, orbiting above Earth's atmosphere, turned up as many as 100 new planets near stars elsewhere in the Milky Way. If the finds are confirmed by the astronomy community, the number of exoplanets will be almost doubled. That shows that solar systems are not unusual.
Doppler Effect...
Astronomers look for a wobble when measuring the light from a distant star. If the star wobbles sufficiently, the color of its light shifts by a small amount. Known as the Doppler effect, the "modulation" or small changes causes the color of light from a star to appear more blue when the star wobbles toward Earth and more red when it moves away. Astronomers use this so-called "radial velocity" to calculate the minimum size of an exoplanet.
The Doppler effect can be observed on Earth in the sound of a train whistle, which changes pitch as it approaches and speeds away.
Other techniques...
Astronomers have other strategies for finding planets, but radial velocity is the only method to have allowed detection of new planets around Sun-like stars.
A different technique uncovered three planets around pulsars, but that method doesn't work with more common stars that could have planets where life could arise. Pulsars are tiny, spinning, dead stars.
Larger planets...
Since the major find in 1995, astronomers have detected dozens of giant worlds. Most are large, the size of Jupiter or larger, and in tight orbits close to their parent stars. Some are so close in that they make complete orbits every few days. They have surface temperatures estimated at more than 1,000 degrees Fahrenheit.
Some of the large exoplanets found so far move around their stars in highly elliptical, oval orbits. That contrasts with Earth and other planets in our Solar System, which follow almost circular paths around the Sun.
The size and orbits of planets could be a problem for planet hunters looking for Earth-like planets because star systems with large Jupiter-size planets in elliptical orbits swooping in near a star would seem to be inhospitable to an Earth-like habitable planet, which probably would need a safe, constant orbit at just the right distance from the star.
Improving data...
Astronomers would like to have better planet data. The radial velocity technique requires collection of data from a star long enough to detect a planet's tug on the star. Data collected in a short time -- less than a year -- finds large planets that orbit a star every few days. Data collected over 12 years could find a large planet about the same distance from its star as Jupiter is from our Sun.
Jupiter is five astronomical units (au) from the Sun. One astronomical unit is the distance from the Sun to Earth.
In 2001, astronomers detected a slightly smaller planet only 75 percent the size of Jupiter in a circular orbit around the star 47 Ursae Majoris. The star is in Earth's sky constellation known as the Big Dipper. That planet orbiting 47 Ursae Majoris travels in a seven-year-long orbit at a distance of 3.7 astronomical units from its star.
The same team of astromomers already had found another planet about 2.5 times the size of Jupiter orbiting in a circular path every three years at 2 astronomical units from the star. In our Solar System, that would be slightly beyond the disatance from the Sun to the planet Mars.
The astronomers suggested that the orbits of the two giant planets would prevent any Earth-like planets from existing in the habitable zone.
At any rate, the discoveries so far have revolutionized planetary science, forcing scientists to revise their theories about how the Universe is structured. All of a sudden, hunting planets has become one of the hottest kinds of astronomy.
Keck Telescope
click image to enlarge
Today, astronomers have bigger telescopes collecting lots more light quickly. A planet hunter now can search for likely-looking stars with a telescope such as one with a 30-ft. mirror in Hawaii known as the Keck. Spotting a good candidate for a faraway solar system, the hunter can turn to the powerful Hubble Space Telescope orbiting above Earth's murky atmosphere. Pointing Hubble toward the candidate solar system, the astronmer will look for a telltale dust ring around the star. The ring of dust could be the genesis of planets.future planet finding missions
Dust rings around a star suggest that unseen planets lurk there. Astronomers say it takes a planet's gravitational pull to organize dust into a ring, in much the same way the rings around planets in our Solar System are produced by so-called shepherd moons. Some have called the planets suspected to be in dust rings around farawy stars "ringworlds."
Smaller planets...
Now, astronomers want to find bodies like our own home world.
Astronomers don't know whether there are smaller Earth-like planets -- with warm temperatures, liquid water running in streams and lakes, and a climate suitable for life -- but they expect to find the answer in the not-too-far future.
Over the next decade or so, space agencies of the United States and Europe plan to launch spacecraft designed specifically to find planets orbiting faraway stars.
The unusual suspects...
One example of a faraway planet lies in a potential solar system 220 lightyears from Earth around a star astronomers call HR 4796A. Infrared light images of that corner of our Milky Way galaxy revealed a dusty disk that looked like hula hoop. Zooming in, it looked like the planet Saturn of our own Solar System. Suddenly, astronomers were very excited about hunting planets.
A lightyear is about 6 trillion miles, the distance light travels in a year. That puts HR 4796A about 1.320 quadrillion miles from Earth.
Astronomers have been finding some distant planets that are nonconformists.Theoretical walls tumbling...
- Previously, astronomers had thought no planet should be where the one orbiting HR 4796A was found. Its star was less than 10 million years old. That's too young for a planet to have developed.
- Many newly-discovered planets are as large as our planet Jupiter, but in orbits closer to their host stars than our planet Mercury is to the Sun. That condition should not exist. After all, a Jupiter-sized planet is supposed to orbit farther away from its star. Otherwise, it would be sucked into the star by the pull of the star's strong gravity.
- Many of the newly-discovered planets are in elliptical orbits around their stars. That's different from the planets of our Solar System, which travel mostly circular paths around the Sun.
The new distant planet observations have forced astronomers to rebuild planetary theory. One explanation already advanced describes "Hot Jupiters" slowly falling in toward their stars, destroying other planets along the way. In another new explanation, the elliptical orbits of those faraway planets might be caused by the gravitational pull of some other so-far-unseen nearby star or planet.
Money hunt...
Astronomers used to find financing planet hunting difficult. That became easier after Swiss astronomers found signs of a planet at the star 51 Pegasi in 1995.
They noticed that the spectrum of light arriving at Earth from the star 51 Pegasi was wobbling. Upon closer examination, the phenomenon seemed to be caused by a large planet orbiting very close to 51 Pegasi. From that discovery, a wobble became a telltale sign for planet hunters and money became easier to find.
Five percent planets...
Five percent of stars surveyed show evidence of planets. Even so, some astronomers are not convinced that an Earth-like planet exists anywhere beyond our Solar System. They see our Solar System as an anomaly. On the other hand, many believe that when stars are formed, planets are formed.
The generation of even more sensitive telescopes coming online in the first decade of the 21st century will have sky time dedicated to planet searches. Scientists are also perfecting new techniques that use less-expensive equipment to locate planets. Proponets say it is very likely we will see direct images of distant planets.
But, will that offer evidence that life as we know it exists on those faraway planets? Could we exist there if we went there? We'll still be wondering... [Where did we come from? Are we alone?]
Smallest exoplanet yet...
One of three planets found orbiting the M-dwarf star Gliese 876 is the the smallest planet so far found outside of our solar system. The star and its planets are 15 lightyears away from Earth in the constellation Aquarius.
It's not only the smallest extrasolar planet yet detected, but also the first of a new kind of rocky world orbiting a star like our Sun. It's probably about seven times as massive as Earth, and about twice as wide.
The small planet requires only 1.94 days to flit all the way around its star. That means it probably is around two million miles from the star. That would make it only about two percent of the distance between Earth and our Sun.
Most of the far-off extrasolar planets found to date have been gas giants like Jupiter. And, in fact, there are two Jupiter-sized planets in the Gliese 876 system.
Because it's so close to the star, the planet's surface probably is 400° to 750°F. That would be too hot to retain gas in the way Jupiter does. That suggests the planet could be mostly solid – rocky with an iron core, a silicon mantle, and maybe a dense steamy covering of water.
A planet with three stars...
To start a 2MB QuickTime movie, click this NASA/JPL/Caltech artist's concept of a sunset seen through the tenuous atmosphere of a moon baking in orbit around HD 188753, the first planet found in a triple star system. The more distant stars dip below the horizon as the gas giant main star sails into view. The landscape is illuminated by sunlight reflected from the planet. Both planet and moon are so hot even shaded areas glow. The first star setting is closest, most massive and hottest, so it is large and white. The second star is farther away, less massive and cooler, thus smaller and yellow. The final star is at the same distance as the second, but less massive and cooler, thus smaller and orange-red. Our Sun is a bit cooler than the hottest star of the triple system.
To view the video, your browser needs the free QuickTime movie player as a helper application or plug-in. Get QuickTime »
For the first time, astronomers are looking at a planet bathed in the light of three stars. From the point of view down on that faraway world, a sun would set over the horizon, followed by a second sun and then a third. The planet, labeled HD 188753 Ab, is the first known to exist in a triple-star system. Dr. Maciej Konacki of the California Institute of Technology at Pasadena, California, found the planet using the Keck I telescope atop Hawaii's Mauna Kea mountain.
HD 188753 Ab is in a class of extrasolar planets known as hot Jupiters – gas giants that zip around close to their parent stars. In this instance, the planet requires a mere 3.3 days to travel around its main star. In turn, that star is circled every 25.7 years by a pair of stars themselves in a 156-day orbit. The three stars are cramped into the same amount of space as the distance between our Sun and its planet Saturn.
The star system, known as HD188753, is 149 lightyears away from Earth in the constellation Cygnus. Its planet HD 188753 Ab is 1.14 times the mass of Jupiter.
NASA says multiple star systems are numerous across the Universe. Probably, more than half of all stars are in multiple star systems. The star closest to our Sun, Alpha Centauri, is part of a trio.
Small Neptunian Exoplanets
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