For years, scientists have hypothesized about the presence of planets outside our solar system, claiming a variety of indirect evidence that planets are scattered across the Milky Way Galaxy. Numerous newborn stars, too young to have formed planets, have been discovered to be encircled by massive disks of material. However, scientists uncovered just one example of a young star whose disc displayed the telltale thinning toward the center that suggests the creation of planets after a decade of searching. The disk around the star Beta Pictoris was discovered in 1984.
At a NASA news briefing on Tuesday, April 21, 1998, two teams of scientists showed infrared photographs showing a disk of luminous material around the star HR 4796A. HR 4796A is a young star located around 220 light-years from Earth. The inner section of the disk produces far less radiation than the outside part, indicating that there is less dust around the star. One possibility is that one or more invisible planets have cleared the region of dust. Despite the fact that the star is just 10 million years old, as compared to our solar, which is around 4.6 billion years old, astronomers think Jupiter and Saturn had already formed by the time our sun reached that age. One team, led by NASA’s Jet Propulsion Laboratory’s Michael W. Werner, discovered their finding using the mid-infrared camera on the 10-meter Keck II telescope atop Hawaii’s Mauna Kea. Ray Jayawardhana and Lee W. Hartmann of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, led a different team that employed a 4-meter telescope at the Cerro Tololo Inter-American Observatory in La Serena, Chile.
These young prospective solar systems provide astronomers with exciting opportunities to observe solar system formation and refine hypotheses about how our own solar system arose. Planets are considered to develop from a spinning disk of dust, gas, and ice around a young star. Larger pieces develop when the material inside the disc collides and clumps together. These fragments continue to smash, forming tiny, then bigger entities that circle the parent star. Small grains of dust may grow into solid planets like Earth, whereas gas and dust accumulations farther out in the disc may become massive worlds like Jupiter or Saturn. These massive planets are responsible for clearing away the majority of the gas and dust in the cloud spinning around the star, leaving a “hole” in the disk’s interior, which subsequently starts to resemble a doughnut.
Dusty discs are connected with two more stars, both of which are still fairly young but are around ten times older than HR 4796A. Researchers discovered dusty disks around two of the brightest stars in the sky, Fomalhaut and Vega, using the James Clerk Maxwell submillimeter radio telescope on Mauna Kea. The finding was discovered the same week that NASA had a news conference to announce the disk surrounding HR 4796A. Both Vega and Fomalhaut, like the disk encircling HR 4796A, have disks that seem edge-on from Earth’s viewpoint.
Fomalhaut looks to be ringed by a disk centered on a region that may have been cleared by planet formation and is believed to be roughly 200 million years old and 26 light-years away from Earth. The brightest radio emissions seem to be roughly as far away from Fomalhaut as the Kuiper belt objects beyond Pluto are from our own sun.
Vega, on the other hand, has a perplexing radio image despite being 350 million years old. The strongest emission, and hence the greatest concentration of material, seems to emanate from a single blob twice the distance between Vega and our sun. One possibility is that the blob is dust circling a large planet at that point. However, the blob might potentially represent radiation from a distant galaxy in the same sky area.
When researchers discovered this “blob,” they returned to Beta Pictoris and, to their amazement, discovered a similar blob in orbit around it. There is another considerably fainter blob just opposite this one. The two blobs might be connected since they were flung out in opposing directions from the disk’s core.
These star systems are excellent representations of the many phases of solar system development. Planets are developing when the quantity of dust and gas around the star decreases. The less dirty the disk surrounding the star, the more probable the dust has clumped together to form bigger entities in orbit around the star. Direct views of these bodies, however, are not yet available. Planets, unlike stars, reflect the light of their parent suns. And, due to their close closeness to their suns in terms of astronomical distances, the light of the star itself overpowers the planets themselves, making a direct observation with existing instruments impossible.
For decades, astronomers have been interested in the search for planets beyond our solar system. With the advancement of increasingly advanced gear, it is now feasible to detect planets in orbit around the sun indirectly. Solar systems might theoretically emerge around every star in our galaxy. However, whether or not the solar system stays stable is determined by the kind of star and the process by which the planets originated. According to one idea, many more stars than we thought formerly had planets in orbit around them; however, the orbits proved unstable, and the planets plummeted to their deaths into their sun. Other stars may have had the necessary disk of gas, dust, and ice, but due to constant meteor bombardment, the bits of material was never able to condense into a body big enough to be designated a planet; or the body may have formed, only to be crushed to pieces by the next meteor impact.
Humans on Earth have long debated whether or not we are alone in the galaxy, if not the whole cosmos. A planet capable of sustaining life as we know it must fulfill a number of conditions; in our own solar system of nine planets, only one fits these criteria. Although the topic is unlikely to be addressed in our lives, several exciting discoveries have been uncovered in the past decade that has brought us closer to an answer than ever before.
Radio astronomers Alex Wolszczan and Dale Frail found planets circling a pulsar, a kind of exploded star, in 1991. Although the finding was significant, it left unanswered the issue of whether planets might exist around sun-like stars, which is a must for those interested in the hunt for extraterrestrial life.
The finding of a planet circling the solar-type star 51 Pegasi was reported on October 6, 1995, by Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland. This finding was nothing short of miraculous, and it altered the course of history. This planet is part of a family that has astounded the astronomy world; it is a Jupiter-mass planet that is so near to its home star that it takes just 4.2 days to complete one orbit. This planet is almost half the size of Jupiter and is barely 5 million miles from its sun, or one-eighth the distance Mercury is from our Sun. Adam Burrows and colleagues at the University of Arizona calculated that this planet was big enough to retain its atmosphere despite its closeness to the sun.
Geoff Marcy of San Francisco State University and colleague Paul Butler of the University of California/Berkeley declared two planets of their own three months later. One of them was a planet of at least 2.3 Jupiter masses that orbited the solar-type star 47 Ursae Majoris. This planet orbits at a distance of 2.1 AU, putting it on the asteroid belt’s inner rim if it were in our solar system. It may not be Jupiter’s twin, but it might be a relative. Butler anticipated that “within one year, more extrasolar planets would be discovered than there are in our solar system,” despite the fact that he and Marcy still had masses of data to sort through and that numerous other research teams were also in the hunt. This comment, far from being pompous, proved to be prescient.
Butler and March discovered a second 51 Pegasi class planet circling the star Rho 1 Cancri in April 1996. Marcy and Butler discovered two more within three months, circling the stars Tau Bootis and Upsilon Andromedae. This data suggests that these so-called “hot Jupiters” exist in around 5% of solar-type stars. The discovery of an eighth planet was announced in April 1997, when a nine-member team headed by Harvard University’s Robert W. Noyes discovered a planet around the star Rho Coronae Borealis.
Nobody knows what these hot Jupiters are like since no one has been able to picture or capture their spectra of them – they were found indirectly via their gravitational influence on their host stars. Because there shouldn’t be enough rocky material near a star to produce a large planet, most scientists believe these planets are massive gas bags comparable to Jupiter and Saturn in composition.
Here’s how the current planet detection system works. Because the star in a solar system is so brilliant in comparison to the reflected light of its planets, it is exceedingly difficult to discover planets surrounding any one star. In the visible light spectrum, our sun, for example, outshines its planets by around one billion times. With existing technology, direct visual observation is almost impossible. As a result, an indirect mode of observation known as the Doppler planet-detection technique is adopted. This entails checking for “wobbles” in the star’s velocity induced by gravitational tugging from the planets. The planet in orbit puts gravitational pull on its sun, dragging it around in a circular or oval path resembling the planet’s own orbit. Although the wobbling motion of the star from a long distance is relatively modest, the Doppler shift of the starlight itself may be observed. As the star wobbles back and forth in relation to Earth, the light waves expand and contract in a cyclical pattern, changing between the red and blue extremes of the spectrum. Astronomers can reconstruct the route of the star’s wobble and calculate the planet’s mass, orbit, and distance from the host star using this pattern, the Doppler shift.
A planetary system is thought to develop from a disk of gas and dust that surrounds a young star. A gigantic planet grows from an ice core with some rock tossed in. When this core reaches the mass of several Earths, its gravity pulls massive amounts of gas from the surrounding protoplanetary disc. However, the extreme heat and wind of the newborn sun prevent ice from accumulating in the inner disk. Furthermore, heat and wind force gas out of the inner disk, preventing gaseous planets from forming there. According to current understanding, gas giants can only develop at the freezing outer section of the protoplanetary disk, at least 5 times the distance between Earth and our Sun. With the discovery of these so-called hot Jupiters, this notion is now being put into doubt.
Most planets, both within and beyond our solar system, including these hot Jupiters, have almost perfectly circular orbits. However, three of the newly identified planets have very elliptical orbits and hence belong to the “eccentric planets” family.
The first of them was found in 1989 by Harvard-Smithsonian Center for Astrophysics scientist David Latham’s group. The object has at least 9 Jupiter masses and circles the star HD 114762. The planet’s eccentric orbit gets it as near to the star as.22 AU and as distant as.46 AU. However, scientists are hesitant to identify this object as a planet since it is far more massive than Jupiter and has such an erratic orbit. This is why Latham was never given credit for finding the first planet to circle another star.
But, if this isn’t a planet, what exactly is it? For the time being, astronomers will refer to it as a brown dwarf. Brown dwarfs are comparable to gas giant planets like Jupiter in composition, but instead of developing from disk-like planets, they grow from collapsing gas clouds like stars. Brown dwarfs do not light like stars because they lack the mass of 80 Jupiter required to spark nuclear processes in their cores. Brown dwarfs are therefore transitional objects that cross the mass gap between stars and planets.
For eight years, the HD 114762 object was the only one of its kind known to science. However, Marcy and Butler discovered a near-twin, an object with a minimum mass of 6.5 Jupiter, circling the star 70 Virginis in January 1996. This object’s orbit ranges from.27 to.59 AU.
It will be difficult to determine if these objects are planets or brown dwarfs. Because there is no conceivable reason why the heaviest planets couldn’t be more massive than the lightest brown dwarfs, mass alone isn’t the determining criterion. Even a spectrum revealing the chemical makeup of their outer atmospheres would probably be insufficient since both brown dwarfs and gas giant planets are mostly made of hydrogen and helium.
The crucial distinction is found deep inside the core. Because a brown dwarf, like a star, develops from a collapsing gas cloud, it retains a gaseous composition all the way to the core. A planet, on the other hand, is formed by accreting material in a disk and has an ice and rock core. However, since we are seeing these things from such great distances, it will be very hard to establish their internal structures, which might be the key to unlocking the puzzle of how they evolved.
Some astronomers feel that eccentricity may be decisive. Mayor and Queloz identified ten brown dwarfs circling solar-type stars, the majority of which had severely eccentric orbits. Binary stars often circle each other in very eccentric orbits. As a result, the high eccentricity of the HD 114762 and 70 Virginis objects may indicate that they evolved in the same manner as stars do, making them brown dwarfs.
According to Marcy and Butler, the least massive of Mayor and Queloz’s ten new brown dwarfs has a minimum mass of 17 Jupiters, making it substantially heavier than HD 114762 and its 70 Virginis neighbors. They argue that the final two objects are either planets or members of a distinct population.
Planet formation models predicted a decade before the hot Jupiters were found that under certain circumstances, gas giant planets would encounter drag forces with their disks, causing them to spiral inward from their origin. However, these models have a depressing conclusion: the planets would spiral all the way toward the star, eliminating themselves as independent entities and getting absorbed by the star’s mass.
The third of these strange worlds is not like the other two. Both Marcy and Butler’s groups, as well as William D. Cochran and Artie P. Hatzes of the University of Texas McDonald Observatory on Mt. Locke in western Texas, independently detected this planet. It has a far lower minimum mass of 1.5 Jupiter and by far the most eccentric orbit of any planet identified to date. This planet’s orbit comes as near as.6 AU to its sun, 16 Cygni B, and as distant as 2.8 AU, around the distance of our own solar system’s asteroid belt. The seasons on this planet would be harsh. However, seasonal fluctuation is governed by changes in distances from its sun rather than the tilt of its axis, as is the case with Earth’s seasons. Furthermore, this planet orbits a triple-star system; 16 Cygni B and 16 Cygni A (which is significantly bigger) circle each other in a cigar-shaped elliptical that takes at least 125,000 years. 16 Cygni C, the system’s third component, is a red dwarf star about half the size of the sun that circles the main pair at a distance of at least 100,000 AU. Nobody knows how this planet came to have such an eccentric orbit, but the simplest hypothesis is that it originated in a circular orbit, but each time 16 Cygni A comes in for a near encounter, its gravity pulls on the planet, gradually tugging the circle into its present elongated shape.
The two Jupiter-like planets discovered by George Gatewood of the University of Pittsburgh circling Lalande 21185, a red dwarf star approximately 8.25 light years distant – five times closer than the next nearest known extrasolar system planet. Although each of the dozen or so planets discovered to date may be regarded as somewhat questionable due to its detection through indirect techniques, Lalande 21185 is by far the most “iffy.” Gatewood employs a method distinct from the other planet seekers in that he does not instantly identify a mass and orbit. He’ll need to keep an eye on the star for many years before he can pinpoint precise masses and orbits. For the time being, Gatewood can only state that Lalande 21185 seems to be orbited by at least two Jupiter-mass planets at distances of around 2.2 AU (again, the inner asteroid belt) and 11 AU (a little beyond Saturn). There are additional suggestions that the third planet circles the star at a considerably greater distance.
However, before theorists go too far, it is important to remember that the searches are most sensitive to huge planets circling near their host stars, which is precisely what scientists are discovering. According to astronomers, the planets discovered are the simplest to locate and hence may not be reflective of what is really out there. To fully comprehend the tale of planets and planet formation, astronomers must locate whole systems, such as the one that may be circling Lalande 21185.
Marcy and Butler have found convincing evidence for second planets around Rho 1 Cancri and Upsilon Andromedae, suggesting that astronomers are making significant progress in this field. No doubt, as technologies and telescopes improve and more teams study stars, many more planet-like objects will be discovered orbiting distant suns. Marcy and Butler have started a second Doppler scan of 400 stars using Hawaii’s 10-meter Keck telescope. Mayor and Queloz just increased the amount of their Northern Hemisphere Doppler study to around 400 stars, and they will shortly begin a Southern Hemisphere scan of 500 additional stars. Doppler surveys of several hundred more stars will begin at McDonald Observatory’s nine-meter Hobby-Eberly Telescope within the next year.
By the year 2000, both the 10-meter Keck telescope and a binocular telescope at the University of Arizona will have evolved into optical interferometers capable of imaging extrasolar planets. NASA intends to deploy at least three space-borne telescopes capable of detecting planets in infrared light. One of these planned telescopes is the Terrestrial Planet Finder, a space-based interferometer that will be able to gather images of prospective candidates for habitable planets circling distant suns beginning around the year 2010.
Interferometry is one of the most exciting and possibly profitable techniques for locating distant planets. This is how it works: Ronald Bracewell of Stanford University demonstrated in 1974 how two tiny telescopes might work together to hunt for huge, cold planets like Jupiter. The planned equipment by Bracewell consists of two one-meter telescopes spaced 20 meters apart. Bracewell demonstrated that he could invert the light waves from one telescope (changing the peaks of the light waves into valleys or troughs) and then combine that inverted light with the light from the second telescope while both telescopes were pointing at the same star. If the images were perfectly aligned, the star’s light would be “canceled out,” removing the glare that hides planets. The spectrum of the planet might so be spotted from a slightly different direction. This gadget is known as an interferometer because it uses light wave interference to determine specifics about a light source.
Bracewell’s proposed telescope would be sensitive enough to identify Jupiter-sized planets, but the identification of Earth-sized planets would need considerably more sensitivity. There’s another issue as well. Although many ground-based sensors are sensitive enough to detect intense infrared radiation emitted by stars, the telescope’s own heat, along with air circumstances, would make detecting a planet very difficult. Another issue to contend with is the background heat generated by our solar system’s cloud of dust particles, as seen in the so-called “zodiacal glow.” Bracewell concluded that this brightness would virtually drown out the signal of a massive planet, much alone one the size of Earth. The obvious option was to orbit the telescope.
Of course, merely discovering an Earth-sized planet in orbit around a star is not enough; the driving force is to discover such a planet that meets the prerequisites for life. Because of the possibility of possessing liquid pools of water, planets comparable to Earth in size and distance from their sun provide the greatest opportunity for carbon-based life. Water is the solvent for biological activities in life. However, the temperature of the planet and/or its distance from the sun are meaningless if the planet lacks enough gravitational pull to retain seas and an atmosphere.
In 1986, Roger Angel, Neville Woolf, and Andrew Cheng of the University of Arizona hypothesized that mid-infrared wavelengths would be the optimum area to hunt for evidence of life on other planets, spectroscopically speaking. This kind of radiation has wavelengths that are 10 to 20 times longer than visible light. Furthermore, a planet emits around 40% more photons, or light particles, at these wavelengths than it does in the visible spectrum, making the planet’s radiation considerably simpler to distinguish from that of its sun. Although there are other prerequisites for alien life, three chemicals must be present in order for life as we know it to exist. These distinguishing molecules are ozone, which indicates a lack of oxygen in the atmosphere, water, and carbon dioxide.
A study of each planet’s infrared radiation was conducted using our own solar system as a test. Only Earth, out of the nine planets in our solar system, exhibited the infrared signal of life. Although Mars and Venus both had carbon dioxide atmospheres, only Earth displayed ozone and abundant water signals. Ozone is a sensitive indicator of atmospheric oxygen; it would have developed on Earth around a billion years before oxygen exhibited its infrared spectral signature.
Roger Angel demonstrated in 1990 that the degree of accuracy required to find smaller planets was achievable with more than two telescopes. Alain Leger and his colleagues at the University of Paris demonstrated that if the sensor were put into orbit at the approximate distance Jupiter is from the sun, the background thermal radiation from particles inside the solar system would be essentially minimal due to the frigid temperature. He also proved that an orbiting interferometer at this distance, with telescopes as tiny as one meter in diameter, could detect an Earth-sized planet.
NASA formed three teams in 1995 to research strategies for identifying extrasolar planets. An international team, comprising Bracewell, Angel, and Leger, was created at the University of Arizona to investigate the issue. Angel and Woolf devised an interferometer using two pairs of mirrors placed in a straight line. They reasoned that since an interferometer of this kind could cancel starlight extremely efficiently, it could reach around 75 meters, a length that they claim provides significant benefits. It would allow scientists to recreate genuine pictures of planets circling stars and see stars across a broad range of distances without having to enlarge or compress the array. Angel and Woolf hypothesized that this technology may detect earth-like planets that would otherwise be invisible by studying them for carbon dioxide, water, ozone, and perhaps methane, another sign of life.
The finding of another Earth-like planet, maybe inhabited by life, might be considered space exploration’s pinnacle accomplishment. Even the discovery of such a planet does not guarantee that the “life” on it will be able to communicate with us; early forms of life on Earth, such as blue-green algae, were not in the habit of broadcasting radio signals proclaiming their existence. If such an Earth-like planet is discovered, the life on it is unlikely to be at our level of evolution.
Despite the fact that the chances of discovering life as we know it are small, the quest continues. Who among us hasn’t looked up at the stars and thought about the possibilities of alien life? After all, the hunt for such life is one of humanity’s most compelling desires.
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