Since the discovery of the first alien planet circling a Sun-like star back in 1995, extrasolar planet discoveries have continued at a breathtaking rate. Almost 800 extrasolar planets have been confirmed so far, and there are more than 3,500 planetary candidates–at last count. Planets are generally thought to be a common result of star-birth, constructed from the remnant dust and gas whirling around in the protoplanetary disks that surround young, fiery stellar objects–and this theory has been strengthened by evidence that has accumulated over nearly two decades. However, in July 2013, astronomers announced that the sharp-edged, slender, and mildly lopsided rings of dust and gas that compose protoplanetary disks, circling faraway stars, may really be the result of interactions between dust and gas, rather than the gravitational effects of alien planet-children–as earlier suggested. If this is true, it will dramatically reduce current estimates of the number of extrasolar planets lurking in distant star systems.
Many young nearby stars, dancing around in our Milky Way Galaxy, sport surrounding disks of dusty debris. These dusty rings orbit active, young, fiery stars at distances approximately equivalent to where the dwarf planet Pluto–and a multitude of other icy objects of its kind–orbit around our Sun in the distant, frigid outer limits of our Solar System called the Kuiper Belt. Some of these dusty rings show surprisingly crisp edges and mildly off-center orbits. Astronomers have frequently attributed these irregular features to the gravitational effects of extrasolar planets that are too faint to be observed directly.
Protoplanetary disks have been observed around many young stars inhabiting youthful stellar clusters in our Galaxy. They form when the baby star is born, and can hang around for about 10 million years. By the time a bouncing baby star reaches what is termed the T Tauri stage in its young life, the disk has cooled off considerably from its earlier searing-hot temperature, and thinned out considerably. T Tauri stars are very active and young variable stars that are less than 10 million years old. These stellar babies possess masses that are similar to, or somewhat less, than that of our Sun, but they sport diameters that are several times greater–however, T Tauri stars are still in the process of shrinking! When our now middle-aged Sun was less than 10 million years old–it is now about 4.56 billion years of age–it went through a T Tauri stage.
By the time a baby Sun-like star has reached the T Tauri stage, extremely small, smoke–like grains of dust, bearing crystalline silicates, have started to condense near the center of its surrounding disk. These dust particles possess a natural stickiness, and readily glue themselves together when they bump into one another in the dense environment of the disk. This process eventually leads to the formation of ever larger and larger objects up to several centimeters in size. The tattle-tale signs of this process have been observed in the infrared spectra of young disks circling faraway stars in our Galaxy. Further aggregation may result in the birth of planetesimals–the precursors of full-grown planets–measuring at least one kilometer across.
Planetesimals are very abundant, and widely dispersed throughout the protoplanetary disk as it lazily circles the fiery, active young star. The asteroids of our Sun’s own family, are the relic chunks of rocky planetesimals–gradually whittling one another down to ever smaller chunks and tidbits. Conversely, the comets are relic icy planetesimals populating the outer regions of a planetary system such as our own. Meteorites are usually fragments of shattered planetesimals that have wandered their weary and tragic way around the “shooting gallery” of interplanetary Space, only to meet their doom when they blast onto a planet’s unfortunate surface. Therefore, meteorites that have pelted our own Earth, can provide valuable information about the long-ago birth and early evolution of own Solar System.
Several studies indicate that planets abound in our Galaxy. NASA’s Kepler Space Telescope detected 3,500 extrasolar planet candidates, and planet surveys utilizing the gravitational microlensing technique achieved the first significant result concerning planet frequency in our Galaxy, indicating that each single Sun-like star in our Galaxy is the stellar parent of–on average–one or more planets in an orbital range of 0.5 to 10 Astronomical Units (AU)–1 AU is the mean distance between Earth and our Sun. Gravitational microlensing is a technique that is based on Albert Einstein’s General Theory of Relativity. Einstein’s theory predicts that light emanating from a remote star can be gravitationally lensed toward our planet as a closer star floats in front of it, and then away over a span of several weeks. The gravitational lensing effect magnifies the light of the more remote star, due to the gravitational bending of its light by the lensing star that is directly in front of it as seen from Earth. This lensing effect enables astronomers to observe remote stars, that would otherwise not be fully observable, because of the light-bending, magnifying effects of the lensing foreground star. If a jumbo-sized, very massive planet circles the lensing star, its gravity will add to or depress the lensed light for a short time interval. So far, astronomers have spotted 20 alien planets using this technique, and have used these findings to calculate our Milky Way Galaxy’s population of planets circling distant stars.
NASA’s highly productive, though ill-fated, Kepler Space Telescope was launched in 2009 and it, along with other space-borne and Earth-bound ‘scopes, succeeded in spotting approximately 270 alien planets by detecting them as they transited–that is, floated directly in front of–the face of their fiery parent stars. Kepler’s primary mission was to determine the frequency of Earth-like planets dwelling in orbits around their stars at a distance where liquid water could exist.
Dusty Mirage?
Dr. Vladimir Lyra, of the California Institute of Technology in Pasadena California, is co-author of a July 2013 study published in the journal Nature, that suggests the number of known extrasolar planets may actually be smaller than currently believed by planet-hunters.
The slightly off-center and surprisingly crisp edges of disks of dusty debris, which orbit distant stars, are usually thought to be the result of gravitational tugs of alien planets that are too faint to be seen directly. Many planet-hunting astronomers attribute these irregularities to a phenomenon akin to the way shepherd moons are thought to sculpt portions of the ring systems that encircle the outer giant planets Saturn and Neptune. For example, the star HD216956 (Fomalhaut) shows one of the most dramatic examples of such irregularities. Some planet-hunters suggest that Fomalhaut’s sharp-edged and eccentric ring of dust, dancing around it, is due to the presence of planet-sized worlds. Fomalhaut (Alpha Piscis Austrini) is an extremely brilliant star. It is the brightest star in the constellation Piscis Austrinus and one of the most brilliant stars in the entire sky. It resides approximately 25 light-years from Earth, and it has long been of great interest to planet-hunters because of its highly suggestive orbiting disk of dust that sports those enticing irregularities.
However,”It’s often easy to see something you cannot explain, and then blame it on something you cannot see,” noted Dr. Lyra in the July 10, 2013 Nature News. Using detailed new computer models that they developed, Dr. Lyra and his co-author Dr. Marc Kuchner, discovered that scientists do not need to blame the presence of planets on the mysterious irregularity of these dusty ring structures. Dr. Kuchner is an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Earlier simulations have generally ignored the effects that any leftover gas–that still hangs around within the dusty debris disks–might have on the swirling dust. However, the team’s new model includes this previously ignored gas-dust relationship, including models that show how gas may jitter the movement of tiny, easily pushed around, dust grains–in a way that has been likened to air resistance. Dr. Lyra and Dr. Kuchner also improved studies of the physics of circling dust.
Instead of producing only circular and spread out disks of dust, the models occasionally churned out well-defined dust rings sporting slightly off-center orbital paths–just like those observed around Fomalhaut and some other stars dwelling in our Galaxy. “We were amazed when we saw the results the models were giving,” Dr. Lyra continued to comment.
Although the theory that the gas–lingering around within dusty debris disks– could influence its construct, had been suggested earlier by some researchers, it was not studied in detail. Dr. Lyra and Dr. Kuchner’s results “show that the [dust-gas] interactions indeed occur as envisioned,” Dr. Douglas Lin commented in the July 10, 2013 Nature News. Dr. Lin is an astrophysicist at the University of California at Santa Cruz. He added that astronomers are now spotting debris rings circling stars that reside beyond our Sun’s immediate Galactic vicinity.
These new findings throw some cold water on the estimates for the total number of planets inhabiting our Galaxy–possibly shifting those calculations to considerably smaller numbers. It can no longer be said with the same degree of certainty that any anomalous ring is the result of a planet.
However, the dust-gas relationship might work in conjunction with alien planets that haunt the dusty debris rings in hidden secret–and so might still contribute to the sculpting of the dusty rings. It is plausible that some of the dusty debris disks sporting such irregular features do indeed host alien planets. “We just can’t prove it,” Dr. Lin continued to comment.