The first exoplanet to be observed circling a Sun-like star was an enormous roaster, spotted by two Swiss astronomers who made the historic discovery back in 1995. Since then, around 1800 exoplanets orbiting stars beyond our Sun have been detected, bringing with them a treasure trove of information describing many rich and strange alien worlds for astronomers to pour over. In May 2014, a team of astronomers at Vanderbilt University in Nashville, Tennessee, announced their bizarre discovery that some faraway Sun-like stars, inhabiting our Milky Way Galaxy, hungrily devour tasty Earth-like planets that circle them in searing-hot, close-in orbits. These “Earth-eaters,” during their development, snack on large quantities of the rocky stuff from which “terrestrial”planets–like Mercury, Venus, Earth, and Mars–are composed.
Trey Mack, a doctoral student in astronomy at Vanderbilt, created a model that estimates the effect this sort of sinister diet has on a parent star’s chemical composition. Mack and colleagues have also used this model to study a distant duo of twin stars that both possess their own set of planetary offspring.
The results of this study were published online May 7, 2014 in the Astrophysical Journal.
After obtaining a high-resolution spectrum for a target star, astronomers can now actually spot the tattle-tale signature of this evil feast.
“Trey has shown that we can actually model the chemical signature of a star in detail, element by element, and determine how that signature is changed by the ingestion of Earth-like planets. After obtaining a high-resolution spectrum for a given star, we can actually detect that signature in detail,” noted Dr. Keivan Stassun in a May 16, 2014 Vanderbilt University Press Release. Dr. Stassum is a professor of astronomy at Vanderbilt.
This new model will enable astronomers to better understand the process of planet formation–as well as helping them in their ongoing and dedicated hunt for Earth-like worlds dwelling beyond our Sun.
Stars are enormous, seething, and searing-hot balls composed of more than 98 percent hydrogen and helium gas. All of the other elements that may exist in the glaring furnace of a star compose less than 2 percent of their mass. In astronomical jargon, all atomic elements heavier than hydrogen and helium are termed metals, and they have coined the term metallicity to define the ratio of the relative abundance of iron to hydrogen in a star’s chemical composition.
Over the past twenty years, astronomers have developed new strategies to help them detect exoplanets in great numbers–and there have been several recent studies that attempt to link stellar metallicity with planet formation. One study, conducted by scientists at Los Alamos National Laboratory in New Mexico, suggests that stars sporting high metallicity are more likely to give rise to planetary systems than stars that are less richly endowed with elements heavier than helium. A second study argues that hot Jupiter planets are seen primarily in close, fast orbits around high metallicity stellar parents, while smaller planets are most frequently observed in orbit around stars with a diverse range of metallicities.
The first alien planet to be discovered in orbit around a distant Sun-like star was a hot Jupiter dubbed 51 Pegasi b–or 51 Peg b, for short. This sizzling distant world proved to be enormous, hugging its parent-star, 51 Pegasi, fast and close. In fact, 51 Peg b orbits its stellar parent at a distance of only 4,300,000 miles–which is only a small fraction of the distance separating Mercury, the innermost planet in our Solar System, from the Sun.
51 Peg b was discovered by Dr. Michel Mayor and Dr. Didier Queloz of the Geneva Observatory in Switzerland, and the existence of such a roasting hot Jupiter surprised astronomers, who believed that Jupiter-like planets could only dwell in the cold, outer regions surrounding their stars–like Jupiter in our own Sun’s family.
Since the discovery of 51 Peg b, almost a generation ago, many other strange and unforeseen alien worlds have been spotted by surprised astronomers, as they orbit around stars that are very similar to our own.
Of Stars And Exoplanets
All stars are born when a very dense blob secreted deep within a cold and dark interstellar molecular cloud–composed of star-birthing gas and dust–collapses under the mighty weight of its own gravity. Many such dark, enormous, and amorphous clouds haunt our Milky Way Galaxy, floating around in spooky silence in the Space between stars.
Brilliant, young stars are surrounded by swirling protoplanetary accretion disks that whirl around them. Baby planets are born from these accretion disks–composed of nourishing fine dust particles and gas. The dust particles that dance around within the whirling disks are very sticky, and cling to one another, forming ever larger and larger objects–from pebble size, to boulder size, to mountain-size, to planet-size. The larger primordial planet-forming bodies that eventually form, termed planetesimals, ultimately collide with one another and merge to create major planets–the full-grown children of the stars that they circle.
Almost 2000 alien worlds have been spotted by planet-hunting astronomers circling distant stars beyond our Sun. Approximately 1790 exoplanets dwell in 1110 planetary systems that include about 460 systems sporting multiple planets–at least, as of May 13, 2014.
The ill-fated, but highly productive Kepler Space Telescope spotted a few thousand candidate alien worlds of which, perhaps, 11% could be false-positives.
Astronomers think that there are at least 100 billion planets inhabiting are starlit, barred-spiral Milky Way Galaxy, with at least one planet-child on average per sparkling stellar parent. Our Galaxy also possibly hosts trillions of rogue–alternatively termed orphan— exoplanets, that are not bound to any star at all, but wander around through interstellar Space bereft of a stellar family. Such unfortunate, lonely worlds were likely unceremoniously evicted from the families of their parent-stars, as a result of catastrophic gravitational interactions with sister planets.
About 1 out of 5 Sun-like stars are thought to possess “Earth-sized” planet offspring, dwelling in their habitable zones–and the closest one is calculated to be situated within 12 light-years from our Solar System. The habitable zone surrounding a star is that comfortable region where liquid water can exist in its life-friendly liquid state–the so-called Goldilocks zone, where it is not too hot, not too cold, but just right for life as we know it to arise, evolve, and flourish. Where liquid water exists, the potential is there for Earth’s kind of life to develop.
For hundreds of years scientists and philosophers alike speculated that exoplanets might exist around distant stars–but they had no way of spotting them, or of knowing their frequency. Several claims of exoplanet detection were made by planet-hunters back in the 19th century, but they were ultimately rejected by other astronomers who were unable to confirm the “discoveries”.
The first confirmed discovery of exoplanets came in 1992, with the detection of several terrestrial-mass planets circling the pulsar PSR B1257+12. Pulsars are not hydrogen-burning, main-sequence stars like our own Sun. They are the dense remains of massive stars that blasted themselves to pieces in the fiery conflagration of supernovae explosions. Pulsars are rapidly spinning neutron stars, that emit regular lighthouse-like beacons of light into Space. The pulsar planets are bathed in a constant shower of deadly radiation–emanating from the stellar corpse that they are doomed to circle–and, as such, they are weird and very unfortunate worlds, likely to be extremely hostile to delicate living tidbits.
51 Peg b was the first exoplanet to be discovered circling a hydrogen-burning star still on the main sequence–like our own Sun!
Earth-Eaters!
The hydrogen and helium that compose still-living stars on the main-sequence are used as fuel for the nuclear fusion reactions that manufacture their sizzling heat and brilliant light. However, stars do carry within them a sprinkling of other atomic elements on their surfaces. By analyzing starlight, astronomers are able to determine which elements are present in a stellar system–and it gives some tattle-tale clues about the kind of planets the system harbors.
To discover more, the Vanderbilt team used telescopes at the Las Campanas Observatory in Chile to peer at a stellar system harboring a duo of Sun-like stars bearing the bland names of HD 20781 and HD 20782. The two sister stars were born from the same dark, frigid molecular cloud of dust and gas–indicating that they should have been born with the same chemical composition. Any differences that might have developed after their starry birth, would have to be attributed to the influence of their planets. The team of astronomers found that one star possesses a Jupiter-like planet in a highly eccentric orbit, while the other sports two considerably smaller Neptune-mass worlds.
The team of astronomers studied the levels of 15 elements in both stars, including calcium, aluminum, silicon, and iron because they have melting points higher than 1,200 degrees Fahrenheit, and are the refractory materials that serve as the building blocks for Earth-like, “terrestrial” worlds. Both stars displayed higher levels of these elements than our own Sun, indicating that they have both consumed large dinners of Earth-like material–an estimated 20 Earth-masses for the star with the two Neptune-sized planets, and 10 Earth-masses for the one with the Jupiter-sized planet.
The Vanderbilt astronomers based their study on the work of coauthor Dr. Simon Schuler of the University of Tampa, who had expanded the examination of stars’ chemical composition beyond their iron content. Mack, Schuler, and Stassum applied this technique to the stellar duo HD 20781 and HD 20782. Both stars are G-class dwarf stars akin to our Sun.
When the team of astronomers analyzed the spectrum of the sister stars, they discovered that not only were the relative abundances of the refractory elements significantly higher than that of our Sun, but also the higher the melting temperature of a particular element, the greater its abundance. This trend provided a precious clue that Earth-like rocky planets had been devoured.
The results of the study support the theory that a star’s chemical composition and the nature of its planetary system are linked.
“Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter. The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets,” Mack explained in the May 16, 2014 Vanderbilt University Press Release.
If a sufficient number of doomed, rocky Earth-like worlds plummet into their fiery parent-star, they will leave behind a sad testimony to their former existence in the form of a special chemical signature that astronomers can spot. “With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect,” Mack added.
Following this line of reasoning, it is not especially likely that the two Earth-eaters possess circling rocky planets. The two Neptune-sized planets orbit their star quite closely–at one-third the distance between our planet and the Sun. The other star’s Jupiter-sized planet spends a great deal of its time in the outer limits of the planetary system–however, its eccentric orbit also brings it sweeping inward very close to its star.
The team of astronomers found that the star hosting the two Neptune-sized planets devoured more terrestrial rocky-planet material than its sister star. The astronomers speculate that this may be because the two planets were more efficient at pushing luckless Earth-like planets into their star, than the lone Jupiter-sized planet was at bullying unfortunate Earth-like planets into the fiery furnace of its own hungry stellar parent.
If the tattle-tale chemical fingerprint of Sun-like G-class stars–that hungrily devour rocky Earth-like planets–turns out to be universal, “When we find stars with similar chemical signatures we will be able to conclude that their planetary systems must be very different from our own and that they most likely lack inner rocky planets. And when we find stars that lack these signatures, then they are good candidates for hosting planetary systems similar to our own,” Mack explained in the May 16, 2014 Vanderbilt University Press Release.
Stassun added that “This work reveals that the question of whether and how stars form planets is actually the wrong thing to ask. The real question seems to be how many of the planets that a star makes avoid the fate of being eaten by their parent star?”
