Death March Of The Planets
By Judith E Braffman-Miller
Strange things happen among the stars–amazing, unforeseen, and bewildering behavior that both baffles and bewitches those who try to understand the long-held secrets lost in starlight. Mysteries are fun, and solving them can provide an unsurpassed delight. One of the most mesmerizing of stellar mysteries is the weird behavior of parent-stars that swallow their own planetary-offspring, as one by one–in a hideous death march of the planets–these doomed worlds spiral fatally inward, and then finally into, the fiery furnaces of their roiling, broiling parent-stars. In classical mythology, the Titan Kronos devoured his own children, including Hades (Pluto), Poseidon (Neptune), and three daughters. Some stars behave like ancient gods. In October and December 2017, two separate teams of astronomers announced that they have found clues that certain stellar parents display tattle-tale evidence of their terrible feast, showing signs of having devoured their own tragic planetary offspring.
In October 2017, a team of astronomers announced their discovery of twin stars–and, after observing them, had come to the startling realization that one of the stars showed tattle-tale signs of having devoured at least a dozen rocky planets. For this reason, the Princeton astronomers named the stellar duo after Kronos and his less well-known brother Krios. The official designations for the two stars are HD 240430 and HD 240429.
In a separate study, published in December 2017, a different team of U.S. astronomers, who had been observing the star RZ Piscium, announced that they have found disturbing clues suggesting that its weird and unpredictable episodes of “winking” may be caused by enormous tattle-tale clouds of gas and dust. It is believed that these clouds are all that remain of the wicked parent-star’s terrible feast–when it tragically devoured its very own wrecked planet-children.
“Our observations show there are massive blobs of dust and gas that occasionally block the star’s light and are probably spiraling into it. Although there could be other explanations, we suggest this material may have been produced by the break-up of massive orbiting bodies near the star,” explained Kristina Punzi in a December 21, 2017 NASA Press Release. Ms. Punzi is a doctoral student at the Rochester Institute of Technology (RIT) in New York and lead author of a paper describing the findings.
Kronos And Krios
In order to understand the sibling stellar duo, dubbed Kronos and Krios, the researchers first had to confirm that the two widely separated stars really compose a binary system. After that, the scientists studied Kronos’ bizarre chemical abundance pattern, according to Semyeong Oh, a graduate student in astrophysical sciences who is lead author on a new paper that describes Kronos and Krios. Oh works with Dr. David Spergel, who is the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation and director of the Flatiron Institute’s Center for Computational Astrophysics. The Flatiron Institute, located in New York, is the intramural research division of the Simons Foundation.
Other co-moving stellar binaries have displayed different chemistries, Oh went on to explain, but none that are as dramatically different as those of Kronos and Krios.
Indeed, most stars that are as richly endowed with metals as Kronos “have all the other elements enhanced at a similar level, whereas Kronos has volatile elements suppressed, which makes it really weird in the general context of stellar abundance patterns,” Oh continued to comment.
In astronomy the term metal refers to all of the atomic elements that are heavier than helium. Only hydrogen, helium, and traces of lithium were formed in the fireball of the Big Bang that marks the moment of our Universe’s birth almost 14 billion years ago. All of the atomic elements heavier than helium were either formed in the nuclear-fusing fires of the stars, or in the supernovae that heralded the explosive “deaths” of massive stars.
The problem is that Kronos showed an unusually high level of rock-forming minerals, including magnesium, aluminum, silicon, iron, chromium and yttrium–without an equally high level of volatile compounds, such as those frequently found in gas form, such as oxygen, carbon, nitrogen, and potassium.
Kronos is already beyond the Galactic norm, Oh noted. She added that, in addition, “because it (Kronos) has a stellar companion to compare it to, it makes the case a little stronger.”
Dr. Jessie Christiansen, an astronomer at the NASA Exoplanet Science Institute at the California Institute of Technology (Caltech) in Pasadena, explained in the October 12, 2017 Princeton University Press Release that “At the moment we are still at the stage of piecing together different observations to determine how and when exoplanets form. It’s difficult to directly observe planet formation around young stars–they are typically shrouded in dust, and the stars themselves are very active, which makes it hard to disentangle any signals from the planets. So we have to infer what we can from the limited information we have. If borne out, this new window onto the masses and compositions of the material in the early stages of planetary systems may provide crucial constraints for planet formation theories.” Dr. Christiansen was not involved in the research.
Dr. Adrian Price-Whelan, a Lyman Spitzer, Jr. Postdoctoral Fellow in Astrophysical Sciences at Princeton, and a co-author on the paper describing Kronos and Krios, commented in the same Press Release that “One of the most common assumptions–well-motivated, but it is an assumption–that’s pervasive through Galactic astronomy right now is that stars are born with [chemical] abundances, and they then keep those abundances. This study is an indication that, at least in some cases, that is catastrophically false.”
Some Like It Hot
Astronomers have suspected, for almost a generation now, that some parent-stars devour their planet-offspring. Indeed, this rather disturbing behavior among the stars was proposed back in 1995, when the first confirmed exoplanet was discovered in orbit around its Sun-like star.
In 1995, when Dr. Michel Mayor and Dr. Didier Queloz of Switzerland’s Geneva Observatory reported the first strong evidence of the existence of a planet, circling a hydrogen-burning main-sequence star like our Sun, this historic discovery was met with both joy and bewilderment. The Swiss team’s observations indicated the presence of a planet, as hefty as our own Solar System’s banded behemoth Jupiter, circling its stellar parent in an orbit far too close for comfort.
The distant parent-star, 51 Pegasi, is situated in the constellation Pegasus. The then-newly-discovered gigantic planet, named 51 Pegasi b, is a mere 4,300,000 miles from its star–a tiny fraction of the distance separating Mercury, the innermost major planet, from our own Sun. 51 Pegasi b orbits its star every 4.2 days!
The problem was that the then-existing theories of planetary system formation proposed that giant Jupiter-like planets could only form at much greater distances from their stellar parents. What was the enormous 51 Pegasi b doing so close to its star?
51 Pegasi is considered to be a neighboring star, a comparatively trifling 42 light-years from our Sun and its family. Even so, 51 Pegasi b was extremely difficult to detect because it was lost in the blinding glare of its far more brilliant stellar parent. No existing telescope could image this planet, even though it is enormous. Only a tiny “wobble” in the motions of 51 Pegasi, carefully monitored over a two year period with a visible light spectrograph at the Observatoire de Haute Provence in Saint Michel, France, betrayed the gravitational tug of this huge planet on its star–thus creating the tattle-tale “wobble”.
In October 1995, a team of astronomers at San Francisco State University and the University of California, Berkeley, confirmed the Swiss team’s discovery from the Lick Observatory’s three-meter telescope poised atop Mount Hamilton in California. The California team observed exactly the same “wobble” as the Swiss team.
The good news is that one of the most profound questions in astrophysics had at long last been answered. Yes, there are planets orbiting stars that are like our own Sun.
The bad news is that the good news challenged existing theories of planet formation. How did this enormous roasting behemoth of a planet get so close to its roiling, broiling star? After all, 51 Pegasi b likely roasts at over 1,800 degrees Fahrenheit–a searing-hot temperature that makes the planet glow red like a toaster coil.
New theories had to be devised to explain how this gigantic scorcher managed to get to where astronomers found it. However, no one knew whether the planet always had been this close to 51 Pegasi, or even what it was made of. Some theorists proposed that 51 Pegasi b is essentially one big molten rock. Others suggested that the planet–like our own Jupiter–is a gas-giant that was born 100 times farther from its star, and was bounced toward 51 Pegasi through a near-catastrophic brush with an undiscovered second planet or companion star.
Yet a third theory postulated that the planet was born at a distance from its star comparable to Jupiter’s average distance from our Sun. According to this theory, 51 Pegasi b gradually lost energy as a result of interactions with the disk of gas and dust from which it was born. The baby planet, alas, was doomed to spiral inward from its distant place of birth to where it now roasts miserably in its close-in orbit.
According to this model, 51 Pegasi b is only one of several planets that were born in the cooler, outer regions of the disk. Most of the other migrating planets crashed to a fiery death within the stellar furnace of 51 Pegasi. However, in the case of 51 Pegasi b, a tragedy was avoided, and the planet was spared the fate of its doomed sister worlds. The behemoth planet did not crash down into the oven of its murderous parent-star. Instead, it was saved–just in the nick of time–from that dreadful fate. So, now, 51 Pegasi b, orbits its parent-star fast and close–baking slowly in its hell-like orbit.
This tragic “death march of the planets” probably occurs over the course of a few hundred thousand years. The fact that a planet could actually survive such a disaster probably depends, some theorists propose, on how late it began to march.
If an entire generation of ill-fated planets plunged to a horrible death in the roiling furnace of 51 Pegasi before 51 Pegasi b came marching along, another generation of planets may now be spiraling in from more distant orbits.
51 Pegasi b was the first of a new class of exoplanets to be discovered. These so-called hot Jupiter worlds rapidly orbit their parent stars–and these “roasters” may be inwardly spiraling gas-giants, doomed to be vaporized in the ovens of their merciless stellar parents.
Death March Of The Planets
RZ Piscium is located approximately 550 light-years from our Solar System in the constellation Pisces. During its mysterious dimming episodes–or “winks”–which can go on for as long as two days, the erratic star fades to become 10 times fainter. The “winking” star manufactures considerably more energy at infrared wavelengths than emitted by stars like our own Sun. This means that RZ Piscium is encircled by a disk of warm dust. Indeed, about 8 percent of its total luminosity is in the infrared portion of the electromagnetic spectrum. This level is matched by only a few of the thousands of stars in our Sun’s neighborhood that have been studied over the past 40 years. This indicates the presence of enormous quantities of dust.
“Our observations show there are massive blobs of dust and gas that occasionally block the star’s light and are probably spiraling into it
These observations, and others, have led some astronomers to come to the conclusion that RZ Piscium is a youthful Sun-like star surrounded by a heavily populated asteroid belt, where frequent smash-ups pulverize the rocky asteroids to dust. However, the evidence supporting this model is far from clear.
An alternative scenario indicates that the strange “winking” star is not young, but is instead older than our 4.56 billion year old middle-aged Sun. According to this viewpoint, RZ Piscium is just beginning its transition into the red giant stage of its existence, transitioning from middle-age to become a senior stellar citizen. Stars like our Sun, when they have burned their entire necessary supply of nuclear-fusing fuel, swell to monstrous proportions to become a bloated red giant star.
A dusty disk, surrounding a young star, would have long since dispersed after a few million years. For this reason, those astronomers proposing that the “winking” star is no longer young needed another source of dust to explain the star’s infrared glow. Because the aging star is ballooning in size, it would destroy any unfortunate planets situated in close orbits. The destruction of these tragic worlds could be the source of the enormous quantity of surrounding dust.
So is RZ Piscium a young star surrounded by a debris disk, or is it a planet-wrecking stellar senior citizen? That is the question! According to Kristina Punzi, the answer is that it is a bit of both.
The team of astronomers studied the “winking” star using the European Space Agency’s (ESA’s) XMM-Newton satellite, the Shane 3-meter telescope at Lick Observatory in California, and the 10-meter Keck I telescope at W.M. Keck Observatory in Hawaii.
A young star is frequently a prodigious source of X-rays. The team used 11 hours of XMM-Newton observations in order to make their discovery that RZ Piscium is, likewise, an abundant source of X-rays. The star’s surface is approximately 9,600 degrees Fahrenheit, which makes it only slightly cooler than that of our Sun. The team also demonstrated that the star is enriched in the tattle-tale element lithium. Lithium is slowly destroyed by nuclear reactions within stars.
“The amount of lithium in a star’s surface declines as it ages, so it serves as a clock that allows us to estimate the elapsed time since a star’s birth. Our lithium measurement for RZ Piscium is typical for a star of its surface temperature that is about 30 to 50 million years old,” explained study co-author Dr. Joel Kastner in the December 21, 2017 NASA Press Release. Dr. Kastner is director of RIT’s Laboratory for Multiwavelength Astrophysics.
Therefore, even though RZ Piscium is young, it is still too old to be encircled by such an abundance of gas and dust. “Most Sun-like stars have lost their planet-forming disks within a few million years of their birth. The fact that RZ Piscium hosts so much gas and dust after tens of millions of years means it’s probably destroying, rather than building, planets,” explained team member Dr. Ben Zuckerman in the same NASA Press Release. Dr. Zuckerman is an astronomy professor at the University of California, Los Angeles.
Observations conducted with ground-based instruments also probed the “winking” star’s environment, discovering evidence that the dust is accompanied by large quantities of gas. Based on the temperature of the dust, which is around 450 degrees Fahrenheit, the astronomers concluded that most of the debris is orbiting approximately 30 million miles from RZ Piscium.
“While we think the bulk of this debris is about as close to the star as the planet Mercury ever gets to our Sun, the measurements provide evidence that material is both falling inward toward the star and also flowing outward,” explained another study co-author Dr. Carl Melis in the December 21, 2017 NASA Press Release. Dr. Melis is an associate research scientist at the University of California, San Diego.
The astronomers believe that the best explanation that accounts for all of the available data is that the star is surrounded by debris left behind as tattle-tale evidence of a stellar disaster. It is possible that the star’s tides may be stripping material from a nearby substellar companion or giant planet, thus manufacturing intermittent flows of gas and dust. Or, alternatively, it is possible the companion has already been completely vaporized. Another possible explanation is that one of the more massive gas-rich planets inhabiting the distant stellar system experienced a catastrophic collision in the astronomically recent past.
A paper reporting the findings was published December 21, 2017 in The Astronomical Journal.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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