Dwelling in our Milky Way Galaxy, about 300 light-years away, there is a small star named HIP68468. This little star is so much like our own Sun that it is often referred to as its “twin”–but if HIP68468 is our own Star’s twin, it is an evil one that is hiding a deep, dark secret of terrible past misdeeds. In December 2016, a team of astronomers from the University of Chicago proposed that the sinister star HIP68468 may have swallowed one or more of its family of orbiting planets. The scientists base their theory on the recent discovery of lithium and refractory elements near HIP68468’s surface. This interesting observation of HIP68468’s weird composition suggests that it had once feasted on some of its own planet-children.
“It doesn’t mean that the Sun will ‘eat’ the Earth any time soon. But our discovery provides an indication that violent histories may be common for planetary systems, including our own,” commented Dr. Jacob Bean in a December 15, 2016 University of Chicago Press Release. Dr. Bean is an assistant professor of astronomy and astrophysics at the University of Chicago in Illinois and co-author of an article published in the journal Astronomy & Astrophysics that describes the new research.
As ghastly as the sinister deeds of our Sun’s evil twin may be, the little star is a valuable tattle-tale, providing precious clues about how planetary systems evolve as time goes by.
Astronomers discovered the first exoplanet orbiting a Sun-like star beyond our own Sun back in the mid 1990s. Now, a generation later, astronomers have discovered a veritable treasure trove filled with exoplanets. Some of these brave new worlds are so alien that astronomers never dreamed that such bizarre planets could possibly exist–until they found them where they were hiding in the glaring, obscuring light of their parent-star. On the other hand, some of the newly discovered worlds, belonging to the families of faraway stars, are eerily familiar, and bear a striking resemblance to the familiar planets inhabiting our own Solar System.
Stellar Earth-Eaters
Some stars like our own Sun are Earth-eaters. This is because, as these stars evolve, they manage to feast on large quantities of rocky material from which Earth-like terrestrial planets are born–and the lingering, undigested traces of these terrible feasts can be found “polluting” the atmospheres of their guilty stellar parents.
For example, in May 2014, Dr. Trey Mack of Vanderbilt University in Nashville, Tennessee, announced that he had devised a model that calculates the effect such an unappetizing, rocky diet can have on an Earth-eating star’s chemical composition–and he used his model to analyze a duo of twin stars that both have their own separate sets of planet-children.
Dr. Mack’s model helps to shed new and valuable light on the process of planet-birthing, as well as serving as a useful tool that astronomers can use as they hunt for Earth-like planets dwelling in the distant families of stars beyond our Sun. In our own Solar System, the inner, rocky terrestrial planets are Venus, Earth, and Mars.
Some faraway stars are composed of more than 98 percent hydrogen and helium–the two lightest atomic elements in the Universe, born in the Big Bang about 13.8 billion years ago (Big Bang nucleosynthesis). All of the other, heavier atomic elements account for a mere 2 percent–or less–of their mass. In the jargon of astronomers, any atomic element heavier than helium is called a metal, and they have coined the term metallicity to describe the ratio of the relative abundance of iron to hydrogen in a star’s chemical composition. Therefore, the term metal does not carry the same meaning for astronomers that it does for chemists.
Ever since planet-hunting astronomers spotted the first batch of exoplanets a generation ago, they have made attempts to link stellar metallicity to planet-formation. In one research study, scientists at Los Alamos Research Laboratory proposed that stars with high metallicities are better able to create planetary systems than those stars that are not as richly endowed with heavy metals. Yet another study concluded that hot-Jupiter-sized planets are primarily observed to be in orbit around stars with high metallicities, while smaller planets are seen orbiting stars with a wider range of metal content.
Hot-Jupiters are gas-giant planets resembling our own Solar System’s resident behemoth, Jupiter. While our Jupiter is located in the cold outer region of our Solar System, hot Jupiters hug their stellar parents in very close roasting orbits. In fact, the very first exoplanet to be discovered in orbit around a Sun-like star was a hot Jupiter dubbed 51 Pegasi b.
Dr. Mack used the research of co-author Dr. Simon Schuler of the University of Tampa in Florida, who expanded the study of the chemical makeup of stars beyond their iron content. Dr. Mack looked at the abundance of 15 specific elements relative to that of our Sun. He was especially interested in elements like silicon, calcium, aluminum, and iron that have melting points higher than 1200 degrees Fahrenheit. That is because these particular elements are the refractory materials that played the starring role of building blocks for Earth-like worlds.
Dr. Mack, Dr, Schuler, and Dr. Keivan Stasson, also of Vanderbilt, decided to apply this technique to a stellar-parent-binary duo dubbed HD 20781 and HD 20782. Both stars should have condensed out of the same cold, giant, dark molecular cloud composed of gas and dust, that are the strange cradles that give birth to new, fiery baby stars. Because both stars of this stellar duo had condensed out of the same molecular cloud, they should both have started out with the same chemical compositions. Furthermore, this particular binary pair is the first one discovered where both stars have a set of planet-children all their own.
When the astronomers analyzed the spectrum of the stellar duo, they discovered that the relative abundance of the refractory elements was significantly higher than that of our own Sun. They also found that the higher the melting temperature of a particular element, the higher its abundance. This trend served as a fascinating tattle-tale clue that Earth-like rocky planets had been devoured by their parent-star. The astronomers then went on to calculate that each of the twin stars in the binary system would have had to devour an additional 10 to 20 Earth masses of rocky material in order to produce the observed chemical signatures. Specifically, one of the parent-stars hosts a Jupiter-sized planet, and this voracious star appears to have consumed an extra helping of 10 Earth-masses–while the second star of the duo hosts two Neptune-sized planets, and has apparently managed to gulp down an impressive feast amounting to an additional 20 Earth-masses.
The results of this study support the theory that a star’s chemical make up, and the characteristics of its circling system of planets, are inextricably bound together. In the case of this particular binary system, it is unlikely that either one of the twin stars hosts terrestrial planets. This is because the star with two Neptune-sized planets is being hugged by its two planet-children very closely–at only one-third the Earth-Sun separation. In the case of the other star, on the other hand, the trajectory of its Jupiter-sized behemoth world grazes its stellar-parent–carrying it closer than the orbit of our Solar System’s Mercury at the point of closest approach. The astronomers suggest that the reason why the star hosting a duo of Neptune-sized planets swallowed more terrestrial material than its twin-star is that its two planets were better at pushing material into their star than the lone Jupiter-sized planet was at pushing material into it.
The Sinister Story Of Our Sun’s Evil Twin
Dr. Bean and his colleagues studied our Sun’s evil twin HIP68468 as part of a multi-year project to hunt exoplanets that belong to the family of solar-twins. However, it is somewhat tricky to draw conclusions from only one system, Megan Bedell cautioned in the December 15, 2016 University of Chicago Press Release. Ms. Bedell is a University of Chicago doctoral student who is co-author of the research and the lead planet-hunter for the collaboration. She added that the team is planning to “study more stars like this to see whether this is a common outcome of the planet formation process.”
Supercomputer simulations indicate that billions of years from now, the accumulated gravitational tugs and pulls between the planets of our own Solar System will cause Mercury to tumble into our Sun, noted Dr. Debra Fischer in the December 15, 2016 University of Chicago Press Release. Dr. Fischer is a professor of astronomy at Yale University in New Haven, Connecticut, who was not involved in the research. “This study of HIP68468 is a post-mortem of this process happening around another star similar to our Sun. The discovery deepens our understanding of the evolution of planetary systems,” Dr. Fischer continued to explain.
Using the 3.6-meter telescope at La Silla Observatory in Chile, the team of astronomers from several continents found its first exoplanet in 2015. The more recent discovery still needs to be confirmed, but it includes two planet candidates–one super-Neptune and one super-Earth. The orbits of the two planets take them amazingly close to their parent-star. Indeed, one of the planets is 50 percent more massive than our own Solar System’s Neptune, but is situated at a Venus-like distance from its stellar host. The other, the first super-Earth to be discovered circling a solar-twin, is three times our own planet’s mass and so close to its parent-star that it takes a mere three days to complete a single orbit.
“These two planets most likely didn’t form where we see them today,” Ms. Bedell commented in the December 15, 2016 University of Chicago Press Release. Instead, she added, they likely migrated inward from the outer regions of the planetary system. Other planets could have been unceremoniously evicted from the system–or devoured by their hungry host star.
The tragic truth is that HIP68468’s composition tells the terrible tale of past misdeeds, and points an accusing finger at this star–whose sinister past seems to have involved feasting on its own planets. HIP68468’s contains four times more lithium than would be expected for a star that is 6 billion years old. Lithium is a chemical element that is a soft, silver white metal (as chemists define the term) belonging to the alkali metal group of elements. In addition, the star also shows a surplus of refractory elements–those tattle-tale metals that are resistant to heat and are abundant in rocky terrestrial planets.
In the searing-hot, blazing, roiling interior of stars like HIP68468 and our own Sun, lithium is consumed over time. On the other hand, planets preserve their supply of lithium because their interior temperatures are too low to destroy the element. Therefore, when a star devours a planet, the ingested planet’s lithium stands out to tell the sinister secret of its tragic demise. The destroyed planet’s lithium deposits in the guilty star’s atmosphere tell all!
Alas, when taken together, the lithium and the consumed rocky planet material lingering in the hot atmosphere of HIP68468 is equivalent to the mass of six Earths.
“It can be very hard to know the history of a particular star, but once in a while we get lucky and find stars with chemical compositions that likely came from in-falling planets. That’s the case with HIP68468. The chemical remains of one or more planets are smeared in its atmosphere,” Dr. Fischer explained in the December 15, 2016 University of Chicago Press Release.
“It’s as if we saw a cat sitting next to a bird cage. If there are yellow feathers sticking out of the cat’s mouth, it’s a good bet that the cat swallowed a canary,” she added.
The team of astronomers continues to keep watch on over 60 solar twins, hunting for more exoplanets. In the future, the Giant Magellan Telescope under construction in Chile, for which the University of Chicago is a founding partner, will be capable of spotting more Earth-like exoplanets in the families of solar twins.
“In addition to finding Earth-like planets, the Giant Magellan Telescope will enable astronomers to study the atmospheric composition of stars at even greater detail than we can today. That will further reveal the histories of planetary systems that are subtly imprinted on their host stars,” Dr. Bean explained to the press.
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