Two's A Crowd For Circumbinary Planets

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All stars are gigantic balls of roiling, searing-hot, mostly hydrogen gas. While we're used to seeing our own Star, the Sun, hanging high above us like a solitary brilliant beacon in our daytime sky, many stars have company. In fact, research conducted over the past two hundred years indicates that at least 50 percent of visible stars are members of multiple stellar systems. A binary is a particular type of multiple star system composed of a stellar duo in orbit around their common center of gravity (barycenter), often appearing to the unaided human eye as a single point of light--and their true multiple identity must be revealed by other means. However, in April 2018, a team of astronomers announced that their new research shows that some sparkling stellar binaries actually have a dark side. Alas, planets in orbit around short-period binaries, which are two stars that are bound tightly together in a close gravitational dance, can be hurled out into space as a result of their parent-stars' evolution.

The new study helps to explain why astronomers have discovered few circumbinary planets--which are alien worlds in orbit around stars that are in turn orbiting around each other--even though they have observed literally thousands of short-term binary stars. Short-term binaries are those with orbital periods of 10 days or less.

These findings also mean that such binary stellar systems are a poor place to send future ground- and space-based telescopes to hunt for habitable planets and life beyond Earth.

There are several different types of binary stellar systems--such as visual and spectroscopic binaries--which have been named according to the methods astronomers use in order to observe them. In a paper to be published in The Astrophysical Journal, lead author David Fleming, a doctoral student at the University of Washington in Seattle, presented the findings at the American Astronomical Society's Division of Dynamical Astronomyconference held from April 15-19, 2018 in San Jose, California. Fleming studies eclipsing binaries which are those where the orbital plane is so close to the line of sight that both stars composing the dancing duo are seen to float in front of each other.

Double, Double Toil And Trouble

The billions upon billions of stars that populate the visible Universe are all mostly composed of hydrogen--which is both the most abundant, as well as the lightest, atomic element. Stars transform their hydrogen fuel, deep within their seething-hot nuclear-fusing hearts, into progressively heavier and heavier atomic elements. The only elements that formed in the Big Bang birth of the Universe almost 14 billion years ago, were hydrogen, helium, and trace amounts of lithium. All of the atomic elements heavier than helium were manufactured in the nuclear-fusingcores of the stars--their hot, roiling, seething interiors progressively fusing the nuclei of lighter atoms into heavier things. However, the heaviest atomic elements of all, such as gold and uranium, form as the result of a supernova blast. A supernova heralds the explosive demise of a massive star.

Stars are born in the billowing, swirling, and very cold dense depths of beautiful, dark, giant molecular clouds. These enormous clouds float ghostlike through our Milky Way Galaxy in huge numbers--and probably haunt other galaxies as well. Even though these dark, cold clouds are mainly composed of gas and dust, they also harbor large populations of stars. The material within the floating, phantom-like clouds clumps together in a variety of sizes. The smaller clumps extend approximately one light-year across. The dense clumps eventually collapse to create protostars. The entire star-birthing process lasts for about 10 million years.

Within the secretive cores of these beautiful and bewitching clouds, smaller blobs of gas continue to collapse, creating what astronomers refer to as stellar embryos. The first batch of stars are born tucked within these embryos, and they sustain themselves by feeding on the ambient gas, thus growing in mass. Ultimately, the stars grow massive enough to be influenced by the gravitational attraction of growing stars nearby. The youthful stars then begin to dance together in a mesmerizing gravitational ballet, pulling on one another. This dance causes changes to occur within the natal cloud. Some of the dancing baby stars will pirouette close together, and this will create bound groups of two, three, or even four (or more) stars. Alas, some other stars will be tossed out of the dance altogether, and be ejected from the cloud. This turbulent and chaotic tug-of-war among young dancing stars scatters them throughout their natal cloud.

The term double star is frequently used synonymously with binary star. However, the term double star also refers to an optical double star. Optical doubles got their name because the duo of stars appear near enough together in the sky as seen from Earth to be almost on the same line of sight.

Binary stellar systems are valuable tools for astrophysicists to use. This is because calculations of their orbits enable the masses of their component stars to be directly determined. From this, astrophysicists can go on to indirectly determine other stellar parameters, such as radius and density. In addition, this also helps astronomers discover an empirical mass-luminosity relationship (MLR) from which the masses of single stars can be calculated.

If the two stars that compose a binary system are close enough together they can gravitationally distort their shared outer stellar atmospheres. In certain instances, these close binary systems can exchange mass. This exchange of mass may cause them to experience stages in their evolution that solitary stars--without stellar company--are unable to attain.

Since the invention of the telescope, many dancing stellar duos have been discovered by astronomers searching the heavens. Some of the earliest examples include Mizar and Acrux. Mizar is a denizen of Ursa Major (the Big Dipper), and it was discovered by the Italian astronomer Giovanni Battista Riccioli (1598-1671) in 1650. However, Mizar may have been discovered even earlier by the Italian mathematician Benedetto Castelli (1578-1645) and Galileo Galilei (1564-1642), the great Italian polymath. The brilliant southern star Acrux, a constituent of the Southern Cross, was found to be a double star by the French mathematician and astronomer Father Jean de Fontenay (1643-1710) in 1685.

The English scientist John Michell (1724-1793) was the first to propose that double stars might be physically attached to each other--and that they probably did not result from only a chance alignment. The German-born English astronomer William Herschel (1738-1822) began scanning the sky for double stars in 1770, and soon afterwards published catalogs listing about 700 double stars. In 1803, Herschel began to observe alterations in the relative positions in many double stars over a period of 25 years, and he went on to conclude that they must be binary systems. However, the first orbit of a binary star was not observed until 1827, when the French astronomer Felix Savary (1797-1841) calculated the orbit of Xi Ursa Majoris. Since then, many more double stars have been catalogued and measured. Indeed, the Washington Double Star Catalog contains more than 100,000 duos of double stars--including optical doubles and binary stars. The Washington Double Star Catalog is a database of visual double stars compiled by the United States Naval Observatory in Washington D.C.. However, the orbits are currently known for only a few thousand of these stellar duos, and most have not yet been determined to be either optical double stars or true binaries.This can be accomplished by astronomers observing the relative motion of the two stars. If the motion is part of an orbit, or if the two stars sport similar radial velocities and the difference in their proper motions is small when compared to their common proper motion, then the duo is likely physical. One of the jobs that remain for visual observers of double stars is to acquire sufficient observational data to either prove or disprove a gravitational connection between the two stars.

The Doomed Planets Of Dancing Stellar Duos

David Fleming and his co-authors considered what would happen when eclipsing binaries orbit one another in a close gravitational ballet--lasting about 10 days, or less. The astronomers wondered if tides--the gravitational forces each star exerts on the other--would have "dynamical consequences" for the star system.

"That's actually what we found", using supercomputer simulations, Fleming noted in the April 12, 2018 University of Washington Press Release. "Tidal forces transport angular momentum from the stellar rotations to the orbits. They slow down the stellar rotations, expanding the orbital period," he added.

The transfer of angular momentum not only causes the orbits of the duo to enlarge, it also causes them to circularize. The transfer of angular momentum actually results in a sea-change. Orbits that were originally eccentric and football shaped instead evolve into perfect circles. Over a very long period of time, the spins of the two dancing stars also become synchronized, as the Moon is with Earth, with each star forever showing the same face to the other--while the other face is forever turned away.

The expanding stellar orbit "engulfs planets that were originally safe, and then they are no longer safe--and they get thrown out of the system," explained Dr. Rory Barnes in the April 12, 2018 University of Washington Press Release. Dr. Barnes is a University of Washington assistant professor of astronomy and a co-author on the research paper. In addition, the ejection of one planet in this way can jostle the system enough to perturb the orbits of other orbiting planets in a kind of cascading effect--ultimately ejecting them out of the system as well, and hurling them into the space between stars.

The "life" of acircumbinary planet is obviously a difficult one. Making the situation even worse for these unlucky alien worlds is a "region of instability" that arises from the competing gravitational pulls of the two stars.

"There's a region that you just can't cross--if you go in there, you get ejected from the system. We've confirmed this in simulations, and many others have studied the region as well," Fleming continued to explain in the April 12, 2018 University of Washington Press Release.

This is what astronomers term the dynamical stability limit. It travels outward as the stellar orbit grows larger and larger, and eventually engulfs unfortunate planets, thus causing their orbits to become unstable. This chain of events ultimately sends them shrieking out of the system altogether.

Fleming continued to explain that another intriguing characteristic of this type of binary system is that circumbinary planets have a tendency to orbit just outside this stability limit--indeed, they "pile up" there. But how these tragic planets manage to get to this region is not well understood--they may migrate inward from farther out in the system or they may have simply been born there.

The astronomers applied their new model to known short-period binary star systems. In fact, Fleming and his co-authors found that this stellar-tidal evolution of binary stars ejects at least one planet in 87 percent of multiplanet circumbinary systems--and frequently even more. However, according to Dr. Barnes, even this high number is probably a conservative estimate, and the true number may be as high as 99 percent.

The team of scientists call the process the Stellar Tidal Evolution Ejection of Planets (STEEP). Future discoveries--"or non-detections"--of circumbinary planets in orbit around short-period binary host stars, the authors write, will "provide the best indirect observational test of the STEEP process.

Kepler 47 currently holds the title of the shortest-period binary star for which a circumbinary planet has been detected. This circumbinary planet has a period of only about 7.45 days. The study co-authors propose that future studies searching for possibly habitable worlds around short-period binary stars should focus on those with periods that are longer than about 7.5 days.

Fleming and Barnes' co-authors are University of Washington astronomy professor Dr. Tom Quinn, post-doctoral researcher Dr. Rodrigo Luger, and undergraduate student David E. Graham. This study used storage and networking infrastructure provided by the Hyak supercomputer system located at the University of Washington, funded by the University of Washington's Student Technology Fee.

The research was funded by the NASA Astrobiology Institute through the University of Washington-based Virtual Planetary Laboratory. Fleming is supported by funding from the NASA Space Science Fellowship Program.

Fleming noted in the April 12, 2018 University of Washington Press Release, that in respect to the hunt for habitable worlds beyond our own, planets orbiting short-term eclipsing binary stars might be attractive targets for closer study because of their edge-on angle showing eclipses.

Nevertheless, Fleming cautioned that "This mechanism tends to kill them. So, it's not a good place to look."