Saturday, June 21, 2014

The Hungry Star That Can't Stop Snacking

When a small star like our Sun has finally begun to use up its necessary supply of hydrogen fuel, it first swells up to hideous proportions to become what is known as a Red Giant star. This very bloated, red-hued relic of what was once a small, sparkling Sun-like star balloons in size to the point that--if it is circled by inner, unfortunate planets--it will engulf them with its extended, searing-hot outer gaseous layers, thus consuming them. In June 2014, a team of astronomers announced at the summer meeting of the American Astronomical Society, held in Boston, Massachusetts, that they had spotted an especially hungry Red Giant star that was about to snack on not only one, but two, doomed planets!

The two tragic worlds, dubbed Kepler-56b and Kepler-56c are destined to be swallowed by their greedy parent star in a "short" time--by cosmic standards, that is. Both planets will perish in about 130 million and 155 million years, respectively.

"As far as we know, this is the first time two known exoplanets in a single system have a predicted 'time of death,'" study lead author Dr. Gongjie Li told the press on June 2, 2014. Dr. Li is of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.

She presented her study at a press conference held at the 224th meeting of the AAS.

The hungry star Kepler-56 is in the process of morphing into a bloated, greedy Red Giant. It has already swollen to monstrous proportions, and is currently about four times our Sun's size. As it grows older, it will continue to expand outward. Not only will the crimson star grow larger, but its tides will get more powerful, dragging its planets inward to their eventual tragic fate.

Even before they are vaporized by their star, the two planets will be subjected to intense heating from their increasingly growing stellar parent. Their atmospheres, if present, will begin to boil away--and the miserable planets themselves will be stretched into egg shapes by intense stellar tides.

The Kepler-56 system is much more than merely a tragic example of what happens at the end of a small star's main-sequence (hydrogen-burning) "life". It also provides a disturbing glimpse into the future of our own Solar System. In about five billion years our Sun will also swell into an angry Red Giant, blowing itself up to hideous proportions, first engulfing Mercury, then Venus--and then, possibly, the Earth.

Sun On Steroids

Our Solar System emerged from the jumbled scraps left over from the ancient, long-dead, nuclear-fusing cores of previous generations of stars. Our Sun came into being in a very cold, dense pocket, secreted within an enormous, dark interstellar molecular cloud. There are many such frigid clouds haunting our large, barred-spiral Milky Way Galaxy, and they serve as the strange cradles of its fiery baby stars. Ultimately, the very dense star-birthing pocket, embedded within the dark molecular cloud--composed mostly of gas, but also containing a pinch of dust--will collapse under the heavy weight of its own gravity to give birth to a brilliant new star. In the secret depths of such enormous, cold, dark clouds, slender and delicate strands of material gradually tangle themselves up together, and merge into clumps that grow for hundreds of thousands of years. Then, it happens--suddenly the dense pocket is sufficiently squeezed, by the crush of gravity, to the point that hydrogen atoms floating around within it begin to fuse. This lights the baby star's fire, and it will continue to rage for as long as the star "lives"!

All of our Galaxy's 400 billion stars, including our Sun, were born this way--through the gravitational collapse of heavy pockets embedded within frigid, dark molecular clouds. These billowing black clouds are dispersed throughout our Milky Way, and they carry within them the gas and dust of long-vanished older generations of ancient stars that perished very long ago.

Our Sun is a middle-aged, main-sequence, rather ordinary small star. It was born about 4.56 billion years ago, and it appears to us in our daytime sky as a large and ferociously glaring golden sphere. There are eight major planets, a multitude of moons and moonlets, and a rich assortment of smaller objects--both rocky and icy--circling our Star, which dwells in the distant suburbs of a typical, though majestic, large Galaxy.

However, in another 5 billion years--or so--our Sun will go Red Giant! A star of our Sun's relatively small mass "lives for about 10 billion years on the main-sequence. However, at present, our Sun and stars like it--that are experiencing an active middle-age--are still vibrant and bouncy enough to go on happily burning hydrogen in their stellar furnaces by way of nuclear fusion. Nuclear fusion progressively manufactures heavier atomic elements from lighter ones, in a process termed stellar nucleosynthesis.

When our Sun, and other stars that are similar to it, have finally burned up their necessary supply of hydrogen fuel, their looks change. They are now old stars. In the heart of an elderly Sun-like star, there is a hidden core composed of helium. The helium heart is surrounded by a shell in which the hydrogen is still being fused into helium. At this point, the shell begins to expand outward, and the core continues to enlarge, as the star grows ever older and older. At last, the helium heart itself starts to shrivel under the heavy weight of its own mass, and it becomes increasingly hotter and hotter until, at long last, it becomes sufficiently hot at the center for a new phase of nuclear burning to begin. At this new phase, the helium is fused to form the even heavier atomic element, carbon. In another five billion years, our doomed Star will sport a tiny and searing-hot core that will be emitting more energy than it currently is. The outer gaseous layers of our Sun will have become red and bloated, and it will no longer be the beautiful, brilliant golden ball that we observe lighting up our daytime sky. The fiery-red, swollen, elderly Sun will have morphed into a Red Giant, with a hideous appetite that will cause it to make snacks of its inner-planet children. The temperature on the surface of this angry, seething crimson ball of gas will actually be quite a bit cooler than that of our Sun's surface today. This explains the comparatively cool red hue--in contrast to a much hotter, sparkling, boiling yellow.

When our Sun goes Red Giant it will still be sufficiently hot to convert the frozen inhabitants of the remote Kuiper belt--such as the dwarf planet Pluto and its kindred icy objects--into tropical paradises. However, this balmy tropical haven of refuge will not last forever. The core of our elderly, dying Sun will continue to shrivel because it is no longer capable of spewing out radiation as a result of the process of nuclear fusion--and it will have reached the end of that long stellar road, because all further evolution will be determined by gravity alone. In the end, our Sun will hurl off its outer gaseous layers into the space between stars--but its core will remain in one piece, and all of the Sun's matter will ultimately collapse into this petite remnant object that is only about the size of Earth. Our Sun will have undergone a sea-change, and in its death-throes will have become a type of stellar corpse known as a white dwarf. This strange, dense relic of what was once our fiery, incandescent Star, will be encircled by an exquisitely beautiful shell of expanding varicolored gases that were once its outer layers--termed a planetary nebula. Planetary nebulae, which surround white dwarfs, got their strange name because earlier astronomers thought that they resembled the planets Uranus and Neptune.

For now, our planet sits quite comfortably--though near, in cosmic terms--to the inner edge of our Star's habitable zone, where water can exist in its liquid state, and therefore life can evolve. The habitable zone will spread increasingly further out as our Star glares ever more brightly. Even now, it is relentlessly, slowly, growing every more ominously, murderously brilliant. In about 2 billion years, if human beings have managed to survive, the tattered remnants of our species will be forced to flee our planet before it is vaporized by our Star. Mars will be the first choice for relocation--for a while, anyway. However, about 3 billion years later, what is left of humanity will have to migrate again, because the Sun will be about to snack on that planet as well. The formerly icy moons of the outer planets may prove to be havens of refuge, at this point--but, by this time, whatever may be left of our species had better know how to travel into interstellar space in search of exoplanet homes. Our Sun will hurl off its outer layers, and morph into a white dwarf with a ghastly, powerful gravitational pull. But before our Star goes finally into that good night, its outer layers will become that beautiful shroud of glimmering, shimmering varicolored gases--a planetary nebula, sometimes called a "butterfly of the cosmos."

The Star That Can't Snack On Just One

Alas, both Kepler-56b and Kepler-56c are considerably closer to their murderous parent star than Mercury is to our Sun. Kepler-56b orbits its star once every 10.5 days, while Kepler-56c orbits every 21.4 days. Both of these doomed planets will, therefore, meet their unfortunate fate much faster than Mercury will about 5 billion years from now. Dr. Li and her team calculated the evolution of both the star's size (using the publicly available MESA code ) and the planets' orbits to predict when the planets will be vaporized.

The lone survivor of what was once a planetary system will be Kepler-56d, which is a gas giant planet orbiting in a 3.3-Earth-year orbit around its star. It will be situated at a safe distance, while its two sister worlds become history.

The Kepler-56 planetary system is also famous for being the first "tilted" system sporting multiple planets to be spotted. The orbits of the duo of inner planet siblings are tipped signficantly from their stellar parent's equator. This proved to be a surprise, because planets are born from the same disk of gas and dust (protoplanetary accretion disk) as the star, so they should orbit in almost the same plane as the star's equator--as do the planets in our own Solar System.

The team was able to better determine the tilt of these planets, compared to previous studies. The astronomers discovered that the most likely tilt was either 37 or 131 degrees.

Dr. Li and her team also studied the inclination of the outer and much more fortunate planet and determined that its orbit is probably tilted relative to its star as well. Future observations should help curious astronomers characterize this interesting system, and eventually explain how it managed to become so skewed.

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.

No comments:

Post a Comment