In a couple billion years, our Sun will be unrecognizable. It will swell up and become a red giant, then shrink again and become a white dwarf. The inner planets aren\u2019t expected to survive all the mayhem these transitions unleash, but what will happen to them? What will happen to the outer planets?<\/p>\n
<\/p>\n Right now, our Sun is about 4.6 billion years old. It\u2019s firmly in the main sequence now, meaning it\u2019s going about its business fusing hydrogen into helium and releasing energy. But even though it\u2019s about 330,000 times more massive than the Earth, and nearly all of that mass is hydrogen fuel, it will eventually run out. <\/p>\n In another five billion years or so, its vast reservoir of hydrogen will suffer depletion. As it burns through its hydrogen, the Sun will lose mass. As it loses mass, its gravity weakens and can no longer counteract the outward force driven by fusion. A star is a balancing act between the outward expansion of fusion and the inward force of gravity. Eventually, the Sun\u2019s billions-of-years-long balancing act will totter.<\/p>\n With weakened gravity, the Sun will begin to expand and become a red giant. <\/p>\n The Sun will almost certainly consume Mercury and Venus when it becomes a red giant. It will expand and become about 256 times larger than it is now. The inner two planets are too close, and there\u2019s no way they can escape the swelling star. Earth\u2019s fate is less certain. It may be swallowed by the giant Sun, or it may not. But even if it isn\u2019t consumed, it will lose its oceans and atmosphere and become uninhabitable. <\/p>\n The Sun will be a red giant for about one billion years. After that, it will undergo a series of more rapid changes, shrinking and expanding again. But the mayhem doesn\u2019t end there. <\/p>\n The Sun will pulse and shed its outer layers before being reduced to a tiny remnant of what it once was: a white dwarf.<\/p>\n This will happen to the Sun, its ilk, and almost all stars that host planets. Even the long-lived red dwarfs (M-dwarfs) will eventually become white dwarfs, though their path is different. <\/p>\n Astronomers know the fate of planets too close to the stars undergoing these tumultuous changes. But what happens to planets further away? To their moons? To asteroids and comets?<\/p>\n New research published in The Monthly Notices of the Royal Astronomical Society digs into the issue. The title is \u201cLong-term variability in debris transiting white dwarfs,\u201d and the lead author is Dr. Amornrat Aungwerojwit of Naresuan University in Thailand. <\/p>\n \u201cPractically all known planet hosts will evolve eventually into white dwarfs, and large parts of the various components of their planetary systems\u2014planets, moons, asteroids, and comets\u2014will survive that metamorphosis,\u201d the authors write. <\/p>\n There\u2019s lots of observational evidence for this. Astronomers have detected planetary debris polluting the photospheres of white dwarfs, and they\u2019ve also found compact debris disks around white dwarfs. Those findings show that not everything survives the main sequence to red giant to white dwarf transition.<\/p>\n \u201cPrevious research had shown that when asteroids, moons and planets get close to white dwarfs, the huge gravity of these stars rips these small planetary bodies into smaller and smaller pieces,\u201d said lead author Aungwerojwit.<\/p>\n In this research, the authors observed three white dwarfs over the span of 17 years. They analyzed the changes in brightness that occurred. Each of the three stars behaved differently. <\/p>\n When planets orbit stars, their transits are orderly and predictable. Not so with debris. The fact that the three white dwarfs showed such disorderly transits means they\u2019re being orbited by debris. It also means the nature of that debris is changing. <\/p>\n \u201cThe unpredictable nature of these transits can drive astronomers crazy\u2014one minute they are there, the next they are gone.\u201d<\/p>\n Professor Boris Gaensicke, University of Warwick<\/cite><\/p><\/blockquote>\n<\/figure>\n As small bodies like asteroids and moons are torn into small pieces, they collide with one another until nothing\u2019s left but dust. The dust forms clouds and disks that orbit and rotate around the white dwarfs. <\/p>\n Professor Boris Gaensicke of the University of Warwick is one of the study\u2019s co-authors. \u201cThe simple fact that we can detect the debris of asteroids, maybe moons or even planets whizzing around a white dwarf every couple of hours is quite mind-blowing, but our study shows that the behaviour of these systems can evolve rapidly, in a matter of a few years,\u201d Gaensicke said. <\/p>\n \u201cWhile we think we are on the right path in our studies, the fate of these systems is far more complex than we could have ever imagined,\u201d added Gaensicke.<\/p>\n During the 17 years of observations, all three white dwarfs showed variability. <\/p>\n The first white dwarf (ZTF J0328?1219) was steady and stable until a major catastrophic event around 2011. \u201cThis might suggest that the system underwent a large collisional event around 2011, resulting in the production of large amounts of dust occulting the white dwarf, which has since then gradually dispersed, though leaving sufficient material to account for the ongoing transit activity, which implies continued dust production,\u201d the researchers explain.<\/p>\n The second white dwarf (ZTF J0923+4236) dimmed irregularly every couple of months and displayed chaotic variability on the timescale of minutes. \u201cThese long-term changes may be the result of the ongoing disruption of a planetesimal or the collision between multiple fragments, both leading to a temporarily increased dust production,\u201d the authors explain in their paper. <\/p>\n The third star (WD 1145+017) showed large variations in numbers, shapes and depths of transits in 2015. This activity \u201cconcurs with a large increase in transit activity, followed by a subsequent gradual re-brightening,\u201d the authors explain, adding that \u201cthe overall trends seen in the brightness of WD?1145+017 are linked to varying amounts of transit activity.\u201d<\/p>\n But now all those transits are gone. <\/p>\n \u201cThe unpredictable nature of these transits can drive astronomers crazy\u2014one minute they are there, the next they are gone,\u201d said Gaensicke. \u201cAnd this points to the chaotic environment they are in.\u201d<\/p>\n But astronomers have also found planetesimals, planets, and giant planets around white dwarfs, indicating that the stars\u2019 transitions from main sequence to red giant don\u2019t destroy everything. The dust and debris that astronomers see around these white dwarfs might come from asteroids or from moons pulled free from their giant planets. <\/p>\n \u201cFor the rest of the\u00a0Solar System, some of the asteroids located between Mars and Jupiter, and maybe some of the moons of Jupiter may get dislodged and travel close enough to the eventual white dwarf to undergo the shredding process we have investigated,\u201d said Professor Gaensicke. <\/p>\n When our Sun finally becomes a white dwarf, it will likely have debris around it. But the debris won\u2019t be from Earth. One way or another, the Sun will destroy Earth during its red giant phase. <\/p>\n \u201cWhether or not the Earth can just move out fast enough before the Sun can catch up and burn it is not clear, but [if it does] the Earth would [still] lose its atmosphere and ocean and not be a very nice place to live,\u201d explained Professor Gaensicke.<\/p>\n \n![\"An](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2019\/01\/interior_of_white_dwarf_v3-1024x768.jpg\")
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