Saturday, October 24, 2015

A Planet In The Throes Of Destruction: A Glimpse Into the Future of Earth

For the first time in astronomical history earthlings are getting a glimpse of what it will be like when their own home planet is destroyed by its star, the Sun, some five billion years in the future. Thanks to the planet- hunting Kepler  Space Telescope we now can put together the process of a Sun-like star's end point - accompanied by the destruction of a planet orbiting it at a distance of 570 light years - in the constellation Virgo.

The star, having reached the end of its evolution and life is now in the process of annihilating its own solar system, with one of the Earth-like planets being vaporized by the star's searing heat and ripped apart by its gravity (as depicted in the artist's conception shown above).  According to the lead author of the study, Andrew Vanderburg, of the Harvard-Smithsonian Center for Astrophysics:

"Every second it's losing up to 10 million kilograms or 22 million pounds of material."

Vanderburg and his colleagues - according to an HSCA release -   also made additional observations using a number of ground-based facilities, including : the 1.2-meter and MINERVA telescopes at Whipple Observatory, the MMT, MEarth-South, and Keck telescopes. Combining all the data, they found signs of several additional chunks of material, all in orbits between 4.5 and 5 hours - generating a comet-like debris field. The main transit was particularly prominent, dimming the star by 40 percent. This disclosed the remnant of the Earth like planet.

The dimming transit signal combined with the comet-like pattern suggests the presence of an extended cloud of dust surrounding  the fragment. The total amount of material is estimated to be about the mass of Ceres, the largest main-belt asteroid in our solar system. It is conceivable that this is the reduced mass that's been left after the white dwarf  rent most of the original mass asunder.  (See below.)

According to Carole Mundell, head of astrophysics at the University of Bath, England:

"It's a glimpse into the future of Earth. It reinforces the idea that we are in a much more hostile environment than we sometimes imagine."

The stable lifetime of the Sun depends on how long before it consumes ninety percent of the hydrogen in its core. Theoretical investigations using data from nuclear reaction rates and cross sections suggest the Sun’s Main Sequence lifetime at 8-10 billion years. Since it already has spent 4.5 billion of those years, there are anywhere from 3.5 to 5.5 billion years remaining. Most astrophysicists put the balance at from 4.8 to 5 billion years.

When only ten percent hydrogen remains, the Sun is no longer able to generate sufficient energy from its core nuclear reactions to balance the weight of overlying layers. According to a well-known physical principle (the virial theorem), the Sun’s core must contract. The contraction converts gravitational potential energy into thermal (heat) energy that heats the core.[1] By now, hydrogen burning has moved to a peripheral shell around the core, and is ignited by the core heating process. The ignition creates radiation pressure that forces the outer shells, layers to expand. This same radiation, however, is now emitted from a much larger surface area. The result of this combination of circumstances is that the Sun becomes a Red Giant. This translates into the Sun increasing its diameter to up to 200 times its present size.

Afterwards, it collapses to a much smaller, but vastly denser object known as a white dwarf. Even if the Earth were to survive the red giant phase (humans, of course, will be long since dead - baked to a crisp possibly a billion years earlier) the powerful gravimetric forces of the white dwarf will claim it and other planets.  This is possible because the gravitational attraction of a typical white dwarf is 350,000 times that of Earth's surface gravity. Thus, if we take Earth's g -value at 10 N/kg (or m/s/s) the typical white dwarf's g-value would be 3,500, 000 N/kg. No ordinary mass would be able to withstand such a gravitational pull.

Thus, the extreme gravity of the dwarf will likely cause the nearer planets to fall toward the white dwarf and disintegrate under enormous gravitational tidal forces. This seems to be the scenario now unfolding in the constellation Virgo and which the artist's conception portrays.

We should count ourselves lucky that we are able to merely observe the death throes of a planet like ours from a great distance. But we should know one day the same cataclysm will befall our own home, Earth.

[1] According to the virial theorem: 2K + W = 0 for any spherical system in equilibrium, where K is the gas kinetic energy (K = 3/2(y-1)U) and W is the gravitational potential energy. From this one can obtain the binding (or total) energy of a star as: E(S)= K + W. Combining the two equations, E(S) = W/ 2 = -K. Thus, the total energy of the star is negative and equal to half the gravitational potential energy, or the negative of the gas kinetic. Hence, if E(S) decreases, K increases, but W decreases, i.e. contraction.

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