http://www.skyandtelescope.com.au 27Red giantWhite dwarfPlanetary
nebulaBillions of years (approx.) Not to scale
8 9 10 11 12 13 14 15NASA / ESA / C. ROBERT O’DELL (VANDERBILT UNIVERSITY)
Sussex, UK). “Everybody knows it’s there, but nobody talks
about it.” Properly treating three-dimensional turbulent
convection in the Sun’s outer layers is critical to creating
viable models of stellar evolution, but it’s almost impossible at
the moment, Smith says. “Even fluid dynamicists can’t fully
describe convection in the laboratory. So to be able to get a
good model of it in stars is asking a very great deal. Nobody
has yet managed it.”Mercury, Venus and Mars
For the inner planets, how much mass the Sun loses — and
how fast it does so — is crucial, for it will determine their fate.
As our star’s bulk decreases, its gravitational tug on the planets
will ease, causing them to drift outward in their orbits.
Mercury won’t move away fast enough, and it will be
caught by the Sun, like a moth that lingers too close to
flame. As our star inflates to engulf the innermost planet,
Mercury’s iron-dominated density will ultimately count for
nothing. Over many orbits within the Sun’s outer layers, the
planet’s orbit will decay due to gas drag. “As it spirals into
the atmosphere, it will be under intense ablation from the
radiation field inside the Sun,” says Gregory Laughlin (Yale).
“Mercury will basically be in a radiation bath.” Eventually the
planet will dissolve in the unfathomable heat.
Venus, the next planet whose orbit the swelling Sun will
overtake, will share the same fate, most models suggest.
Again because of gas drag, Venus will sink farther and farther
into the Sun’s embrace, eventually becoming so hot that
it, too, will dissolve. As Mercury before it, Venus will have
reached its virial temperature. This is the tipping point beyond
which the energy of motion in an object’s molecules exceeds
its gravitational binding energy, and the body disintegrates.
For Earth, Kacper Rybicki (Polish Academy of Sciences) and
Carlos Denis (University of Liège, Belgium) calculated the
virial temperature to be about 300,000° Celsius. At that
temperature, all molecules comprising the planet begin
moving faster than its escape velocity, or, in Earth’s case,
faster than 11 km/sec. “If everything’s moving faster than 11
kilometres per second, that thing won’t hold together by its
gravity, and then you no longer have a planet,” says Adams.
“You just have a bunch of particles.”
What about Mars? Most models agree that it will escape
the Sun’s clutches. But while, about 6 billion years from now,it will receive as much warmth as Earth does today, the Red
Planet will remain a lifeless world. It’s so dessicated already,
it has no magnetic field, and its gravity is too low to hold a
substantial warm atmosphere, Laughlin says. In 6 billion
years, its internal geology will be even more inactive than it is
today. “Mars just isn’t a very alive world right now, and it will
be even more dead,” he says.The Blue Planet
Earth is the wild card: No one knows whether it will survive
physically or not. Our planet may be sucked into the Sun’s
atmosphere late in the AGB phase, or it may survive the
encounter. It depends, again, on exactly how much mass the
Sun will shed in these late stages.
Whether or not Earth pulls through in one piece, is kind
of a moot point: Our planet will be toast long before that.
Alas, the choice is between no Earth or Earth as “a charred
ember of its former self,” as Adams puts it. Over the next
billion years, the Sun’s luminosity will increase by about 10%.
Although modest compared to what’s coming during the red
giant phases, this jump will be a death knell for life on Earth.
That much extra energy pouring out from the Sun will
melt the polar ice caps and begin to evaporate the oceans.
Water vapour is a powerful greenhouse gas, and with soSGREAT EYE IN THE SKY The Ring Nebula, also known as Messier
57, possibly resembles the planetary nebula the Sun could spawn late in
its existence as it shrinks to a white dwarf.