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Planets in Binary Systems

jjswaim schrieb/wrote/a écrit:

> I recall reading that the current view is that 'Life' could not exist
> on planets in a multiple star system, only in a single star system.


Hello Julia and List,

BOSS A.P. (1999) The Birth of Binary Stars (Sky & Telescope, June 1999,
pp. 32-38):

Planets in Binary Systems, p. 38: Forging multiple suns is one thing,
but what about planets? Most theoretical work on planet formation has
been done in the context of explaining our own solar system, a
single-star scenario. Very little is known about how planets would arise
in a binary system. But they can and do occur, because about a third of
all extrasolar planets found to date reside in binary systems. Evidently
binary stars are quite hospitable to the formation of giant planets. The
creation of planets probably doesn't occur during a cloud's collapse to
protostars but rather somewhat later.
Naturally, theorists have wondered for decades what life on a world with
two suns would be like. (Science-fiction legends Isaac Asimov and Robert
Silverberg took this multiple-star notion to the extreme - a planet in a
six-sun system - in their classic, Nightfall.) We do know which
planetary orbits would be stable, however, so we can rule out certain
orbital configurations, such as those with radii comparable to the
distance between the component stars.
New computer simulations by Matthew J. Holman (Harvard-Smithsonian
Center for Astrophysics) and Paul A. Wiegert (York University) have
refined where planets should and should not be found in a binary system.
As they report in the January 1999 issue of the Astronomical Journal,
planets remain in long-term stable orbits if they either occupy an orbit
close to one member of the binary or reside a great distance beyond the
coupled suns (effectively orbiting their center of mass). The exact
zones of orbital stability are defined by what Holman and Wiegert term a
critical semimajor axis, which typically is 10 to 30 percent of the
distance between the binary's stars. This value is relatively unaffected
by the ratio of the stars' masses, but it varies dramatically if the
stars orbit one another in circular or highly eccentric orbits.
As an aside, Holman and Wiegert ask, "If our own solar system had a
solar-mass companion in an eccentric orbit, how large would its
semimajor axis need to be for the Sun's planets (excluding Pluto) to
survive?" The answer, they find, depends on the companion's orbital
inclination. A second sun orbiting near the ecliptic would have to
remain at least 400 a.u. away on average. For an over-the-top orbit
inclined 75° or more, the standoff distance is more like 1,000 a.u.
Even though it's now relatively easy to simulate what can happen in a
multiple-star system once it forms, our models have a way to go before
we'll fully understand how planets get into a binary bind in the first
place. But we theorists are not disheartened. To the contrary, we are
ready to take up the challenge. Our exploration of the "binary universe"
is just getting under way.


Best wishes,

Bernd

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