Sky & Telescope - July 2022 - 40

Distance (a.u.)
PAINTINGS: LUCIEN RUDAUX / SUR LES AUTRES MONDES 1938; HABITABLE ZONE:
GREGG DINDERMAN / S&T, SOURCE: TOBIAS MÜLLER (INST. FOR ASTRONOMY &
ASTROPHYSICS, UNIVERSITY OF TÜBINGEN) / HTTP://ASTRO.TWAM.INFO/HZ-PTYPE
Strange Exoplanets
When you have a double star, both stars
shine on the planet. But all photons are not
created equal, Haghighipour explains. They
likely come from two distinct types of stars,
so they'll carry distinct energies. As a result,
the planet's atmosphere responds differently
to each star's photons. When the photons
hit the molecules in the atmosphere,
they produce different chemical reactions,
warming the planet in different ways.
Astronomers need to estimate how each
source will contribute to global temperatures
separately, then combine the effects.
For planets in S-type configurations,
the presence of a companion star widens
the habitable zone a little. Things are more
complicated in circumbinary systems,
where insolation changes dramatically over time and on
several time scales.
First, stellar eclipses can reduce insolation by a few percent
over a period of hours every few days. How much depends on
which star is hidden from view and on the stars' luminosities,
which in some cases can be very different from each other.
Second, the stars change position relative to each other over
the course of their mutual orbit, typically with a period of
days or weeks.
Third, there's the planet's own orbit. If the exoplanet follows
an elongated path around the stars, the insolation can
be many times higher when the planet is closer to the stars
than when it's farther away. This cycle can last from 50 to
1,000 days for known circumbinary planets. Last, there's precession,
which moves the planet around on a cycle that lasts
from tens to hundreds of years.
There's no doubt that the concept of seasons takes on a
completely different meaning for circumbinary planets. Irregular
seasons have implications for habitability, since any life
forms would need a great tolerance for climate swings. These
variations are not equally severe for all circumbinary planets,
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Distance (a.u.)
CHANGING SEASONS As the stars in the binary Kepler-16 orbit each other, the habitable zone around them changes shape. Due to that shift, the
planet's orbit moves in and out of the conservative habitable zone with time.
40 JULY 2022 * SKY & TELESCOPE
Optimistic
habitable
zone
Conservative
habitable
zone
SHADOWSCAPE French astronomer Lucien
Rudaux (1874-1947) illustrated the shadow effects
that might occur in the landscape of planets with
two suns in his comprehensive 1938 book Sur les
Autres Mondes (About Other Worlds).
though. Some planets, like Kepler-453b,
have nearly circular orbits and stay within
the habitable zone of their stars throughout
their year. Although life as we know it
couldn't get a foothold on these gas giants,
perhaps they have moons where organisms
could eke out a living.
Beyond Science Fiction
Over the past 100 years, several thinkers
and artists have envisioned what a planet
with two suns could look like. From George
Lucas's Tatooine to the ocean planet in Stanisław Lem's 1961
novel Solaris, science fiction has readily embraced them. They
also appear in paintings of landscapes where two suns cast
shadows of different colors, created nearly a century ago by
French astronomer Lucien Rudaux.
Now, such work has to contend with comparisons to real
worlds in our galaxy.
Astronomers' picture of these worlds is still in sketch
form. TESS and projects now in the works, like the European
Space Agency's PLATO mission, will ensure more discoveries
down the line. And if exoplanet discoveries have taught us
anything so far, it's that anywhere planets can form, they do.
" The closest stellar system to us is a multiple stellar system,
and it's known to have at least one planet, " Rafikov says.
" Half of the stars are in binaries. " We must therefore try to
understand what's happening around this half of the stellar
population, long ignored by many exoplanet hunters. Maybe
something there will be worth another Nobel Prize.
¢ JAVIER BARBUZANO is a freelance writer in circumbinary
orbit around his two little daughters, Clara and Sofia.
Planet
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Sky & Telescope - July 2022

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