Sky & Telescope - January 2023 - 15

As stars grow older, they and their planets can
affect each other in strange and violent ways.
efully
A
stronomers have confirmed the existence of more
than 5,000 exoplanets to date. The vast majority of
these - approximately 95% - orbit stars in an evolutionary
stage akin to that of our Sun, a stage known as the
main sequence. Main-sequence stars actively fuse hydrogen
into helium in their dense, hot cores. Stars spend the vast
majority of their nuclear-burning lives in this stage: In the
case of a Sun-like star, the main sequence comprises 90% of
the total stellar life cycle.
But what about the 5% of known exoplanets found around
aging stars? These stars bear exotic names like subgiants, red
giants, and white dwarfs, and they have evolved to have vastly
different sizes, temperatures, and luminosities than they did
when they were young. Any planets around these stars have
therefore experienced a dramatic change in circumstances,
which in many cases leads to their destruction.
With so few examples of these mature planetary systems
in hand, it's difficult for us to know how late-stage planetary
evolution unfolds. Nevertheless, the systems we do have serve
as critical benchmarks in our quest to trace the complete
planetary life cycle. These systems are teaching us that the
evolutionary process is in some ways a symbiotic one: Not
only do aging stars affect their planets, but the planets also
affect their stars.
Red and Puffy
A star's birth mass largely determines how its evolution
proceeds; there isn't a " one size fits all " pattern. But of stars
confirmed to have at least one exoplanet, the average mass
is essentially that of the Sun, and nearly all those host stars
(99%) weigh in at less than two solar masses. So the life cycle
10
12
2
4
6
8
2
4
Current orbit of COROT-7b
6
Star's size
8
10
Time (billions of years)
COROT-7B This illustration (left) shows the roasted surface of a
super-Earth that closely orbits the Sun-like star COROT-7. The star will
swell and engulf the planet's orbit in 13 billion years (above) - but the
star will likely eat the planet much sooner than that, depending on how
quickly tidal forces make the planet spiral closer. (Age is approximate.)
sk yand tele scope .o r g * JANUARY 2023 15
12
14
16
Size of COROT-7 (solar radii)
Current age
ARTWORK: ESO / L. CALÇADA, PLOT: GREGG
DINDERMAN / S&T, SOURCE: M. SOARES-FURTADO

Sky & Telescope - January 2023

Table of Contents for the Digital Edition of Sky & Telescope - January 2023

Contents
Sky & Telescope - January 2023 - Cover1
Sky & Telescope - January 2023 - Cover2
Sky & Telescope - January 2023 - 1
Sky & Telescope - January 2023 - Contents
Sky & Telescope - January 2023 - 3
Sky & Telescope - January 2023 - 4
Sky & Telescope - January 2023 - 5
Sky & Telescope - January 2023 - 6
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Sky & Telescope - January 2023 - Cover3
Sky & Telescope - January 2023 - Cover4
Sky & Telescope - January 2023 - SA1
Sky & Telescope - January 2023 - SA2
Sky & Telescope - January 2023 - SA3
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Sky & Telescope - January 2023 - SA9
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