Sky & Telescope - January 2023 - 17

How much time does
Earth have left in the
habitable zone?
Sun-like stars will spend more than
Star's lifetime
(billion years)
10-15
10 billion years fusing hydrogen in
their cores, but that doesn't mean
Earth-like planets have 10 billion
years of clement conditions: The
liquid-water zone moves outward
through the planetary system as the
star ages. Earth has only a couple
billion years left in the Sun's habitable
zone, and it might even start to lose
its atmosphere 1 billion years from
now, as the planet's core cools and
our global magnetic field weakens.
Total time Earth will spend
in the habitable zone
(billion years)
6-8
Earth
Sun
Earth and the Sun formed
(billion years ago)
Time Earth has left in
the habitable zone
(billion years)
4.6
much we astronomers still have to learn about this important
stage of star-planet evolution, especially regarding the effects
on the star itself. Although a planet is tiny compared with its
star (Jupiter contains 0.1% as much mass as the Sun does), it
could still affect its host in observable ways.
For example, stars may occasionally swallow a world more
or less whole. How deep will the planet penetrate the star
before it fully falls apart? Does this depth change the details
of the star's fate? Might dense companions avoid disassociation
altogether, surviving inside the bloated star? Preliminary
work suggests that in rare cases, the planet might go so deep
that it passes into the star's inner layers and triggers the ejection
of the star's outer envelope of gas, which we'd see as a
boost in infrared light from the aging star.
The decay of the planet's orbit could also affect the star.
The planet's angular momentum has to go somewhere,
because the laws of nature require that the angular momen3.
Asymptotic Giant Branch: 12.7 billion years
Sun shrinks and expands again, shedding half its mass.
Sun shrinks and expands again, shedding half its mass.
3. Asymptotic Giant Branch: 12.7 billion years
1.75-3.25
tum be conserved. A likely recipient is the star's spin. Calculations
suggest that a Jupiter-mass world could increase a
Sun-like star's spin by 60%. Even a Neptune-size planet could
spin up its star by 10%.
One effect we already have evidence for is on the star's
surface chemistry. Theory shows that an engulfed planetary
body leaves long-standing, telltale chemical fingerprints on
the star's surface - even in cases in which the companion is
not fully disassociated. We can only detect such contamination
signatures if we have a good idea of the host star's original
composition. Luckily, nature is generous, providing environments
like wide-orbiting binary stars, co-moving groups,
and stellar clusters where such constraints are possible.
One particularly useful engulfment tracer is the lithium-7
isotope, which, on average, is 60 times more abundant
(per unit mass) in planets than in stars. Astronomers have
detected strong signatures of lithium enrichment in some
4. White Dwarf: >12.7 billion years
4. White Dwarf: >12.7 billion years
Sun becomes a white dwarf. After 2 billion years, its cooling rate slows,
enabling a long-lived habitable zone.
Sun becomes a white dwarf. After 2 billion years, its cooling rate slows,
enabling a long-lived habitable zone.
The habitable zone, which had shrunk down to cover Jupiter and
Saturn's orbits, expands again out to the Kuiper Belt.
The habitable zone, which had shrunk down to cover Jupiter and
Saturn's orbits, expands again out to the Kuiper Belt.
The white dwarf has a surface temperature similar to the current Sun,
but it's about 1/10,000 as luminous. The habitable zone lies roughly
0.01 a.u. from the star. Over many billions of years, it slowly shrinks.
The white dwarf has a surface temperature similar to the current Sun,
but it's about 1/10,000 as luminous. The habitable zone lies roughly
0.01 a.u. from the star. Over many billions of years, it slowly shrinks.
sk yand tele scope .o r g * JANUARY 2023 17
AGING HABITABLE ZONE: TERRI DUBÉ / S&T
Sky & Telescope - January 2023

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