- 4289 - LIFE ON EXOPLANETS? - The dawn of the era of extrasolar-planet (or exoplanet) discovery provides us with the first bits of hard information to use in reconsidering the likelihood of life elsewhere. The evidence so far does not alter the improbability that any Extra-terrestrial Intelligence exists near enough to us to matter.
------------------------- 4289 - LIFE ON EXOPLANETS??
- The first known
extrasolar planet, a planet orbiting around a normal star other than our Sun,
was “51 Pegasi b”, discovered 16 years ago. Today, there are more than 1,500
known extrasolar planets, about 431 with confirmed detections and reasonably
well- determined parameters (such as radius, mass and orbital characteristics).
-
- It is therefore
much more crucial for theology, philosophy, politics and popular opinion to
ponder how humanity understands itself if we might be effectively alone in the
universe, humanity being a species that is rare, precious and neither
irrelevant nor cosmically insignificant.
-
- Life, specifically
intelligent life, not just microbes, could be ubiquitous in a universe that is
as spacious and rich as ours; it is possibly teeming with Earthlike planets
hosting life. Perhaps intelligent beings are the inevitable product of life and
evolution.
-
- Today we know that
Mars has no artificial canals, and that this assertion was unsupported wishful
thinking. We anticipate that all
planetary systems will have a set of rocky inner planets, with atmospheres
produced by outgassing, weathering and escape, for the same reasons that our
own rocky inner planets have atmospheres.
-
- Judging from our
own example, the chances seem good that one of these inner planets will orbit
its star at the “right” distance. The
single most remarkable finding of the new research on extrasolar planets is
that an enormous variety of systems exist, a diverse range of often-bizarre
environments that is considerably broader than had usually been imagined before
the first one was discovered.
-
- More than 50
likely Earth-sized planets have been spotted so far, 2023. Earthlike planets,
with signs of liquid water and a congenial atmosphere, have as yet fallen below
the detection threshold, although in the next few years, with the added
patience it takes to measure a few of their yearly transits, it is reasonable
to think that some will be found.
-
- It may turn out
that our own solar system is average, but,
we know now that at least some planetary systems are unlike ours.
Meanwhile, the results make it possible to improve models of planet formation,
which in turn offer improved guidance on planets in general.
-
- First, only the
existence of intelligent beings is relevant. Primitive life may yet be
discovered on Mars; perhaps even multicellular animals will be found on a
nearby extrasolar planet.
-
- These
revolutionary discoveries would help us reconstruct how life on Earth evolved,
but unless a species is capable of conscious, independent thought and has the
ability to communicate, we will still be alone, with no one to teach or learn
from, no one to save us from ourselves (and no one to battle against).
-
- Intelligent life,
for the purposes of this discussion, means life able to communicate between
stars; this implies having something like radio technology. Our own society, by
this definition, is only about 100 years old.
-
- If intelligent
life is common in a universe that is 13.7 billion years old, then surely we are
among the youngest forms in existence.
The fact that there is no other known intelligent life indicates that
the assumption is wrong, intelligent life is not common.
-
- The second
important caveat derives from two features of the world that were unknown at
the time. The first is relativity, the
fastest any signal can travel is the finite speed of light. The second is the
expanding nature of the universe,
presumably the result of a “big bang” creation event.
-
- Distant galaxies
recede from us at an accelerating rate. Even if ETI is infinitesimally rare, in
an infinite universe, every physically possible scenario will exist. Stephen
Hawking and other physicists argue for the existence of “many universes,”
thereby inflating the notion of infinities and life forms.
-
- The universe is
getting bigger and expanding away from us. In fact, for purposes of
communication the limit is even stricter. The universe is not simply expanding,
it is accelerating outward. Light sent
from Earth today can never even catch up to galaxies whose light has taken
about 10 billion years to reach us.
-
- Even though they
are well within our cosmic horizon, such galaxies are forever beyond our reach
and receding quickly. Even if the universe lasts forever, any aliens there will
never enjoy our stray transmissions of I Love Lucy.
-
- The finite speed
of light also sets a practical limit on closer stars. Most stars in our Milky
Way galaxy, and presumably its billions of planets, are hundreds of thousands
of light-years away, so it will take hundreds of thousands of years for any ETI
there to see our signals, and that long again for us to receive a reply. To be
alone for all practical purposes means to be without any communication, or even
the knowledge that any signal is coming, for a very long time.
-
- How long before we
feel such solitude? 100 human
generations; subjectively this seems like practically forever. Because one
generation corresponds to 25 years (and at least one round-trip of messages is
necessary). This limits the following
estimates to stars closer to Earth than 1,250 light-years.
-
- We know a lot
about the stars in this neighborhood and so we can be quantitative. If we
choose to examine a smaller volume, say, that accessible within one lifetime,
the chances of success go down by a factor of a million, because the number of
stars is proportional to the volume of space and scales with time (distance)
cubed, but, we will have a yes-or-no answer sooner. However, if we expand the
search volume and the probabilities of success, the wait time goes up.
-
- One way to figure
the odds is to use the Drake Equation, a set of multiplicative factors tracking
the various phenomena thought to be necessary to get to intelligent life. It is
not a mathematical formulation of a physical process, and every researcher who
uses it breaks down the individual terms somewhat differently, but all estimate
the same thing, the number of civilizations around today.
-
- At its simplest,
the result is a product of five terms: the number of suitable stars, the number
of suitable planets around such a star, the probability of life developing on a
suitable planet, the probability that life evolves to be intelligent and the
typical lifetime of a civilization compared to the lifetime of its star.
-
- The Sun lies in a
cavity of interstellar gas, called the Local Bubble, which extends over roughly
600 light-years. It in turn is located in Gould’s Belt, a spur of stars, star
clusters and molecular clouds between two of the Milky Way galaxy’s spiral
arms, stretching from the Orion nebula to the Ophiucus-Scorpius clouds and on
to the Perseus clusters, a distance of about 1,200 light-years in its longest
dimension.
-
- The approximate
number of stars per cubic light-year here is 0.004, to within a factor of two,
or about 30 million stars of all types in a volume of radius 1,250 light-years.
This result provides a first factor in the Drake Equation considering the
distance limit that has been set.
-
- The first thousand
extrasolar planets discovered were the easiest to find in part because they are
either large or have orbits close enough to their stars that their multiple
transits in front of their stars can be observed, confirmed and studied in a
few years.
-
- In their
statistical review of 1,235 Kepler planetary candidates (planets not yet
completely confirmed) that orbit in less than 50 days. There has not, however, been quite enough
time to find Earthlike planets. Indeed, most of the stars studied have no
planets of any kind yet detected, but in a few more years we may know more
about them.
-
- The website for
Exoplanet Data Explorer at http://exoplanets.org regularly updates the
confirmed results.
-
- The “rare Earth”
hypothesis expresses the idea that Earthlike planets genuinely suitable for
intelligent life are few and far between. Paleontologist delineate a set of
familiar conditions that planets must satisfy for intelligence to prosper: stability, habitability and water, planetary
mass and planetary composition.
-
- To meet the
stability condition, the host star must be stable in size and radiative output
for the billions of years it takes for intelligence to evolve. Our Sun is among
the less-common types of stars. Over 90 percent of stars are smaller than the
Sun, many with less than one-tenth of the Sun’s mass.
-
- It may be hard for
a planet around a small star to evolve intelligent life because small stars are
cooler and their habitable zones, the range of distances where the temperatures
allow water to be liquid, lie closer to the star.
-
- When a planet is
in this closer region, it tends to become gravitationally (tidally) locked to
the star, with one side perpetually facing the star. (Tidal locking keeps one
face of the Moon pointing toward Earth.) But then half of the planet will be in
the dark and cold, and the other half at constant noon. Life seems improbable
in such a place, although some argue that life could develop in the in between
zones with intermediate conditions.
-
- At the other
extreme, stars more massive than the Sun are also probably unsuitable; bigger
stars burn hotter and live shorter lives. Stars with twice the mass of the Sun
exist in a stable, hydrogen- burning, or “main sequence,” phase for only a few
billion years, about 18 percent as long as the Sun’s lifetime, yet, billions of years more than this were needed
for evolution of intelligent life on Earth.
-
- Stars of more
than about eight times the Sun’s mass will die as supernovae after only tens of
millions of years. Fewer than about 10 percent of all stars are in a nominally
acceptable range of masses, from about 0.7 to 1.7 solar masses.
-
- A star’s age also
matters. Stars that are too young will not have had time for life to evolve;
older ones are a problem because a star’s luminosity increases with time (the
Sun will be 40 percent more luminous in another 3.5 billion years), and thus
the location of its habitable zone changes.
-
- Another concern is
that most stars have a companion star orbiting; about two-thirds of solar-type
stars are binaries. Their planets might orbit one star, or the other, or both,
but these situations raise a flag because the changing gravitational influence
of an orbiting companion star potentially could disrupt the long gestational
period of a planet in a habitable zone.
-
- The second
condition for intelligent life, habitability and water, further explores the
concept that a suitable planet must reside in the habitable zone of its star or
have some other mechanism to maintain liquid water. The orbit must be stable as
well, sufficiently circular or otherwise unchanging, so that it remains
suitable for billions of years.
-
- The single most
remarkable result from the discovery of extrasolar planets is their variety:
systems that have extreme elliptical orbits, giant planets orbiting very close
to their stars (called “hot Jupiters”) and other unexpected properties.
-
- It is important to
stress again that technology is only just now able to detect Earth-sized
planets. The presence of hot Jupiters in a system does not exclude the
existence of Earthlike planets farther away in the star’s habitable zone, it
just makes it more complicated.
-
- Planets are thought
to form far from a star by the gradual coalescence of dust grains in a
protoplanetary disk into larger and larger bodies. Once formed, these planets
generally tend to migrate into closer orbits as they interact with material in
the disk. As they migrate, such planets would presumably disrupt small bodies
that might have been in the habitable zone.
-
- Another factor is
the eccentricity of a planet’s orbit, a measure of the closest distance of the
planet to the star compared to its largest distance, which thus determines the
annual variations it receives in stellar illumination. Severe orbital
variations do not preclude liquid water but could inhibit the development of
biological systems.
-
- An eccentric
orbit also increases the likelihood that in a system of similar planets, one
occasionally might be chaotically disrupted. Earth’s orbit is nearly circular.
Of the 431 extrasolar planets currently known with confirmed and published
orbital parameters, only 11—2.2 percent—have eccentricity values less than that
of Earth; 20 percent vary in their stellar distances by a factor of two during
their year, and 50 percent vary in stellar distance by 20 percent.
-
- A related
parameter is the obliquity of a planet, the angle between its spin axis and the
axis of its orbit around its star. Earth’s obliquity, 23.5 degrees, is the
consequence of a massive collision it had with a giant object early in its
existence, which created the Moon.
-
- The approximate
stability of Earth’s obliquity is maintained by torque from the Moon. This
apparently ideal value of obliquity ensures that the climate on Earth’s surface
over the course of a year is neither too hot nor too cold, as first one pole
points slightly toward the Sun during Earth’s yearly orbit, and then the other
receives more daylight.
-
- Scientists have
estimated that if Earth’s obliquity were as high as 90 degrees, a substantial
part of its surface would become uninhabitable. No other planet in our own
solar system has such a stable, much less congenial, obliquity; that of Mars
seems to have varied chaotically between about 0 and 60 degrees (but is
currently about 25 degrees).
-
- Current models of
Earth-sized planet formation suggest that high obliquity angles should be
common, the result of collisions from all directions in early stages of
formation, although the subsequent evolution of obliquities is less well
understood. The rotation axes of stars can also be tilted with respect to the
orbital plane. Measurements of exoplanets suggest so far that stars hosting hot
Jupiters also have large obliquities, probably the result of strong gravitational
perturbations in these extreme systems. -
-
- Again, the current
set of observed extrasolar planets represents the tip of the iceberg; most
extrasolar planetary systems could be different. However, the explanations
advanced so far to explain these observed parameters are general; whatever
tends to produce highly elliptical orbits, for instance, is presumably at work,
in some fashion, in other, still-unknown systems.
-
- The third
condition is planetary mass. A suitable planet must be massive enough to hold
an atmosphere, but not so massive that plate tectonics are inhibited, because
that would reduce geological processing and its crucial consequences for life.
-
- Current estimates
are that planets smaller than about 0.4 Earth masses are unsuited for long-term
atmospheres; if a planet is bigger than about 4 Earth masses, assuming it is
rocky, then planetologists estimate it will be unable to produce the plate
tectonics thought necessary to refresh the atmosphere with volcanoes or other
processes associated with the carbon cycle.
-
- The frequency
with which such planets occur is still not known, but the first indications
from the Kepler satellite are that they are abundant: For planets in close
orbits, about 13 percent are Earth-sized, and there is evidence that in larger
orbits their numbers would be greater, especially in systems around smaller
stars.
-
- Then there is
planetary composition. A suitable planet obviously must contain the elements
needed for complex molecules (carbon, for example), but it also needs elements
that are perhaps not necessary for making life itself but that are essential
for an environment that can host intelligent life: silicon and iron, for
example, to enable plate tectonics, and a magnetic field to shield the planet’s
surface from lethal charged winds from its star. The core of Earth remains
liquid because of the presence of radioactive elements, whose heat keeps the
iron molten and energizes Earth’s internal temperature structure.
-
- The relative
abundance of the elements is not uniform throughout the galaxy, however,
meaning that some regions may be incapable of hosting intelligent life. The
need for radioactive elements means that a supernova, the primary source of
radioactive elements, must have exploded in the vicinity of a suitable planet
relatively recently (but not so close or so recently as to be dangerous).
-
- The Local Bubble
in which the Sun resides is thought to have been caused by supernovae
explosions, and so it might be the case that some key elements present in the
Sun and Earth are deficient in other regions of our 1,250 light-year zone.
-
- Finally, many
planets may exist in open space, having been forcibly ejected from their
stellar systems by multibody gravitational interactions. It is a stretch to
imagine life on them developing and evolving to intelligence without the
radiant energy of a stable star.
-
- Even under ideal
conditions life might not develop easily. The fact that life has not yet been
created in the laboratory means that it is not trivially generated. Mars lies
approximately in the habitable zone, but it hosts no civilizations. Even if
life were the inevitable outcome of chemical processes on every planet with
liquid water, there is no evidence that such fertility occurs quickly.
-
- On Earth life took
about a billion years to form and another few billion years to produce us. If
sometimes the chemistry runs slower by a factor of two or three, or evolution
is sidetracked, it may be too late: A Sunlike star would have swollen in size
to fill the orbit of Earth.
–
- Believers in ETI
sometimes counter that some life might evolve from noncarbon-based forms much
more efficiently. Besides the absence of any plausible evidence for such a
possibility, most agree that at a minimum intelligence requires complexity.
Even if their brains are not made with DNA, such aliens must evolve over
timescales long enough for comparably complex organs to mature.
-
- The development of
intelligent life appears to require more than just planetary suitability.
Consider the unlikely accidents, perhaps essential? perhaps incidental?, that
facilitated humanity’s evolution. A gigantic collision early in Earth’s history
created the Moon, knocking Earth’s axis over enough to make the obliquity and
salutary seasons we enjoy, but not quite enough to shatter Earth entirely.
-
- Meanwhile the Moon
that was produced generates Earth’s tides and stabilizes Earth’s wobble. A few
billion years later, the dinosaurs, which had successfully dominated the planet
for 100 million years, were fortuitously wiped out by another, smaller asteroid
so powerful it destroyed them all, yet did not kill off the mammals. -
-
- Many scientists
have noted that there were roughly 15 mass extinctions, six of them
catastrophic, on Earth before humans emerged on the scene, underscoring the
complex, tumultuous, and perilous history of our evolution.
-
- Many other
contingent conditions on Earth enabled life to thrive. Water is essential, but
if Earth had much more water in its oceans, there would be no landmass for fish
to crawl onto and evolve into toolmakers.
-
- Our evolution was
so random that it could probably never repeat; British paleontologist Simon
Conway Morris argues that convergence of life on Earth toward humanity was
inevitable, but only because of our extraordinarily perfect environment. -
-
- The main
uncertainly in Drake’s original formulation is the longevity of an intelligent
civilization, because if it typically survives for only a short time (recall
that our own radio-based civilization is only about 100 years old), then very
few must be around now.
-
- Drake now guesses
that “only about 1 in 10 million stars has a detectable civilization,” so in
our 100-generation volume of space comprising 30 million stars, there might be
two others.
-
- What are your
estimates? There will be no civilization if a star is too large or too small,
if a planet’s orbit or obliquity is wrong, if its size or chemical composition
is unsuited, if its surface is ill equipped, if its geologic and meteoritic
history is too inauspicious, if the powerful chemistry needed to generate the
first life forms is too intricate or too slow, if evolution from proteins to
intelligence is too often aborted or directed into sterile tangents, or if
civilizations die off easily.
-
- If we are to have
company in our volume of the galaxy, the likelihood on average for each of
these conditions has to be pretty high, better than 20 percent. If the
probability of some, such as the chances for life to form, evolve or survive,
is much smaller, then even if the others are 100 percent certain, it is
unlikely there are any stars near us hosting intelligent beings.
-
- The anthropic
principle is the name given to the observation that the physical constants in
the cosmos are remarkably finely tuned, making it a perfect place to host
intelligent life. Physicists offer a “many-worlds” explanation of how and why
this might be the case.
-
- A misanthropic
principle could also be applicable.
This term is to express the idea that the possible environments and
biological opportunities in this apposite cosmos are so vast, varied and
uncooperative (or hostile), either always or at some time during the roughly
3-to-4 billion years intelligent life requires to emerge, that it is unlikely
for intelligence to form, thrive and survive easily.
-
- To recognize this
conclusion is to have a renewed appreciation for our good fortune, and to
acknowledge that life on Earth is precious and deserves supreme respect. Even
if we are not unique in the universe, though we may not know one way or the
other for eons, we are fortunate.
-
- An awareness of
our rare capabilities can spur deeper humility and an acknowledgment of a
responsibility to act with compassion toward people and our fragile
environment. The ongoing discovery of
amazing new worlds, including Earthlike cousins, will refine our understanding
of and perspective on our planet and its salutary environment.
-
-
December 27, 2023
LIFE
ON EXOPLANETS? 4289
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