Transcript What happened to Pluto?

What happened to Pluto?
Jeremy P. Carlo
Columbia University
August 2006 IAU decision:
• (Short version) Pluto is no longer a planet.
• Public Reaction:
Not good...
• What happened?!
August 2006 IAU decision:
• (Long version)
• We’re going to go back and look at the
basics, to try and understand the rationale
behind the decision.
What is a planet?
• To start let’s consider this simple question.
• We’ll look at the question as a function of
history, and how it has been answered in the
past.
The Ancients
(before ~1500 AD)
• The Ancients had no telescopes
• Planets: Objects which
(all naked-eye observing).
wandered across the sky.
• They noted that the stars remained • Identified with gods.
“fixed” relative to each other,
• The Seven Ancient Planets:
though they all went around the
– The Moon
sky every ~24 hours...
– The Sun
• ...except for exactly seven, the
“wanderers” (planets).
– Mercury
• (There were also random objects
– Venus
like comets and novae, but that’s
– Mars
another story for another talk...)
– Jupiter
– Saturn
The Ancient Solar System
• Geocentric - Earth at center
(pretty reasonable to assume)
• The seven “planets” orbit the
earth in circles, surrounded by
the “fixed stars.”
• Appeared to explain the
observed motion of the
planets... fairly well.
• But Mercury and Venus
followed strange paths in the
sky, and the Sun and Moon just
looked different from the
others.
The first modern astronomers
(~1500-1600)
•
•
•
•
•
By the 1500’s, Tycho Brahe’s
naked-eye observations made it
clear the old model wasn’t working
too well.
Copernicus proposes a Suncentered (heliocentric) model.
Still the same seven “planets” (plus
the Earth), only rearranged.
Earth is the third planet, with the
Moon going around it. Everything
else orbits the Sun.
Orbits not circles, but ellipses
(Kepler’s modification to better fit
Tycho’s data)
The Solar System ~1600
• Sun
1. Mercury
2. Venus
3. Earth
Moon
4. Mars
5. Jupiter
6. Saturn
Dist. from Sun (AU)
0.3
0.7
1.0
1.5
5
10
• Planets - the six known
objects that orbit the sun.
• If it goes around a planet,
then it’s a moon.
• Officially six planets plus
one moon, all known since
ancient times.
• Planets move in elliptical
orbits around the sun
according to Kepler’s Laws.
The Known
Planets,
1600
Mercury
The Known
Planets,
1600
Venus
The Known
Planets,
1600
Earth and Moon
The Known
Planets,
1600
Mars
The Known
Planets,
1600
Jupiter
The Known
Planets,
1600
Saturn
The First Telescopes
(1610-1700)
• Telescope invented by Hans
Lippershey in 1607, mostly for
military use.
• Galileo was the first to observe
the sky with a telescope.
• Craters on the moon
• Sunspots
He later became blind...
• Phases of Venus
The planets show “discs” like the
Sun and Moon
• Four moons around Jupiter!
Io, Europa, Ganymede Callisto
Galileo
Galilei
First discovery of solar system bodies
besides the seven known to ancients.
First objects to orbit something other
than the Sun or the Earth.
Orbited Jupiter in agreement with
Kepler’s Laws!
The Solar System ~1700
• Sun
Dist. from Sun (AU)
1. Mercury
2. Venus
3. Earth
Moon
4. Mars
5. Jupiter
+4 moons
6. Saturn
+5 moons
0.3
0.7
1.0
1.5
5
10
• Still six planets.
• Galileo discovers 4 moons of
Jupiter in 1610 - the first
objects found to orbit
something other than the earth
or sun.
• In 1655 Christiaan Huygens
discovers rings around Saturn
as well as its largest moon,
Titan
• In the 1670’s and 1680’s, four
more moons around Saturn are
found - Tethys, Dione, Rhea
and Iapetus.
Io
Europa
Ganymede
Callisto
Jupiter’s Moons (to scale)
Saturn and its largest
moons (not to scale)
The 1700’s...
A dry spell and then a shocking discovery!
• After about 1690, there were no
new solar system discoveries
(other than comets, etc.)
• But all telescopes up to this
point were rather small, until
William Hershel came around.
He liked to think BIG...
Herschel’s
“40-foot”
telescope
• Many discoveries - comets,
nebulas, star clusters...
• But in 1781 he found an object
that moved in the sky, a new
“wanderer...”
– Kepler’s laws placed it beyond
the edge of the solar system,
twice as far out as Saturn
– A new planet! Uranus...
– For the first time in recorded
history!
• And in 1787, two moons were
found orbiting Uranus - Titania
and Oberon.
Uranus
First planet discovered since ancient times (1781)
The Solar System 1800
• Sun
Dist. from Sun (AU)
1. Mercury
2. Venus
3. Earth
Moon
4. Mars
5. Jupiter
+4 moons
6. Saturn
0.3
0.7
1.0
1.5
5
10
+7 moons
7. Uranus
+2 moons
20
• The family is growing.
Seven planets!
• Two new moons around
Saturn - Mimas & Enceladus
• The solar system is doubled
in size, very nearly
overnight!
• So a new planet was
discovered. Can we perhaps
find others? We’re about to
find out...
1801: Deja vu all over again?
• In 1801 Giuseppe Piazzi finds
yet another “wanderer.”
• This time Kepler’s Laws place
it between Mars and Jupiter
• Ok, not as exciting as finding
something beyond the known
edge of the solar system, but
we’ll take what we can get.
• Two problems:
– Ceres is rather dim. Really dim
for how close it is, actually.
– It doesn’t show a “disc” like all
the other planets, but appears
starlike, “asteroidal” at all
achievable magnifications
• This new find, named Ceres,
must be really small.
• But an even bigger problem
soon became apparent: Three
more similar objects were found
over the next six years: Pallas,
Juno and Vesta.
• One small planet, maybe, but
four?
• Luckily, no more “asteroids”
followed, at least for a while...
The Solar System 1810
• Sun
Dist. from Sun (AU)
1. Mercury
2. Venus
3. Earth
Moon
4. Mars
5-8. Ceres, Pallas,
0.3
0.7
1.0
1.5
2-3
Juno & Vesta
9. Jupiter
+4 moons
10. Saturn
+7 moons
11. Uranus
+4 moons
5
10
20
• Getting kind of crowded eleven planets?
• Of course this all hinges on
counting the four “asteroids” as
planets, despite their small size.
• It all came crashing down in
1845 when a fifth “asteroid”
(Astraea) was found, and more
soon followed.
• These “asteroids” aren’t really
planets at all, but instead form a
class of different objects.
The Solar System 1845
• Sun
Dist. from Sun (AU)
1. Mercury
2. Venus
3. Earth
Moon
4. Mars
The Asteroid Belt
5. Jupiter
+4 moons
6. Saturn
+7 moons
7. Uranus
+4 moons
0.3
0.7
1.0
1.5
2-3
5
10
20
• Back to seven planets. Whew!
• But we must set some limit too small, you’re not a planet.
• But how small is too small?
• Perhaps this “asteroid belt”
consists of objects that failed to
completely coalesce into a fullblown planet?
• Meanwhile, work continued
toward discovering more
planets, and a surprise was in
immediate store...
1846: The year of the mathematicians
• Up to now, every discovery was
made by accident - somebody
looking in the right place at the
right time.
• The 1801 discovery of Ceres
proved to be disappointing.
• But some scientists noted that
there were anomalies in the
orbit of Uranus (which by this
time had completed nearly one
orbit)
• Could there be an eighth planet
causing these perturbations?
• John C. Adams in England and
Urbain Leverrier in France,
both independently proposed
the existence of a planet beyond
Uranus.
• Johann Galle of Germany
looked at the predicted location
in 1846, and sure enough a new
planet was there!
• International collaboration!
• Neptune, the first object to be
discovered “on purpose,” in a
triumph for the predictive
power of science.
Neptune
First planet found “on purpose” (1846)
The Solar System 1846
• Sun
Dist. from Sun (AU)
1. Mercury
2. Venus
3. Earth
Moon
4. Mars
The Asteroid Belt
5. Jupiter
+4 moons
6. Saturn
+7 moons
7. Uranus
+4 moons
8. Neptune
+1 moon
0.3
0.7
1.0
1.5
2-3
5
10
20
30
• Eight planets, and this time for
real!
• Of course, two weeks after
Neptune was discovered, its
largest moon Triton was also
discovered.
• In the same spirit, by the late
1800’s it appeared there were
further orbital perturbations as
well...
• Can we do it again?
Early 1900’s: The Search for
Planet X
• It became apparent that there
were further perturbations in
Uranus’ orbit. Could there be
another planet, beyond
Neptune? Percival Lowell
thought so, and initiated a
search for “Planet X.”
• Although Lowell’s search
failed, a far more
comprehensive search was
taken up by Clyde Tombaugh.
Clyde Tombaugh in
1930
Paydirt!
• In January 1930, Tombaugh
found what he was looking for.
Above: Tombaugh and blink comparator
Right: Discovery images of Pluto (arrow),
January 23 and 29, 1930
The Solar System 1930
•
Sun
1. Mercury
2. Venus
3. Earth
Dist. from Sun (AU)
0.3
0.7
1.0
+1 moon
4. Mars
1.5
+2 moons
The Asteroid Belt
5. Jupiter
2-3
5
+9 moons
6. Saturn
10
+9 moons
7. Uranus
20
+4 moons
8. Neptune
30
+1 moon
9. Pluto
36
• Nine Planets! (Finally!)
• But Pluto clearly was smaller
than Uranus and Neptune.
• Originally it was guessed it was
about the size of the Earth.
• But even that figure proved too
optimistic as new data came in.
Pluto
Planet X at last? (1930)
The distinctions get blurred
• Up to now everything has been tidy, neat
and orderly.
• Clear demarcations between planets,
moons, asteroids & comets.
• But the distinctions were about to get
blurry...
Just how small is Pluto?
•
•
•
•
•
Originally Pluto was thought to be
about the size of the earth.
But Pluto’s mass was continually
lowered from its discovery in 1930
until the 1970’s.
The last straw came when James
Christy discovered Pluto’s moon
Charon in 1978, enabling an exact
mass determination:
Pluto = 1/500 Earth mass
Not only was Pluto (by far) the
smallest planet, it’s also smaller
than at least seven known moons!
But still bigger than the asteroids...
Pluto and Charon
And it isn’t alone either...
•
•
•
•
In 1992, a new trans-Neptunian
object (TNO) was found, 1992
QB1.
1992 QB1 was smaller than Pluto,
but still rather large, and
presumably similar to Pluto in
origin and composition.
It was believed that Pluto and 1992
QB1 were the prototypes of a new
class of objects filling the transNeptunian Kuiper Belt.
Indeed, 1992 QB1 was merely the
first of many TNOs to be
discovered, although Pluto
remained the largest.
• Attractive suggestion: Pluto is
not a planet, but merely the
largest of the Trans-Neptunian
Objects (TNOs), as Ceres is the
largest of the asteroids.
• This creates a new class of
objects, the TNOs, analogous to
the asteroids.
• But Pluto loses its long-held
planetary status...
Very unpopular with the public!
My Very Educated
Mother Just Served Us
Nine Pizzas
My Very Educated
Mother Just Served Us
NOTHING!
What happened to Pluto!!!
My Very Educated
Mother Just Served Us
NOTHING!
What happened to Pluto!!!
This proposal proved very unpopular, and
didn’t get far...
Pluto and the largest TNOs, 2003
• Pluto’s still the biggest TNO
(but the gap is closing...)
• (artist’s conceptions of Sedna and Quaoar)
Enter “Xena” (2003 UB313)
• Discovered by Michael Brown of Caltech in 2003, but not
realized until 2005. Named 2003 UB313, unoficially
nicknamed “Xena.” (X: Tenth Planet?)
• Subsequently found that it’s at least as big as Pluto!
• And it has its own moon, “Gabrielle.”
• Clearly, if Pluto qualifies as a planet, then so does “Xena.”
• But there could be hundreds of other objects bigger than
“Xena.”
• Do all those get to be planets too?
• What do we do?
A matter of size...
The Nine Planets, to scale
Pluto’s the smallest (by far!)
Pluto vs. the largest moons
even the moons are bigger!
Pluto vs. “Xena”, 2005
strike three for old Pluto?
(artist’s conceptions)
What do we do?
• Pluto is the smallest planet (by a wide margin)
• It’s also smaller than at least six moons.
• And it’s not even the largest trans-Neptunian object
anymore!
• And there could be hundreds, maybe thousands, more
of those!
• Dilemma:
– Either remove Pluto from the list of planets, or
– “Xena” and every one of the hundreds of other large
TNOs get to be planets too.
Toward a scientific definition
• So far we’ve gotten by on a wing and a
prayer, without a formal definition of what a
planet was.
• It was obvious what a “planet” was, for the
most part.
• But now we need a scientific definition.
• The IAU convened in Prague in August
2006 to tackle this topic.
Setting a minimum size
• The simplest proposal is to set a minimum
diameter for planethood.
• But how much? 1000 miles? Kilometers?
Fathoms? Cubits? Or 500, 750, 2000, 200, 5000?
• Getting astronomers to agree is like herding cats...
• ..especially upon such an arbitrary definition.
• Does Nature provide a “yardstick?”
• It turns out She does...
It’s all about the shape
• One simple idea:
– An object large enough to form into a sphere under
gravity, is a planet.
• This is actually a lot more significant than it sounds.
• Sphericity implies that the object is dominated by
gravity, rather than microscopic intermolecular
forces.
• It is also associated with stratification of the interior,
which leads to the possibility of geological activity.
Roundness
• Roundness also makes it more likely a
planet will hold an atmosphere, which is
necessary for liquid water... which is
necessary for life and for all sorts of
interesting chemistry.
• So roundness is really a significant
characteristic!
Who’s round?
• All nine planets are round.
• Ceres (the largest asteroid) is round. But
Vesta (the second largest) is significantly
elliptical. Presumably the rest of the
asteroids are elliptical as well.
• Many moons are also round.
• Although we haven’t seen them up close, a
large number of KBOs are almost certainly
round as well.
433 Eros
~30 km across
253 Mathilde
~60 km across
951 Gaspra
~10 km across
Typical small asteroids
with irregular shapes
A Typical Comet
• Halley’s Comet
• ~16 km largest
dimension
• Also irregularly
shaped!
• The largest moons (plus 2 planets)
• All round!
• How do intermediately-sized objects look?
• Asteroid 4179 Toutatis
• ~4 km across
• Clearly not round!
• Asteroid 243 Ida (& Dactyl)
• ~30 km across
• Clearly not round!
• Jupiter’s moon Amalthea
• ~170 km across
• Clearly not round!
• Saturn’s moon Hyperion
• ~280 km across
• Still not round!
• Neptune’s moon Proteus
• ~400 km diameter
• Getting there, but not quite.
• Saturn’s moon Mimas
• ~400 km across
• Round!
• Uranus’ moon Miranda
• ~470 km across
• Round!
• Saturn’s moon Enceladus
• ~500 km across
• Round!
• Saturn’s Moon Tethys
• ~1000 km across
• Round!
What about the large asteroids?
• Ceres - the largest
asteroid.
• Vesta - the secondlargest asteroid.
• 950 km diameter
• Round!
• 600 km largest dimension
• Not round, but somewhat
elliptical.
How big is big enough?
• While there is some dependence on
composition, it appears the minimum size
for roundness is somewhere near 500 km.
• In particular, rocky objects probably have to
be a little bigger to become round than do
icy objects.
• 400-600km covers the transition region.
And the TNOs?
• Unfortunately we have no good photographs of any TNOs
- the best we have are the ground-based and Hubble
images of Pluto.
• Diameters are generally based upon visual magnitude and
assumed surface reflectivity (albedo).
• It appears at least one large TNO, 2003 EL61, is not round
(likely due to its very rapid rotation), but almost all the
others almost certainly are.
The avalanche of round objects
• Not only do we have the nine existing
planets, but we pick up at least twenty
moons, at least one asteroid (Ceres), and at
least a half-dozen known (and who knows
how many unknown) TNOs.
• That’s 40 planets (and counting..)
• We must come up with a stricter
definition...
Eliminating the moons
• Let’s add a second condition:
– Any round object which orbits a planet, is a moon and
not a planet.
• This was a definition proposed (and rejected) by the IAU.
• Under this definition we have the nine existing planets,
plus asteroid Ceres, Pluto’s moon Charon**, “Xena” and
some unknown number (but likely at least half a dozen)
number of trans-Neptunian objects.
**I’ll explain...
The Third Condition
• Subsequent IAU proposal:
– In addition to the first two criteria, a planet must also
“have cleared the neighbourhood around its orbit.”
– Anything that meets the first two criteria but not the
third is termed a “dwarf planet.”
• This proposal was voted upon and accepted.
• End result: Pluto, Charon, Ceres and “Xena”
(now officially named Eris) become dwarf planets,
and we’re back down to 8 full-fledged planets.
A definition, finally!
• A planet is a celestial body that:
– (a) is in orbit around the Sun;
– (b) has sufficient mass for its self-gravity to overcome
rigid body forces so that it assumes a hydrostatic
equilibrium (nearly round) shape;
– (c) has cleared the neighbourhood around its orbit.
Planet: satisfies all three of (a), (b), (c)
Dwarf Planet: satisfies only (a), (b)
Small Solar System Body: only (a)
The Solar System Today
Pluto, the Planet (1930-2006)
Pluto, the Planet (1930-2006)
134340 Pluto, the Dwarf Planet (2006-??)
Addendum: Alphabet Soup
• TNOs, KBOs, SDOs...
• What are they?
• Classifications of Pluto-like objects
(PLO’s?) in the outer solar system.
• Primary Subdivisions:
– Cis-Neptunian Objects
– Neptune’s largest moon Triton (special case)
– Trans-Neptunian Objects
• Cis-Neptunian Objects
– Neptune Trojans - captured KBOs?
– Centaurs - scattered KBOs?
• Trans-Neptunian Objects
– Kuiper Belt Objects
• Resonant KBOs
• Non-resonant KBOs
– Scattered Disc Objects
– Oort Cloud Objects
• Triton - captured KBO?
• Relationship to comets?
Cis-Neptunian Object
• Cometlike objects (but much larger) orbiting
within or at the orbit of Neptune.
• Neptune Trojans - analogous to Jupiter’s Trojans follow or lead Neptune in its orbit by 60 degrees.
• Centaurs - large TNO-like objects orbiting
between Saturn and Neptune.
Prototype: 2060 Chiron
• Probably have same or similar origin to TNOs.
Special Case: Triton
• Triton is the largest
moon of Neptune.
• It orbits Neptune
retrograde, extremely
unusual (unique) for
such a large object.
• Theory is that it is a
captured object similar
to Pluto.
• If so, it is thus far the
only such object seen
up-close.
TNO: Trans-Neptunian Object
• An object which orbits the sun with an average
distance greater than Neptune.
• Pluto was the first (and still prototype) TNO.
• TNOs can be subdivided into several groups,
depending on orbit.
– Kuiper Belt Objects (KBOs)
– Scattered Disc Objects (SDOs)
– Oort Cloud Objects
KBOs: Kuiper Belt Objects
• Members of a class of objects existing just past
Neptune, average distance 30-50 AU.
• Prototype: Pluto.
• Orbits are dominated by Neptune’s effects.
• Sharp cutoff after about 48 AU.
• Generally low inclination orbits (<30º)
• KBOs can be further subdivided:
– Resonant KBOs
– Non-resonant KBOs
Resonant KBOs
• Orbital periods are a rational fraction of
Neptune’s:
• 3:2 (e.g. 3 Neptune orbits to 2 of the object’s orbit)
– Orbital period = 165 years x 3/2 = 248 years
– Prototype: Pluto
– These objects are called “Plutinos”
• Other resonances: 2:1 (“twotinos,” 5:2, etc.)
• These occur because of Neptune’s repeated
gravitational influence.
• The orbits are usually somewhat elliptical and
moderately inclined.
Non-resonant KBOs
• No relation between orbital period and Neptune’s.
• Prototype: 1992 QB1 (“cubewanos”)
• Tend to have more closely circular orbits than
their resonant counterparts.
Scattered Disc Objects (SDOs)
• More distant members of the TNO class, past the
Kuiper Belt edge.
• Beyond about 50 AU, Neptune’s gravitational
effects are minimal.
• Prototype members: Sedna, Eris (both >75 AU).
• Often have very elliptical orbits (Sedna goes out to
~1000 AU)
• Tend to have high inclinations (Eris is 45)
The Pluto-like Objects
Oort Cloud Objects
• The Oort cloud is a hypothetical reservior
for comets, etc., >10,000 AU away.
• No objects in the Oort cloud have been
directly observed.
• Could the scattered disc and Kuiper belt be
inward extensions of the Oort cloud?
Relationship to Comets?
• The Pluto-like objects have very similar
composition to comets, although they’re much
bigger.
• Could comets simply be the smaller members of
this class of objects (presumably there must be
oodles of them, as-yet undiscovered), pushed into
the inner solar system by gravitational
interactions?
• Two basic types of comets:
– Short-Period Comets (50-100 years)
– Long-Period Comets (1000+ years)
Short-Period Comets
• Orbit of Comet Halley, a typical short-period
comet.
• Its aphelion: right about at the Kuiper belt!
• Of course we only know about it because it
periodically comes close to the earth.
• Many other comets share these properties!
Long-Period Comets
• There are also long-period comets (such as
Hale-Bopp and Hyakutake) which have
orbits that take them well beyond the known
reaches of the solar system.
• Could these objects come from the Oort
cloud?
Outer Solar System Objects
• All these objects - comets, KBOs, SDOs,
centaurs, etc. - are related to one another.
• All share the same origin in the birth of the
solar system, and have similar compositions.
• Clearly what we’ve seen so far is only the
tip of the iceberg, and much more is to be
discovered!