Download What is a planet? Why? How?

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 What is a planet?
 Why?
• Does it matter what you call an object?
• Does the public care so much?
• Were scientists made fun of, but not HP Computer
Company?
 How?
• International Astronomical Union
• Coming to a consensus
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 Come up with your own definition of a planet
based on what you know
 Test your definition against new discoveries
 Keep in mind those questions about science
as you do this:
• Bias?
• Definition open to debate and being changed?
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 Why?
 Because you will see that the argument in the
summer of ’06 was really a continuation of an
argument that started back in 1801…
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 Look up into the sky, what do you see?
• Stars
• Sun
• Moon
• Some planets (Mercury, Venus, Mars, Jupiter, Saturn)
 All “move” around the Earth but:
• Stars stay in fixed positions compared to each other
• The Sun moves around compared to the stars
• The Moon moves around compared to the stars
• The planets move around compared to the stars
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 So have two groups:
• “Planets” meaning:
• Sun
• Moon
• The “bright” planets (Mercury, Venus, Mars, Jupiter,
Saturn)
• Stars meaning:
• Everything else
 Happy Happy?
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 What makes the Sun different from the
planets?
 What makes the Moon different from the
planets?
 OK, so make that four groups:
Sun
Moon
Stars
The Bright Planets
 Based upon a combination of how they
move and what they look like
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 Galileo saw things in the solar system never
seen before
 Four large moons of Jupiter
• First seen by Galileo
• He named them the “Medici Stars”
• Who the heck are the Medici?
• These “Medici Stars” don’t go around the Earth!
 Do we now need another group?
 Note your technology drives your questions
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 Sun
 Planets
 Moons
 Stars
 Based upon a combination of where they
are, how they move,…
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Let the average distance between the Earth
and the Sun equal 1, then
Planet
Mercury
Venus
Earth
Mars
???
Jupiter
Saturn
???
Distance
0.4
0.7
1.0
1.5
5.2
9.5
Series
(0 + 4) / 10 = 0.4
(3 + 4) / 10 = 0.7
(6 + 4) / 10 = 1.0
(12 + 4) / 10 = 1.6
(24 + 4) / 10 = 2.8
(48 + 4) / 10 = 5.2
(96 + 4) / 10 = 10.0
(192 + 4) / 10 = 19.6
The series was found in 1766
Called the Titus–Bode Law
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 William Herschel, an English astronomer
was doing an all sky survey
 Found the planet, Uranus
• He called it “George’s Star”
• Who the heck was George?
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Let the average distance between the Earth
and the Sun equal 1, then
Planet
Mercury
Venus
Earth
Mars
???
Jupiter
Saturn
Uranus
Distance
0.4
0.7
1.0
1.5
5.2
9.5
19.2
Series
(0 + 4) / 10 = 0.4
(3 + 4)) / 10 = 0.7
(6 + 4) / 10 = 1.0
(12 + 4) / 10 = 1.6
(24 + 4) / 10 = 2.8
(48 + 4) / 10 = 5.2
(96 + 4) / 10 = 10.0
(192 + 4) / 10 = 19.6
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 ???
=
(24 + 4) / 10 = 2.8
 The asteroid Ceres was found in 1801, its
average distance from the Sun is 2.8
 The asteroid Juno was found in 1804, its
average distance from the sun is 2.7
 The asteroid Vesta was found in 1807, its
average distance from the Sun is 2.4
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 Herschel questioned are these objects really
planets.
• Note he was the only other person to have found a planet
• Could this bias his opinion?
 As more and more asteroids were discovered,
people wondered can you really have hundreds
of planets.
 Scientists decided that these weren’t planets but
called them asteroids.
 But for 50 years Ceres was called a planet!
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 Start with a big cloud of gas and dust
 Starts to collapse under its own weight
 Heats up and spin’s up as it collapses
 Stuff in center forms star
 Stuff in orbit around star clumps together to
form larger and larger objects -> planets
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 They are thought to be
“failed small (Earth-like) planets”
Total mass 0.0006 Earth Mases
 Either
• Never got big enough to become a “real planet”
• Or got all the way up to  Mars sized
• Big enough iron core forms
• Water on the surface
• Life?
• Then a massive collision breaks up the planet down to
its core
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Ceres largest main belt asteroid, It is
the only one big enough to be round
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http://www.youtube.com/watch?v=z0tj1yi6DSs
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 People noticed that they could not predict
accurately where Uranus should be in the
night sky
 John Adams and Urbain Le Verrier,
independent of each other came up with the
idea that maybe there was another planet
beyond Uranus that was pulling on Uranus
and changing its path.
 Scientists looked where they predicted this
planet to be and Neptune was found.
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 Let the average distance between the Earth and
the Sun equal 1, then
Planet
Mercury
Venus
Earth
Mars
Asteroids
Jupiter
Saturn
Uranus
Neptune
Distance
0.4
0.7
1.0
1.5
2.1–3.5
5.2
9.5
19.2
30.1
Series
(0 + 4) / 10 = 0.4
(3 + 4)) / 10 = 0.7
(6 + 4) / 10 = 1.0
(12 + 4) / 10 = 1.6
(24 + 4) / 10 = 2.8
(48 + 4) / 10 = 5.2
(96 + 4) / 10 = 10.0
(192 + 4) / 10 = 19.6
(384 + 4) / 10 = 38.8??
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 Scientists noticed that they could not predict
accurately where Neptune should be in the
night sky
 So scientists made a prediction that there
should be a planet beyond Neptune.
 Scientists looked for another planet and Pluto
was found.
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 Scientists noticed that Pluto did not have
enough mass to affect Neptune’s orbit
 Went back and checked and found that there
was no problem with Neptune’s orbit
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 Another object was found near Pluto’s orbit
and a little bit bigger (maybe) than Pluto!
• Is this a planet?
• Or is this the problem with the asteroids all over
again?
 Pluto is one of the largest members of
another belt of objects called the Kuiper belt.
• ½ rock, ½ ice (by volume)
• Think long orbital period comets that never get
close to the Sun
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 Astronomers decided they needed to
decide what a planet is.
 International Astronomical Union formed
a committee
• International, so no more George or Medici
• Meets every two years
 Committee report was in 2006 (IAU meets
every two years)
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 A planet is a celestial body that:
• Self-gravity is strong enough for it to be nearly
round in shape
• Which is in orbit around a star but:
•
•
Itself is not a star
Not a satellite of a planet
 Advantages
• Simple
• Covers high and low mass objects
• Works well for other solar systems
• Pluto stays in
• But the riff-raff comes in
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Ceres largest main belt asteroid, it is
the only one big enough to be round
Eris and its moon Dysnomia. Eris was
formally known as 2003 UB 313
or Xena. Dysnomia was formally
known as Gabrielle.
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 A planet is a celestial body that:
• Is in orbit around the Sun
• Self-gravity is strong enough for it to be nearly
•
round in shape
Has cleared the zone around its orbit
 A “dwarf planet” is a celestial body that:
• Is in orbit around the Sun
• Self-gravity is strong enough for it to be nearly
•
•
round in shape
Is not a satellite of a planet
Has not cleared the zone around its orbit
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 IAU met again in 2008. What is a planet was
back on the agenda.
 Special class of dwarf planet: Plutoid
• In orbit around the Sun at a semimajor axis greater
•
•
•
than that of Neptune
Near-spherical shape
Not cleared the neighborhood around their orbit.
Satellites of plutoids are not plutoids themselves,
even if they are near-spherical in shape
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 Upper limit to the mass of a planet
• Can an object change from being a star to being a
planet?
 Lower limit to the mass/size of a planets
 Planets in orbits around other stars
 Free–floating planets or can an object stop
being a planet?
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 Or why is the Sun not a planet?
• Stars shine (give of visible light) but planets do not
• Fusion!
 OK, so what is fusion?
• Lighter atomic weight nuclei are fused into heavier
atomic weight nuclei
 What makes fusion hard to achieve & What is
needed for fusion?
• Like charges repel
• High pressure & high temperature overcome this
• With enough mass can meet these two conditions
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 Elements do NOT all start to fuse at the same
temperature and pressure
 Most common element in the universe is
hydrogen
• Normal hydrogen nucleus has one proton
• “Heavy Hydrogen” or deuterium one proton plus
•
one neutron
Deuterium fuses at a lower temperature & pressure
than normal hydrogen
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 Brown dwarfs
• Massive enough that fusion started for deuterium
• But not massive enough for normal hydrogen fusion
• Fusion takes place only over a short time period for
•
•
•
a brown dwarf
Brown Dwarfs start at about 13 x Jupiter mass
If a brown dwarf is in orbit around another star and
fusion is not actively occurring, is it a planet?
Does your definition cover this case?
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 Has to be round?
 Why are planets round?
 So the composition of an object will determine
in part if it is a planet, icy objects will become
“round” at a smaller size than rocky objects
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2005 FY9
Makemake
2003 EL61
Haumea
Hiʻiaka
Namaka
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 Use a minimum size or radius?
• Jupiter’s moon Ganymede is larger than
Mercury
 Atmosphere?
• Mercury does not have an atmosphere
• But Titan, a large moon of Saturn, has an
atmosphere thicker than either the Earth’s or
Mars’
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 Currently (as of last night)
http://exoplanet.eu/
• 861Exoplanets
• 677 Planetary systems
• 128 Multiple planet systems
 Current methods work best for finding large
planets (Jupiter or larger) that orbit close to the
star
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How do astronomers find extra-solar planets?
http://science.howstuffworks.com
/planet-hunting1.htm
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How do astronomers find extra-solar planets?
Kalas et al. (2000)
Direct Imaging
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 Finding objects larger than Jupiter but
closer to the star than Mercury caused
some problems
• Don’t think they formed there
• Formed in outer part of their solar system and
•
•
moved inwards.
A planet in the way could be thrown out
So if a planet sized object formed near a star but
no longer orbits a star is it a planet?
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http://ciera.northwestern.edu/rasio/
UpsAndPR/JPG/UpsAnd.mov
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