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Is there anything wrongwith this picture?

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 Distance SeriesMercury 0.4 (0 + 4) / 10 = 0.4Venus 0.7 (3 + 4) / 10 = 0.7Earth 1.0 (6 + 4) / 10 = 1.0Mars 1.5 (12 + 4) / 10 = 1.6??? (24 + 4) / 10 = 2.8Jupiter 5.2 (48 + 4) / 10 = 5.2Saturn 9.5 (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 Distance SeriesMercury 0.4 (0 + 4) / 10 = 0.4Venus 0.7 (3 + 4)) / 10 = 0.7Earth 1.0 (6 + 4) / 10 = 1.0Mars 1.5 (12 + 4) / 10 = 1.6??? (24 + 4) / 10 = 2.8Jupiter 5.2 (48 + 4) / 10 = 5.2Saturn 9.5 (96 + 4) / 10 = 10.0Uranus 19.2 (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 Distance SeriesMercury 0.4 (0 + 4) / 10 = 0.4Venus 0.7 (3 + 4)) / 10 = 0.7Earth 1.0 (6 + 4) / 10 = 1.0Mars 1.5 (12 + 4) / 10 = 1.6Asteroids 2.1–3.5 (24 + 4) / 10 = 2.8Jupiter 5.2 (48 + 4) / 10 = 5.2Saturn 9.5 (96 + 4) / 10 = 10.0Uranus 19.2 (192 + 4) / 10 = 19.6Neptune 30.1 (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 member 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 isthe only one big enough to be round

Eris and its moon Dysnomia. Eris wasformally known as 2003 UB 313or 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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2003 EL61HaumeaHiʻiakaNamaka

2005 FY9Makemake

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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) 843 Exoplanets• http://exoplanet.eu/

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?

Direct Imaging

Kalas et al. (2000)

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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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