Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- October 2, 2006 Dr. Uwe Trittmann Welcome to

Today’s Topics Pluto, Armageddon, etc. – Solar System News The Night Sky in October

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Pluto’s Demotion Since August 2006, Pluto is not a planet anymore, but a “dwarf planet” The International Astronomical Union (IAU) voted on a new definition of the term “planet” at its triennial meeting in Prague There is hope: lots of astronomers are not happy with the new definition, and could vote to change it in 3 years …

Vote: Should Pluto be a “full” planet? Yes No

An Inventory of the Solar System

The Terrestrial (inner) Planets Small, dense and rocky Few moons, no rings Mercury Venus Earth Mars

The Jovian (Outer) Planets Large, gaseous, lots of moons, rings Jupiter Uranus Saturn Neptune

Moons (Satellites) Satellites or Moons orbit around planets, which orbit around the Sun  Satellites thus orbit the sun! Satellites can be big or small, rocky or icy, dark or light, spherical or irregularly shaped

Extreme Example: Titan Titan is the only moon in the solar system known to have an atmosphere Infrared picture shows surface details

Uranus’ Moons Proteus is one of Neptune’s moons Earth’s moon

Asteroids, Comets and Meteors Debris in the Solar System

Asteroid Discovery First (and largest) Asteroid Ceres discovered New Year’s 1801 by G. Piazzi, fitting exactly into Bode’s law: a=2.8 A.U. Today more than 100,000 asteroids known Largest diameter 960 km, smallest: few km Most of them are named about 20 of them are visible with binoculars

Asteroid Types C-type (dark, carbonaceous), 75% of all Asteroids S-Type (brighter, silicate) 15% Other (e.g. M-type with nickel/iron) 10% Gaspra: S-type Ida: S-type; with moon Dactyl

How to detect Asteroids Fast moving Icarus is a line on this long- time exposed photo

The Solar System: Top View

Side view: Inclination of Orbits Orbits (here: Mars) are very slightly tilted with respect to the sun-earth plane  Planets appear close to the path of the sun in the sky, the ecliptic

Why is Pluto not a planet anymore? – The Definitions (1) A "planet"1 is a celestial body that:planet –(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, and –(c) has cleared the neighbourhood around its orbit. (2) A "dwarf planet" is a celestial body that:dwarf planet –(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) shape2, –(c) has not cleared the neighbourhood around its orbit, and –(d) is not a satellite. (3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".Small Solar System Bodies

Cleaning up the Neighborhood Small objects are forced out of the inner Solar System by gravitational pull of bigger planets Small planetesimals collide and form planets -- or are thrown out!

Why should we change the definitions? There were no clear definitions New data / new discoveries –Many new trans-Neptunian objects found –Many new exo-solar planets discovered  It’s the way of science: we build our view of the world based on observations; if it is more convenient we modify our theories, terms, etc. to better represent the world around us.

The candidates size-wise: Eris, Pluto, Ceres (and Earth)

Brown dwarfs: Big planets or small Stars

Two types of planets

More candidates …

Is the demotion outrageous? – Not unprecendented! History of our view of the solar system –Pre-1450 AD: Earth in center (not a planet!), 7 planets (including Sun & Moon!) –Copernicus: Sun and 6 planets –W. Herschel (1781): Sun and 6 planets (add Uranus) –Piazzi (1801): Sun and 7 planets (add Ceres) –A few years later: Sun and 11 planets (add 3 more asteroids: Pallas, Juno, Vesta) –1846: a dozen planets (add Neptune)

Demotion of the “Minor Planets” In the mid-1800s the asteroids were considered minor planets (too many of them by then)  8 Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Pluto is different! Pluto is only 2200 km in diameter Much smaller than the Earth’s Moon

Pluto Sought by Percival Lowell, discovered by Clyde Tombaugh in 1930 –Existence discovered based on perturbations of Uranus’s and Neptune's orbits –The calculations were wrong, but it was there anyway! Satellite Charon discovered in 1978 –Mutual eclipses of Pluto and Charon reveal sizes and masses Resolution 1“ Space Telescope: distance 0.9“

Pluto Less dense than our Moon About 1/5 as big as the Earth About right for a Jovian moon Maybe a Kuiper belt object Icy material like Triton, one of Neptune’s moons Best picture of Pluto so far

Pluto’s strange Orbit Very far out there: 40 A.U. Pluto’s year = 248 Earth years Orbit inclined 17° w.r.t. ecliptic Very eccentric orbit: Perihelion: 30 A.U. (inside the Neptune orbit!) Aphelion: 50 A.U.

Pluto’s eccentric Orbit

Is the demotion the final word? Problems with the new definition abound –Example: if trans-Neptunian objects exist, then Neptune has not cleared his neighborhood  Neptune is not a planet?! Many astronomers are not happy with the new definitions; less than 500 voted!  Stay tuned!

Observing the Planets

Planetary Motions The sky seems to revolve around us because of Earth’s rotation Additionally, planets move with respect to the fixed stars, that’s why they are called planets (greek: wanderers) Due to the planet’s movement in their orbit, and Earth’s orbital motion, this additional motion – the apparent motion of the planet as seen from Earth - looks complicated.

Apparent Planetary Motion Motion as seen from Earth, which itself is revolving around the Sun.

The heliocentric explanation of retrograde planetary motion

Inner and Outer Planets Inner Planets: closer to sun than Earth –Mercury & Venus –Always close to sun in the sky Outer Planets: further from sun than Earth –Mars, Jupiter, Saturn, Uranus, Neptune, Pluto –Best viewing when opposite of sun in the sky

Inner Planets Inner planet Earth superior conjunction inferior conjunction western elongation eastern elongation

Outer Planets Outer planet Earth conjunction opposition quadrature

Close Outer Planet Outer planet Earth Size of planet varies a lot as Earth moves

Far-Out Planet Outer planet Earth Size of planet varies little as Earth moves

Mercury Color: yellow-golden Brightness: up to –1m Size: 10” When to observe: several times a year for short periods Difficulty: pretty tough, innermost planet, always very close to the sun

Venus Color: white Brightness: up to –4.5m Size: up to 40” When to observe: all year, except for period around superior conjunction; either west of the sun (morning star), or east of the sun (evening star) Difficulty: very easy

Mars Color: orange Brightness: up to –2.2 m Size: up to 25” When to observe: about every 2 years Difficulty: very easy around opposition

Mars Opposition 2005 Date of opposition: November 7, 2005 Constellation: Aries Date of closest distance: October 30, 2005 Closest distance to Earth: million km (43 million miles, or AU) In 2003 (historically close): 55.8 million km

Jupiter Color: yellowish-white Brightness: up to –2.5m Size: 40” When to observe: most of the year, except for some months around conjunction Difficulty: easy, moons visible in binoculars

Jupiter & Moons

Saturn Color: yellowish Brightness: up to –1.5m Size: 20” When to observe: most of the year, except for some months around conjunction Difficulty: easy, rings and moons visible in small telescopes

Saturn & Moons

Uranus Color: greenish Brightness: around 5.7m Size: 4” When to observe: most of the year, except for some months around conjunction Difficulty: challenging, with binoculars

Neptune Color: greenish Brightness: around 7.8m Size: 2.5” When to observe: most of the year, except for some months around conjunction Difficulty: challenging, good binoculars

Pluto Color: white Brightness: 14m Size: star-like, no disk When to observe: most of the year, except for some months around conjunction Difficulty: very tough, outermost planet, always very far away, very faint; big telescope and several nights to identify

The Night Sky in October The sun is past autumn equinox -> longer nights! Autumn constellations are coming up: Cassiopeia, Pegasus, Perseus, Andromeda, Pisces  lots of open star clusters! Saturn is visible later at night

Moon Phases Today (Waxing Gibbous, 75%) 10 / 6 (Full Moon) 10 / 13 (Last Quarter Moon) 10/ 22 (New Moon) 10 / 29 (First Quarter Moon)

Today at Noon Sun at meridian, i.e. exactly south

10 PM Typical observing hour, early October Uranus at meridian Neptune Moon

South- West High in the sky: The summer triangle

Due North Big Dipper points to the north pole

High up – the Autumn Constellations W of Cassiopeia Big Square of Pegasus Andromeda Galaxy

“PR” Foto Actual look

East Perseus and Auriga with Plejades and the Double Cluster

South Planets –Uranus –Neptune Zodiac: –Capricorn –Aquarius

Mark your Calendars! Next Starry Monday: November 6, 2006, 7 pm (this is a Monday ) Observing at Prairie Oaks Metro Park: –Wednesday, October 27, 6:45 pm –Friday, January 26, 6:30 pm Web pages: – (Obs.) – (Physics Dept.)

Mark your Calendars II Physics Coffee is every Wednesday, 3:30 pm Open to the public, everyone welcome! Location: across the hall, Science 256 Free coffee, cookies, etc.

Outlook: Armageddon from Outer Space – Apophis is coming! Asteroid Apophis is making a very close encounter with Earth on Friday the 13 th April 2029 (very close, but certain miss) If things go really bad that day, it’ll return exactly 7 years later and will crash into Earth! Stay tuned!

Impact on Earth Most probably caused the extinction of the dinosaurs

Impact Craters Quebec's Manicouagan Reservoir. Large meteorite landed about 200 million years ago. The lake, 70 km in diameter, now fills the ring. Barringer Crater, AZ 1.2 km diameter, 0.2 km deep; produced by impact about 25,000 years ago

Tunguska ~30 m body struck Siberia in 1908 Energy equal to that of a 10 Megaton bomb! Detonation above ground; several craters

Frequency of Impact Events