Physics 101 - Astronomy - Spring 2019

Class notes for day 18, March 26, 2019

I used three powerpoints today: day18meteors.pptx, day18SSformation.pptx, and day18exo.pptx.

Ch. 14 describes the smaller fragments left over after the formation of the solar system, which may fall to Earth as meteorites, and then it goes over the formation of the Solar System. Then we get a discussion of similar planetary systems around other stars, called extrasolar planetary systems (and the planets themselves are usually called exoplanets).

See the Day 17 notes for some short comments about meteors and see the PowerPoint for some pictures. Also look thru the material in Ch. 14.

Formation of the Solar System

My PowerPoint went over more detail than Ch. 14 in the OpenStax book, because I wanted to describe where all the elements above helium came from. We will see this material again later in the book.

The heavy elements in the solar system were formed in an earlier generation of stars.
The early Universe contained only hydrogen, helium, and traces of lithium.
Heavier elements were created in the core of stars as they “burned” the hydrogen and helium into carbon, oxygen, neon, calcium, magnesium, silicon, and iron.
These were then expelled into space by
- stellar winds (happening with our sun now, see this picture of the sun and its outer atmosphere)
- planetary nebulae (not planets, but they had a similar appearance to early astronomers with small telescopes) - see slides in the file day19SSformation.pptx
- nova and supernova explosions, which also create elements up to the region around iron in the periodic table.
When we study the lifetime of stars, we will learn that:
Planetary Nebulae are shells of gas blown off of stars like our Sun at the end of their lifetime.
A Nova is an explosion on a white dwarf, but only a small amount of material on the surface of the white dwarf explodes.
A Supernova is an explosion that destroys the entire star and expels almost all of it into space.

Major facts that any theory of solar-system formation must explain

Each planet is relatively isolated in space.
The orbits of the planets are nearly circular.
The orbits of the planets all lie in nearly the same plane.
Direction of planet’s movement in orbit is same as sun’s rotation.
Direction of planet’s rotation is same as sun’s rotation. (*usually*)
Direction of the various moon’s revolution is same as planet’s rotation.
The planetary system is highly differentiated.
Asteroids are very old, and not similar to terrestrial planets or Jovian planets.
The Kuiper belt is a group of asteroid-sized icy bodies orbiting outside the orbit of Neptune. (KBO – Kuiper Belt Objects)
The Oort Cloud is composed of icy cometary objects that do not orbit in the same plane as the planets (the ecliptic).

A Theory of Solar System Formation:
A spinning gas cloud condenses to a much smaller size, and begins to rotate much faster due to conservation of angular momentum. A protostar forms in the center and planets may form in orbit around it.
This process explains the fact that all the objects in the Solar System tend to rotate (CCW) in the same way (or ‘sense’).
The planets are different from each other. Those close to the star (or Sun) are rocky, whereas those further away have large amounts of light gases like hydrogen, helium, methane, water, ammonia, etc.
Differentiation is due to the temperatures in the Early Solar Nebula, forming different types of planets at different distances from the Sun. The inner solar system is closer to the early Sun, and so it is hotter. Volatile gases are not condensed on the nearby planets due to the heat from the Sun and ended up condensing in the Jovian planets further out.
This is similar to a process in chemical plants called distillation or fractionation.

Last part of Ch. 14 - Exoplanets

Exoplanets are planets around other stars outside the Solar System (exo- basically means "outside"). Obviously, they are very far away and hard to detect. They have only been seen recently using new techniques. Even so, we have a list of over 3000 confirmed exoplanets.

Evidence for these comes from several sources:

- Distortion of protoplanetary disks, sometimes called proplyds.
- Doppler shift of the light of the star, indicating radial velocity shifts.
- Astrometry to measure stellar wobble.
- Detection of dimming of the star, indicating a transit by a planet.

I showed a few examples in class.

The star Fomalhaut has a planet, imaged in visible light:
http://apod.nasa.gov/apod/ap081114.html

The star Beta Pictoris has a nebula forming around it, as shown in your book and also this Astronomy Picture of the Day.
http://apod.nasa.gov/apod/ap971128.html
and we see evidence of a planet around this star.
http://apod.nasa.gov/apod/ap100703.html

With more recent work, we know about hundreds of exoplanets. We will study these more, later in the course. Additional material on exoplanets was included in the PowerPoint day19exo.pptx.

Optional: If you are interested in learning more (this is material that won't be on the exam) I have put some links here at the bottom of these notes.

For more about extrasolar planets, see these web sites:
http://exoplanets.org/
http://exoplanet.eu/
http://en.wikipedia.org/wiki/List_of_stars_with_confirmed_extrasolar_planets
NASA and the JPL (Jet Propulsion Lab, in Pasadena, California) has an "official" webpage on Planet Searches.
http://planetquest.jpl.nasa.gov/ has a list.
http://exoplanetarchive.ipac.caltech.edu/ has a list of exoplanets.
So it's hard to keep count, since the exoplanets are being discovered frequently now.

NASA launched a satellite to look for exoplanets:
http://kepler.nasa.gov/
http://www.nasa.gov/mission_pages/kepler/main/index.html
Earth-size planets with temperatures suitable for life have been found by the Kepler spacecraft.

First direct image of an exoplanet (in infrared): http://apod.nasa.gov/apod/ap050510.html

Multi-planet star system (Keck infrared image): http://apod.nasa.gov/apod/ap081117.html

Kepler-11 has six planets:
http://apod.nasa.gov/apod/ap110203.html 
How common are Earth-sized planets?
http://apod.nasa.gov/apod/ap130112.html 
A protoplanetary disk forming around a star
http://apod.nasa.gov/apod/ap141110.html 
For fun, you could try Super Planet Crash (a game)
http://apod.nasa.gov/apod/ap150112.html 

http://exoplanets.org/ describes the current search and has summary information about searches for extrasolar planets (exoplanets). They have a page describing the method of observing the Doppler shift of the light from a star to deduce that it has an orbiting planet. As an example of how the Doppler method might be used to discover a planet around a star, our own Jupiter's gravitational pull causes the Sun to wobble around in a circle with a velocity of 12 meters per second, and this could be detected by observers living around other stars. Here is a webpage with an animation of the Doppler method.
http://www.astronautica.com/detect.htm