chapter 21.3 evidence that planets form around other stars & 21.4 planets beyond solar system & 21.5

how do planets form?

• the gathering together of gas and dust particles in orbit around a newly created star

• each dust particle is heated by the protostar and radiates in the infrared region of the spectrum

• before any planets form, we can detect such radiation from all of the spread-out individual dust particles that are to become planets

• we can detect the silhouette of the disk if it blocks bright light coming from a source behind it

• planets must form in 3-30 millions years

radiation and our search for planets

• once the dust particles gather together and form a few planets/moons, the majority of dust is inside planets

• radiation can now be detected from the outside surfaces

• the amount of infrared radiation is therefore the greatest before the dust particles combine into planets

  • search for planets begins with the search for infrared radiation from the material required to make them

disks around protostars

• essential to star formation

• nearly all very young protostars have disks

• range in size from 10-1000AU

• mass contained: 1-10% of Sun’s mass

• observations of the disks can reveal presence of interstellar molecules, such as benzene, ethane, acetic acid and formaldehyde

• how disks change with time can be used to estimate how long it takes for planets to form

age of protostar and subsequent qualities of disk

• less than 1-3 million years old: disk extends all the way from very close to the surface of the star out to tens or hundreds of AU away

• old protostars: disks with outer parts that still contain larges amounts of dust, but the inner regions have lost most of their dust. in these objects disk looks like donut

  • inner, dense parts of most disks disappear by the time the star is 10 million years old

accretion

• the process where a celestial object grows in mass by gravitationally attracting and accumulating surrounding matter, typically gas and dust

• can drive the rapid growth planets

exoplanet

• planet outside of the solar system

• most exoplanet detections are made using techniques where we observe the effect that the planet exerts on the host star

  • both the star and planet revolve about their common center of mass

doppler method of detecting planets

hot Jupiters

• giant planets orbiting very close to their stars

transiting planets

• transit: when the orbital plane of a planet is tilted or inclined so that it is viewed edge-on, we will see the planet cross in front of the star once per orbit, causing the star to dim slightly

superearths

• most common planets

• radii between 1.4-2.8 of Earth

mini-Neptunes

• radii between 2.8-4.0 Earth radius