Lecture 16: Exoplanets I

Question 1

what is astrometry and what are its limits?

Answer 1

all objects in a planetary system orbit their common centre of mass

limited by what the telescope is able to detect

Question 2

how to determine a stars radial velocity

Answer 2

when given wavelength change:

Δ𝜆/ 𝜆= v_r/ c

Question 3

how to determine a planets semimajor axis

Answer 3

when given period of orbit and stars mass:

a= ³√GM_star/ 4𝜋² P²_planet

Question 4

how to determine the mass of a planet

Answer 4

when given a stars radial velocity, mass, and a planets period:

M_planet= M_star V_star P_planet/ 2𝜋a_planet

Question 5

what is transit technique?

Answer 5

when a planet moves between us and its parent star, it will block some starlight

the amount blocked lets us calculate the radius of the planet

Question 6

how can you work out a planets density, and hence, composition?

Answer 6

by combining radial velocity and transit measurements

Question 7

how to calculate density from mass and radius

Answer 7

density= mass/ volume

get volume from V= 4/3𝜋r³

Question 8

what is microlensing?

Answer 8

when a foreground star can gravitationally focus light from a background star

if the foreground star has an orbiting planet this may produce an anomaly

can detect low mass planets

Question 9

what are the limits of direct imaging?

Answer 9

if the host star is not very bright, it is sometimes possible to directly image the planet

until recently, only worked on dim stars and fairly bright planets

Question 10

how do you calculate a planets radius?

Answer 10

R_planet= R_star √fraction of light blocked

Question 11

what is the radial velocity/ doppler technique?

Answer 11

as a star orbits the centre of mass, its spectral lines will be shifted alternately toward the blue and red