Extrasolar planets Detection and habitability

October 23rd 2014

Detecting exoplanets

• Primary methods :

• Direct imaging

• Transit

• Radial velocity

Direct imaging

• Planets are much fainter than their host star since they do not emit their own light

• By blocking the light from the star, it can be possible to reveal planets

Transit• Looking at the planetary

system from the side

• The passage of the planet create a very small dip in the luminosity of the star

Transit• Looking at the planetary

system from the side

• The passage of the planet create a very small dip in the luminosity of the star

Transit• Looking at the planetary

system from the side

• The passage of the planet create a very small dip in the luminosity of the star

Transit• Looking at the planetary

system from the side

• The passage of the planet create a very small dip in the luminosity of the star

Transit

• What other factor could influence the shape of the light curve?

The size and period of the transiting planet

Radial velocity• Gravity means that the star is

pulling on the planet, but the planet is also pulling on the star

• This results in both going back and forth around their mutual center of gravity

• This change in speed means the spectral lines from the star are going to vary due to Doppler shift if there’s a planet

• The more massive the planet, the larger the effect

In the lab• Using real transit and radial velocity data taken at UVic, you’re

going to estimate the parameters for HD209458b5 EXTRASOLAR PLANETS: DETECTION AND HABITABILITY 26

1 EXTRASOLAR PLANETS 2

the star, the eclipse of the star by the planet, and also the variations in theradial velocity of the star. We can change the planet’s mass, period, radiusand the orbital inclination by clicking on the buttons in the button panelseen in Figure 1.

Figure 1. The modelling program.

The second panel shows the blue planet orbiting the red star. The ellipseis the path of the planet around the star. Click on the [Orbital Inclination]button to change how much the orbit is inclined to the line of sight. If theinclination is just right, the planet will pass in front of the star and blockout part of the star’s light making the star appear fainter.

The third panel shows a plot of how the brightness of HD209458 changesas a function of time. Every 3.525 days the star becomes very slightly fainter.We watched the dimming of HD209458 one summer with our 20 inch tele-scope and our observations are the points plotted in the third panel. Thedepth of the eclipse depends on the size of the planet; the bigger the planet,the more light it blocks and the deeper the eclipse. We can find the radius ofthe planet from the depth of the eclipse. Change the radius of the planet us-ing the [Planet Radius] buttons so that the line agrees with the data. Change

Figure 9: The modeling program.

Record the radius of the planet and the inclination of the orbit.The bottom panel is a plot of the radial velocities of HD209458 as observed by Geo↵ Marcy

with the world’s largest telescope. A sine curve has been drawn through the data points, but itdoes not quite agree with them. The more massive the planet the larger the reflex motion of thestar and the larger the amplitude of the radial velocities. So we can measure the mass of the planetby measuring the amplitude of the radial velocity variations. Change the amplitude of the radialvelocity curve by clicking on the [Planet Mass] button.

We can also change the position of the data points along the graph by changing our guess of theperiod of the planet’s orbit around the star. Change the period by clicking on the [Period] buttonsand you will find that the observed points move left or right a little bit. From Kepler’s Law weknow the semi-major axis of the orbit of the planet around the star depends on the period of theorbit. Change the period and see that the Radius of the Orbit changes as well. Change the planetmass and the period to best fit the data points.

Record your best estimate of the mass, period, and semi-major axis of the orbit ofthe planet.

Would you be able to detect a Jupiter mass planet in a one year orbit? Click on the [Stop]animation box and then click on the Period value and change it from 3.52 to 365 days. Click [Start]twice and the animation will start again with the planet at about 1 Astronomical Unit from thestar. Increase the mass of the planet until it is at 1 Jupiter mass. Increase the radius of the planetuntil it is at 1 Jupiter radius. Set the Inclination to 90 degrees so that eclipses must occur.

Would you be able to observe radial velocity variations (uncertainty of ±4 m

sec

) oreclipses for a Jupiter sized planet in an Earth like orbit?

Would it be possible to detect planets like the Earth? Set the Period in the simulator to 365days and the radius of the planet to 0.1 Jupiter and the mass of the planet to 0.01 Jupiter Masses.

Habitable zone

• Suppose that all life needs liquid water

• There is only a certain zone not to close and not to far from the host star where liquid water is possible

• This is the habitable zone

Habitable zone

• The temperature of the star is a factor that influence how far this zone is going to be

• Which is going to have a closer habitable zone, a red or a blue star?

• The other important factor is the presence of an atmosphere to store heat and warm the surface through greenhouse effect

Thermal equilibrium

• On medium timescale, Earth’s average temperature is not changing (longer than day to day, but shorter than global warming)

• The Earth is in thermal equilibrium with its environment

• Energyabsorbed = Energyradiated

Blackbody temperature

• A blackbody absorbs all incoming light and reemits the energy as a continuum spectrum

• The blackbody spectrum varies depending of its thermal equilibrium temperature (think of red vs. blue stars)

Albedo

• Real objects or planets like the Earth are not perfect blackbody, they reflect some of the incoming light

• The fraction of reflected light is called albedo. It is 0.4 (or 40%) for the Earth. In other words, the Earth absorbs 60% of the incoming light

Surface Typical albedo

Fresh asphalt 0.04

Worn asphalt 0.12

Conifer forest 0.10

Bare soil 0.17

Green grass 0.25

Desert sand 0.40

Ocean ice 0.6

Fresh snow 0.85

Blackbody temperature• A planet twice the distance from

the Sun receives 4 times less light (4πr2)

• A fraction of the light is reflected

• The energy absorbed is going to bring the planet to its thermal equilibrium temperature or blackbody temperature

TBB =

✓FL

AD2

◆1/4

Greenhouse effect5 EXTRASOLAR PLANETS: DETECTION AND HABITABILITY 31

Figure 11: Illustration of the principal of the greenhouse e↵ect. On the left, incoming solar radiationis absorbed by the ground and re-radiated to space. On the right, incoming solar radiation isabsorbed by the ground and re-radiated, but then captured by the atmosphere. The atmospherethen re-radiates this energy, but both upward and back down toward the ground, increasing theincoming radiation the ground sees over the case with no atmosphere.

where TBB

is the Blackbody temperature of the planet. This calculation only considers a singlelayer, perfect greenhouse gas atmosphere - the simplest possible case. In reality, the e↵ect dependson other factors, like the e�ciency of the greenhouse gas and the number of atmosphere layers youconsider.

Ein (surface) = Eout (surface)Atmosphere absorb IR but

let visible light through

Ein = Eout

Ein (surface) = Eout (surface) = Esun + 0.5 x Eatm

Eatm = Eout (surface)

Ein (surface) = 2 x Esun

Greenhouse effect5 EXTRASOLAR PLANETS: DETECTION AND HABITABILITY 31

Figure 11: Illustration of the principal of the greenhouse e↵ect. On the left, incoming solar radiationis absorbed by the ground and re-radiated to space. On the right, incoming solar radiation isabsorbed by the ground and re-radiated, but then captured by the atmosphere. The atmospherethen re-radiates this energy, but both upward and back down toward the ground, increasing theincoming radiation the ground sees over the case with no atmosphere.

where TBB

is the Blackbody temperature of the planet. This calculation only considers a singlelayer, perfect greenhouse gas atmosphere - the simplest possible case. In reality, the e↵ect dependson other factors, like the e�ciency of the greenhouse gas and the number of atmosphere layers youconsider.

Ein = Eout

Simply by adding an atmosphere that absorbs the IR emitted by the surface, we double the

energy the surface actually absorbs

Tw/ atm = (2)1/4 ⇥ TBB ⇠ 1.19⇥ TBB

In the lab

• One bottle filled with air, one filled with CO2

• The black cardboard serves as our blackbody surface, absorbing visible light and emitting IR

• Should we see a difference in temperature? What do you predict?

5 EXTRASOLAR PLANETS: DETECTION AND HABITABILITY 33

AIR(most thermal radiation escapes)

100% CO2(most thermal radiation captured by CO2)

Light absorbers(black paper)

Light source

ther

mom

eter

therm

om

eter

visible light visible light

thermal radiation thermal radiation

Figure 12: Experimental setup for testing the greenhouse e↵ect hypothesis.

Experimental Setup

Experimental Materials:

• 2 2-liter plastic bottles with labels removed, half-wrapped in black paper.

• 1 filled with CO2

• 1 filled with normal air

• 2 bottle caps with thermometer probes attached.

• 1 high-power light bulb in stand.

Procedure:

1. Check to make sure both your bottles are clean and dry, and have black backing paper attachedto one side of them.

2. Make sure that your temperature probes are sealed in tightly, such that your CO2 has notleaked out.