Topic 6 The Detection Of Exoplanets Flashcards
What is drakes equation?
This equation proposed by frank drake predicts the number of civilisations, N, broadcasting their existence in our galaxy.
N = Rb T
Where Rb is the rate (number per unit time) at which broadcasting civilisations (civilisations that broadcast in radio waves) appear in the galaxy and T is the time over which they broadcast.
What is required of an object if it is to be detected?
- Reflects radiation
- Emits radiation
- Refracts radiation
- Blocks (absorbs or occults) radiation
- Affects the motion of a nearby observable body
Flux (brightness) equation
F = L / 4πd^2
Where L is luminosity and d is the radius of the body.
For a planet the equation will be
F = L / 2πd^2
As only half the planet is lit.
What **astronomical magnitude **? (Equation)
This is the brightness of us, Stella object as seen from earth. A difference of five magnitude is equal to a brightness change of a factor of 100. A negative magnitude means the object is brighter than the brightest star such as the Sun, moon and a few planets.
m = 2.5log10(b / bo)
Where b is the brightness of the star and bo is the brightness of an object with magnitude 0 (2.29x10^-8 W m^-2)
What is spatial resolution?
This is a telescopes ability to distinguish between 2 separate object.
α = λ / D
Where λ is wavelength and D is the aperture of the optical device.
α will be in radians.
What other factors can effect spatial resolution?
The distortion due to atmosphere (seeing) can also effect spatial resolution.
Equation for angular measure
β = a / d
Where β is the angular size in radians, a is actual size and d is distance to the object.
Why do objects with contrasting brightnesses cause a problem for telescopes?
A large ratio is a problem for resolution because of the unavoidable optical effects and seeing that cause a start to appear as a fuzzy disk rather than a point on any astronomical image. This will cause the radius of the disc to be much larger than the actual disc of the star in the image covering any potential planets.
How can bad seeing and large brightness ratios be overcome?
Adaptive optics can be used to compensate for fluctuations in the seeing conditions.
Coronagraphs can physically block out the light of the star but not that of the planet in optical telescopes.
You can mathematically subtract the image of a nearby comparison star from the signal of the target star and if light remains it signifies a planet .
Why may infrared be a better wavelength to search for planets?
Earth like planets in mid radiation in the infrared spectrum whereas sunlight stars emit radiation in the visible spectrum therefore in the infrared spectrum the brightness ratio is far smaller.
What are the disadvantages of observing in infrared?
Due to infrared wavelengths being smaller, it would mean that a telescope with the same size optic would have a poorer resolution, that is the minimum separated angle will be larger.
earths atmosphere also absorbs infrared radiation making it hard to make ground based observations in this wavelength. This can be overcome with space based telescopes.
What is an interferometer?
This is a form of radio astronomy where multiple low resolution radio images are combined to form one high resolution telescope. This method allows you to remove the light from a star by combining a delayed signal with a non-delayed signal, causing the crest and troughs of the radio waves from the star to be cancelled out.
this would leave the Spectre from the planets allow you to identify the absorption lines from its atmosphere.
What is occultation?
This is a method of planet hunting whereby you measure a small dip in brightness of a star caused by the transit of a planet in front of it. If the dip in brightness occurs on a regular basis, it can be assumed that planet is orbiting the star.
Fraction of a stars light blocked by an exoplanet equation
The fraction, fp, of a star’s light that is blocked out by an extra solar planet of radius Rp is therefore:
fp = (Rp / R*)^2
What is gravitational lensing?
This is the effect caused by light reversing a curved region of space time due to the gravity from a massive object. This can distort the image of what is behind it and in some cases cause brightness amplification.
What is an Einstein ring?
In the event of microlensing when the star lens and observer are in a perfect line Star will appear as a ring of light around the lens.
If the alignment is not perfect, it will result in just two images of the source either side of the lens.
What are the drawbacks of using microlensing to discover exoplanets?
The chance passage of the star and its planet in front of a background star, which is necessary for microlensing to occur, is unpredictable and unrepeatable. It is not usually possible to make follow-up studies of exoplanets discovered by this method.
Centre of mass and radii equation
M1a1 = M2a2
Where M is mass and a is radius from centre of mass of two objects orbiting cachothe.
What is astrometry?
The science of accurately measuring the position of stars is known as astrometry. This can be used to detect the wobble of a star caused by a planet orbiting it.
Doppler shift equation.
Δλ / λ = Vr / c
Change in wavelength / wavelength = velocity / speed of light
If an object is moving away the velocity and Δλ will be positive, if the object is moving towards you the velocity and Δλ will be negative.
What is radial velocity?
This is a component of an object velocity along the observers line of sight.
How is radial velocity linked to actual velocity?
Vr = vcos(θ)
Where θ is the angle between the line of site and the velocity of the object.
