Physics Update Course Summer 2013


Dr Sarah Casewell

Observational astronomer

Department of Physics and Astronomy,

University of Leicester

Dr Casewell’s research interests are in white and brown dwarfs. She has published research on topics as diverse as: the first T dwarfs in the Pleiades, the white dwarf initial mass-final mass relation and new L dwarfs in Blanco 1, the Hyades and Praesepe.

She is currently researching brown dwarfs in close binaries with white dwarfs and is investigating how the atmosphere of the brown dwarf changes with irradiation.

Our system is quite unique in that it has predominantly small planets.


However a big planet doesn’t necessarily mean a heavy planet.


If it were possible then Saturn would float in a bath of water because it has such a low density.

The maximum size for a celestial body to be classed as a planet is thirteen Jupiter masses or less.


One of the first people to suggest there could be exoplanets was Giordano Bruno (1548 – February 17, 1600). He was an Italian Dominican friar, philosopher, mathematician, astrologer and astronomer. His cosmological theories went beyond the Copernican model: he proposed the Sun was essentially a star, and that the universe contained an infinite number of inhabited worlds populated by other intelligent beings. The Roman Inquisition found him guilty of heresy and he was burned at the stake. After his death he gained considerable fame, particularly among 19th- and early 20th-century commentators who, focusing on his astronomical beliefs, regarded him as a martyr for free thought and modern scientific ideas.

1991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar PSR1257+12 – Variations in arrival times of pulses suggests presence of three or more planets – Planets probably formed from debris left after supernova explosion.

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1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & Didier Queloz using the “Doppler Wobble” method. Most successful detection method by far, but other methods like transits is now very successful.

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A planet which is of similar proportions to Jupiter exerts a slight Gravitational pull on its parent star. This causes the parent star to “wobble” and the velocity of this wobble depends on the mass of the planet and the distance at which it orbits from the parent star. Currently, velocities of giant planets have been measured in the region of 1-100m/s.

901 exoplanets have been found so far.

Direct imaging

Directly images the planets but there are some problems because it only works for sufficiently large planets that are well-separated from their parent star and you need to subtract stellar contribution (using a coronagraph) before planet can be seen.


In 2008 3 planets were imaged around the star HR8799. The system is 130 light years away (40pc). The three planets have separations of 24, 38 and 68AU from their sun. In comparison, Jupiter is at 5AU and Neptune at 30AU from our Sun. The masses of the planets are 7 Jup, 10M Jup and 10M Jup.

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Fomalhaut (alpha Piscis Austrini)

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2Mjup planet imaged inside disk. The system is believed to be 200Myr old. Like our early solar system as shown in the above right picture.



Planet Hunting – RV

Radial velocity is the velocity of an object in the direction of the line of sight (i.e., its speed straight towards or away from an observer). In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity. The spectroscopic radial velocity is the radial component of the velocity of the source at emission and the observer at observation, as determined by spectroscopy. Astrometric radial velocity is the radial velocity as determined by astrometric observations (for example, a secular change in the annual parallax).

Planets orbit the Sun because the Sun is very large and has lots of mass, so planets are gravitationally attracted to stars, just like the moon and satellites are gravitationally attracted to the Earth.

But the planets have some mass too!

The Sun is also gravitationally attracted to the planets – the Sun actually wobbles a little bit as the planets go around it.

This happens to other stars with planets too. When the star moves towards us, the lightwaves coming from the star are slightly squashed and the light appears slightly bluer – we call this blueshifted. When the star moves away from us, the lightwaves are slightly stretched, or redshifted.

The Star + planet orbit have a common centre of gravity. As the star moves towards the observer the wavelength of light shortens (is blueshifted) and as it moves away the wavelength of light increases (is red-shifted).

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456 planets have been detected by the Doppler Wobble including 160 multiple systems.

Planet hunting – transit

Sometimes, if we are lucky, planets can cross in front of their parent stars as seen from the Earth. This is called a transit. As the planet crosses in front of the star, it blocks some of the star’s light. If we measure the amount of light coming from the star, we can measure how much light is blocked by the planet, which can tell us the size of the planet.

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The above picture on the left shows the change in brightness when the planet passes in front of the star. The above picture on the right shows the effect of the planet passing behind the star.

From the methods outlined, we can often compute various parameters of the planetary system, for example mass and radius of planet, and distance from the parent star. If we know the spectral type of the parent star and we know the distance to the planet, we can work out whether or not the planet is in the habitable zone of the star. This is highly dependent on stellar spectral type.

In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its photosphere (the atomic excitations that are most prominent in the light), giving an objective measure of the photosphere’s temperature. Light from the star is analysed by splitting it with a diffraction grating, subdividing the incoming photons into a spectrum exhibiting a rainbow of colours interspersed with absorption lines, each line indicating a certain ion of a certain chemical element. The presence of a certain chemical element in such an absorption spectrum primarily indicates that the temperature conditions are suitable for a certain excitation of this element. If the star temperature has been determined by a majority of absorption lines, unusual absences or strengths of lines for a certain element may indicate an unusual chemical composition of the photosphere.

SuperWASP is the UK’s leading extra-solar planet detection program comprising of a consortium of eight academic institutions which include Cambridge University, the Instituto de Astrofisica de Canarias, the Isaac Newton Group of telescopes, Keele University, Leicester University, the Open University, Queen’s University Belfast and St. Andrew’s University. SuperWASP consists of two robotic observatories that operate continuously all year around, allowing us to cover both hemispheres of the sky.

Wide Angle Search for Planets (by transit method). The first telescope was located in La Palma and the second in South Africa. Operations started in May 2004. More than 65 new planets have been detected by WASP with 231 by transit experiments in total.

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SuperWASP monitors about 1/4 of the sky from each site. That means millions of stars, every night!


The successor to SuperWASP is NGTS (Next-Generation Transit Survey). It is a wide-field photometric survey designed to discover transiting exoplanets of Neptune-size and smaller around bright stars (magnitude V<13).

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It is easier to detect an “Earth” sized planet if its sun is small.


Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft, named for the Renaissance astronomer Johannes Kepler, was launched on March 7, 2009.

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Specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets”,

It is searching for Earths by the transit method. It aims to find an Earth around a Sun-like star with a one year orbit. Three transits are needed to confirm the presence of the planet. So the mission will last at least three years…

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As of July 2013, Kepler has found 134 confirmed exoplanets in 76 stellar systems.

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Detection biases: lots of large close in planets, which are more likely to

a) transit the star from the perspective of an observer

b) affect the star’s radial velocity in a measurable way.

So, as yet, no other ‘solar systems’ found with small, rocky planets out to about 2 AU and gas giants at 5 AU and beyond.



So if we want to find out even more about planets outside our solar system, we need to start looking at their atmospheres.

This is not straightforward as often all we can see from Earth is a single, unresolved point of light that includes radiance from the star and any planets orbiting it.

We use a method called transit spectroscopy to study the atmospheres of extrasolar planets.


We need to use the UV to IR part of EM spectrum as stars emit light mostly in visible region and planets are cooler, so emit in IR. Gases absorb IR light and clouds scatter UV – IR light. Comparing light across these wavelengths can tell us lots about atmospheres!


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The atmosphere of this planet is highly absorbing in red wavelengths, but it is transparent in blue wavelengths. So in a blue filter the planet looks opaque, but the atmosphere is still transparent. In a red filter, however, the atmosphere also looks opaque.

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So, in wavelengths where the atmosphere is opaque (i.e gaseous absorption lines) the measured planet radius is apparently increased during primary transit.

In secondary transit, what is effectively measured is the emission spectrum (light at all different wavelengths) from the dayside of the planet:

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Opacity varies as a function of wavelength. Different wavelengths are sensitive to radiation emitted/reflected at different altitudes.

By carefully monitoring the total amount of light coming from a star/planet system over time, we can work out what the atmospheres of exoplanets are made of.

It can be tricky though:

You need a very high signal-to-noise (and these signals are tiny);

Stellar activity (star spots) can cause problems;

It is much easier for larger, hotter planets than smaller, cooler ones;

Degeneracy (where two or more different states of a system have the same energy level) is the bane of the atmospheric physicist’s life. Several different atmospheric scenarios provide an equally good fit to data.

Over to you…

Kepler archive data is available to everyone.

Planethunters website

Kepler website has a huge amount of resources so…. Can you spot a planet?

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Planet/Not a Planet (1)?


Planet/Not a Planet (2)?


Kepler-8b is the fifth of the first five exoplanets discovered by NASA’s Kepler spacecraft, which aims to discover planets in a region of the sky between the constellations Lyra and Cygnus that transit (cross in front of) their host stars.


Planet/Not a Planet (3)?


Planet/Not a Planet (4)?

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Planet/Not a Planet (5)?


Planet/Not a Planet (6)?



1) Not a planet. It is an eclipsing binary.

2) A planet.

3) Not a planet. It is a white dwarf.

4) Not a planet.

5) Not a planet.

6) A planet. It is Kepler 22b with a temperature of 262 K. Radius of 2.3 Rearth Period P = 289 days. Mass M < 0.11 MJup

Kepler-22b is an extrasolar planet orbiting G-type star Kepler-22. It is located 600 light years away from Earth in the constellation of Cygnus. It was discovered by NASA’s Kepler Space Telescope in 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star.


I have to admit I was rubbish at this exercise. H Hare

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