I'm an astrophysics postgraduate and my goal is to study exoplanets as a profession. Extra-solar planets or exoplanets are planets outside of our solar system. I've always been interested in astronomy and have been gradually veering towards the exoplanets direction. It is a relatively new field of astronomy and there is so much more to learn about it. So here's a page to share some of the things that have been discovered so far. I'll update the news section as often as I can. The rest is just general information about exoplanets.

Latest News
A Brief History
Techniques for Discovering Planets
Planet Formation
The Possibility of Life


Latest News

The Hubble Space telescope has found 16 planet candidates in a field of 180,000 stars located 26,000 light years away in the central bulge of our galaxy. Only two of these planets could be confirmed by follow up measurements (using the doppler wobble tecnhnique with the VLT - Very Large Telescope in Chile) due to the large distance of the planets from us. Five of the new planets showed properties never seen before and have inspired a new category for exoplanets called USPPs (Ultra Short Period Planets). These planets orbit their parent stars (which are red dwarfs so they are smaller and cooler than our own Sun) with a period of less than a day.

The Spitzer infrared Space Telescope has discovered O-type stars, the largest types of stars - up to 100 times the mass of our Sun, can create too hostile an environment for planets to form around nearby smaller stars. The heat from the O star heats the proto planetary disks around other stars and essentially evaporates the material. The intense winds from an O star then blow away the material so the smaller star no longer has the ability to form planets.

SuperWASP (Wide Angle Search for Planets) has discovered two transiting planets, named WASP-1b and WASP-2b. They are both hot Jupiters and orbit their stars at 2.5 and 2 days respectivly. The planets were confirmed by the doppler shift using SOPHIE, a French instrument. SuperWASP is based in La Palma in the Canary Islands and also at Sutherland Observatory in South Africa. Together they have eight cameras which can survey the entire sky to search for dips in the light curves of stars.

An object (CHXR 73 B) 12 times the mass of Jupiter has been discovered orbiting a red dwarf. It is thought to be a brown dwarf because a large planet couldn't form out of the stars accretion disk at the distance that CHXR 73 B is from the red dwarf. Hot Jupiters can only form up to three billion miles from a star and this is nearly 20 billion miles from the red dwarf. It is about 2 million years old.

It was announced yesterday by the International Astronomical Union (IAU) that Pluto is no longer a planet. It is now what is known as a dwarf planet. There has been conflict for some time over the true nature of Pluto, as it is quite an oddball compared to the other eight planets. Also the discovery of further big Kuiper Belt Objects such as Sedna and 2003 UB313 ('Xena') added to the conflict, especially since 'Xena' is larger than Pluto. This definition is only applicable for objects in our solar system as we do not yet know enough about exoplanets to include them in the definition. The conclusion that the IAU came to yesterday is as follows: (taken from the IAU website)

Resolution 5A:
(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

Resolution 6A:
Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.



A twin planetary system with no parent star has been discovered by Ray Jayawardhana (University of Toronto) and Valentin D. Ivanov (ESO). Single planets like these have been found before, but never a binary system. Theses free floating planets are known as planemos. One planemo is about 7 times the mass of Jupiter and the other is twice that. They were imaged in the infrared using a 3.5 metre telescope in Chile. The name of the system is Oph1622. It is not entirely sure how these can form.

A Brief History

The search for exoplanets using astrometry began in the mid twentieth century, the most notable search being done by Peter Van de Kamp. He was certain that there were two planets around Barnard's Star. Although this has never been proven, his work paved the way for further research. The first unofficial exoplanet was found around the pulsar PSR 1829-10 by Andrew Lyne. It is generally not considered the first exoplanet because of its unusual parent star. The first proper planet was discovered in 1995 by Michel Mayor and Didier Queloz around the star 51 Pegasi. Since then there have been over 200 planets (as of October 2006) discovered and this number is constantly increasing.

Techniques for Discovering Planets

Astrometry was the first technique used to try and find exoplanets but it was never very successful. Astrometry is the direct measurement of the "wobble" of a star. A star will wobble if its centre of gravity is shifted from the centre of the star by a large object orbiting it. Detecting this is very difficult but should be possible in the future using interferometry. To make use of astrometry, we need really big telescopes. However that is not always possible because the bigger they are, the more expensive they are. Interferometry helps solve this problem. This combines the power of two or more telescopes at a given distance. Thus the area is bigger and it will have a better angular resolution. Several terrestrial telescopes have been proposed that will be able to use this to find exoplanets, such as the VLT (very large telescope) and OWL (Overwhelmingly Large Telescope). Some proposed space based projects are SIM (Space Interferometry Mission), TPF (Terrestrial Planet Finder) and Darwin.

Radial velocity (also known as Doppler wobble) uses the same basic idea as astrometry. It is measuring the stars wobble except in an indirect method. As the star moves back and forth its light is redshifted as it moves away from the observer and blueshifted as it moves towards us. These redshifts indicate that a planet may be present. This technique, along with astrometry can only be used for large planets orbiting with short periods. A period is the amount of time it takes a planet to orbit its star. If a planet is too small or far away from its parent star, it will not affect the centre of gravity of its parent star and thus they cannot be detected. We can currently measure radial velocities down to 1m/s. If we were to detect a planet as small as the Earth, we would need to be able to detect radial velocities down to 10cm/s.



Of the 200 known planets, about 30 have been discovered by the transit method. This is when the planet, its star and the Earth are directly aligned. Thus when the planet orbits its star, it casts a shadow over the star. This can be observed in our own solar system with Venus and Mercury. However we cannot actually see these distant transits. Instead we have to detect the slight dip in the light of the star as the planet passes in front of it. This is usually between 0.3 and 3 %. Once again this is only possible with planets of high mass and low orbital period and for transiting planets; negligible eccentricity.

With transiting planets it is possible to get much more data than if it was observed via RV (radial velocity). For example if the mass and radius are already known, then other properties such as density and surface gravity can be found. Also because the inclination (i.e. it has a "flat" orbit. E.g. all the planets in the solar system lie in the same plane but the Kuiper Belt Object, Pluto, is inclined at 13 degrees) is zero or close to it, reflected spectra of the star can easily be found in the orbital phase of the planet. These spectra are copies of the stellar spectrum and would contain absorption lines of heavy elements that are only produced in stars. If you were to search for a reflected spectrum in a planet found by RV, you would have to search over all possible inclinations, as inclination is hard to determine with these planets. Another interesting thing that can be determined from transiting planets is sunspots on the parent star. If the planet passes in front of a sunspot it causes a deviation on the light curve. This enables the sunspot to be monitored and thus the activity of the star.



Microlensing is gravitational lensing on a smaller scale. Gravitational lensing happens when an object of large mass such as a galaxy has such large gravity that it can bend the light of an object behind it. The observer then sees the source object as a distorted image. It can appear as a ring (known as an Einstein Ring - this can be seen when the source, lens and observer are directly aligned), arcs or multiple images.

With microlensing the lens is usually a star towards the galactic centre. Because a star has much less mass than a galaxy, the only effect observed is the brightening of the source object, which is usually another star. This can be represented on a light curve. If there is a planet present orbiting around the source star it can be observed as an anomaly in the light curve. If the planet is at a distance greater than the Einstein Ring, then a spike appears in the light curve. If the planet is at a distance less than the Einstein Ring, then a dip appears in the light curve. Microlensing is the only current technique is not reliant on large planets. It is possible to detect Earth size planets. However these microlensing events cannot be predicted or repeated and it is only by chance that we can find these planets. 4 planets have been found to date using this method, the smallest of which is only 5.5 Earth masses.



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Planet Formation

Compared to exoplanetary systems found so far, our solar system seems to be the exception to the rule. But this could just be because we are still incapable of detecting Earth size planets. All planets are formed from the accretion disk around a young star. Bits of dust clump together and collide with each other until they eventually form objects of significant size. In our solar system the inner planets are small because there was less dust to be accumulated in those small orbits. The giant planets formed further out. They have larger orbits which allowed for more dust to accumulate. Also because of the increased distance from the Sun, when the giant planets were forming, they not only formed from dust but from ice grains too. When a planet is several times the mass of the Earth, it then has enough of a gravitational pull to attract lighter elements and that is what forms a gas giant.

When the first planet was discovered, it caused much confusion. How could a planet so large orbit its star so closely? From our own solar system we were sure that gas giants cannot form so close to a star, because there is not enough material in the protoplanetary disk. Hot Jupiters are so called because they are similar in size to Jupiter, i.e. gas giants, and because they are very close to their stars making them "hot." These planets can have periods of just a few days.
It is now thought that these gas giants do in fact form at a distance from their parent star and then migrate towards the star. This can happen if several large planets are in unstable orbits because of the excessive gravitational attractions and one is "pushed" out of the system, meaning the remaining planets would move inwards. It is also thought it can happen due to spiral density waves. The planets' orbit around the star causes spiral density waves in the accretion disk. These waves travel away from the planet and thus it loses angular momentum, which in turn changes its orbit and causes it to migrate towards the star.
Another element of confusion is how Hot Jupiters can exist so close to the star without evaporating. 15% of known planets are closer than 0.1 AU ( 1 Astronomical Unit is the distance from the Sun to the Earth) to their parent stars. In fact these planets are evaporating and have a very short life span. Eventually just the core of the planet is left, which is called a Chthonian planet.

Super Earths are also known as Failed Jupiters. These are planets similar in size to the core of a gas giant. They have reached a mass that is great enough to start attracting gases such as hydrogen and helium, but fail to do so. This is because a star's accretion disk only lasts for between 5 and 10 million years. When the star is finished forming, the solar wind is switched on which proceeds to cause any remaining material in the disk to be removed from the immediate area. Super Earths are planets which would have been gas giants has the accretion disk of the star remained for a longer time.

If a Hot Jupiter gets to about 0.013 times the mass of the Sun (i.e. 13 Jupiter masses), there is a lot of doubt over whether it is still a planet or not. There is a type of celestial body called a Brown Dwarf, which is somewhere in between a hot Jupiter and a red dwarf (small star). The lower limit on the brown dwarfs is very uncertain, as there is no sure way to tell if it is a large planet or a small brown dwarf. A brown dwarf is a "failed star". It starts its life similar to an ordinary star, i.e. collapsing dust and gas, but do not have sufficient core temperature to fuse hydrogen to helium. The temperatures range from 300 to 3000 Kelvin where as a red dwarf would be about 4000K.
A star above 0.08 solar masses can rapidly fuse lithium into two helium nuclei by colliding with a hydrogen nucleus. Therefore one way to test to see if it is a brown dwarf is to see if lithium shows up in the spectrum. A red dwarf will have little or no lithium, as it depletes quickly once fusion starts. In a brown dwarf between 0.075 and 0.08 solar masses it is possible for a small amount of fusion to take place, but it quickly dies out. So the upper limit on brown dwarfs is more defined than the lower the limit, though there is still a margin of error.
Brown dwarfs are not very luminous and become less so as they age, so it is very difficult to detect them in the visible spectrum. One way of finding them is by using the radial velocity method. Another way to find them is to look at white dwarfs. A white dwarf is a dead star that no longer has nuclear fusion in its core. It emits mostly in the visible and ultraviolet range of the spectrum. So if a white dwarf seems to have a lot of infrared radiation associated with it, it could mean that it is actually in a binary system with a brown dwarf, since brown dwarfs emit mostly in the infrared.

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The Possibility of Life

The main reason the search for exoplanets is so exciting is because there is hope that we might find life on other planets. Although it would be naive to think that we are completely alone in the universe, due to the sheer size of it, any life out there will be few and far between. There are so many factors that made it possible for life to begin and evolve on our own planet, that finding a planet that recreates these conditions will prove difficult. All we have to go on is our own planet but it is possible that life can exist on other types of planets. The factors that are believed to have led to life on our planet are as follows:

The most important thing in looking for a planet with life on it is if the planet lies in the habitable zone of its star, as we do. The habitable zone is the area around a star where it is possible for liquid water to exist and the temperature is not too hot or not too cold, earning it the nickname the "Goldilocks Zone." The correct temperatures to sustain liquid water can only be found between 119 and 245 million kilometres from our Sun. Our planet is at 149.6 million kilometres from the Sun. The radius of the habitable zone is proportional to the size of the star. Eventually as the Sun expands into a red giant, the habitable zone will move outward and eventually Earth will no longer be able to sustain liquid water.

The type of star is very important. For example for a planet orbiting a pulsar, it is thought life cannot exist due to the strong magnetic fields. If a planet is orbiting a giant star, any life that forms would not have long to evolve due to the shorter life time of larger stars. Also Hot Jupiters cannot form around red dwarfs because of the insignificant core mass in the accretion disk of the stars.
It is possible that we could come across a planet that had life on it that is now extinct. For example, eventually our Sun will get so hot that plant life will no longer be able to exist and the Earth will become dusty and rocky. This is turn will mean a reduction in oxygen. Although prehistoric animals survived in lower oxygen levels than today, eventually the temperature will become so hot that they will not survive.

Jupiter acts as a sort of shield for the Earth. Most of the comets and asteroids that come from further out in the solar system are either captured or destroyed by Jupiter's immense gravity. The most famous example is the comet Shoemaker-Levy 9 which crashed into Jupiter in 1994. Because of Jupiter we suffer from far fewer impacts that we should. Other planets in other systems may not be so lucky as to have a large mass planet further out than them and this would influence the survival rate of any life there.
Our moon is the biggest in the solar system relative to planet size. It is roughly 1/6 the size of the Earth. Because of this, its gravity influences us, which can be seen in the tides of the oceans. We are currently tilted at an angle and rotate around it once every 26,000 years, which is not noticeable in our lifetimes. Without a large moon, our planets axis would not be stable. The axis of the Earth would change randomly. Because we have a stable axis, we have regular seasons, which is essential for crop growth etc. The lack of a moon does not necessarily rule out life on a planet, it just means it would be make for a very difficult life on that planet.

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