Search for Other Worlds
By: Peter Starr
Date: 12th June 2004
INTRODUCTION
In 1961, Frank Drake formulated the Drake Equation, estimating the probability of the existence of civilisations in the Galaxy that possess the technology to communicate with Earth. One of the key variables of the Drake Equation is the fraction of stars that contain planets (Fp). In 1961 the only known planets were orbiting our sun, therefore Fp could range between 1 in 100 billion to 100 percent. Considering our galaxy contains 100 billion stars and the universe contains many billions of galaxies, the probability is that there must be other solar systems. The challenge is being able to detect them.
The only planets directly observed exist in our Solar System. No planets have been observed directly orbiting other stars. This is due to the fact that stars are very far away and any reflected light on any planet would be negated by the glare of the host star. The difference in brightness is about a billion times. Our solar system is therefore the only solar system we know about. We know that nine planets revolve around the sun in the same direction, they lie in a single plane, they have almost circular orbits, small planets lie close to the Sun, larger planets are further away from the Sun and some planets have moons in orbit around them. Is this configuration typical? If other solar systems exist, do they have similar characteristics to ours? To determine this, we need to know how our solar system formed as well as developing technology to detect exosolar planets. If all stars and solar systems form in a similar way to our star and solar system, we can assume that most stars have planets with similar properties to those in our solar system.
Modern theory is that the solar system formed from a cloud of dust and gas. The cloud condensed into a rotating flat disk with the protosun forming at the centre under contraction. As the protosun contracted it became hotter. The inner region of the disk, only the high melting point compounds could condense like iron and rock. These formed the planetisemals which collided together to form the rocky planets. In the outer region of the disk, the temperature was cooler. Ice condensed and became part of the planets. Because of the large area, the planets grew large and had enough gravity to hang onto gaseous material such as hydrogen and helium which was plentiful in the solar nebula. Icy bodies like Pluto and the Kuiper belt escaped collision due to the even larger area of the solar system at those distances. Many of the objects were flung out of the solar system due to the gravitational affects of Jupiter and Saturn. Many were locked into synchronous orbits by the effect of Neptune.
This theory explains the characteristics we see in the solar system very well. The theory explains how all other stars are formed. As the ingredients in our solar system are found throughout the galaxy in clouds of gas and dust where we see new stars being born, we can assume that these stars will form planets as well. Disks around new stars called proplyds have been observed, so it can be safe to assume our solar system is not unique. Many have been observed in the Orion Nebula.
However not all stars are similar to the sun. They come in a great variety of sizes, different metalicities and half are in doubles or higher multiples. These different environments may affect how different solar systems form if at all. Higher mass stars evolve more quickly and may not have had the time for planets to accrete. Double stars may have no planets at all as one of the stars may kick out any forming planets by their gravity from the other star. This is similar to why no planet formed where the asteroid belt is. Jupiters gravitational influence left a pile of rubble which is now the asteroid belt. An analysis was done on the Alpha Centauri system [15]. The Alpha Centauri system contains 3 stars, Proxima and Alpha Centauri A and Alpha Centauri B. A and B are on average 23 AU apart and circle a common point every 80 years. Ignoring Proxima Centauri which is 10 000 Au from the centre of the system, and placing B in our solar system at 23 AU and remove Jupiter and Saturn, planets could exist however their orbits would only be stable for a billion years. The planets are unlikely to form as the binary would affect the orbits and eccentricities of planetesimals and would lead to disruption rather than accumulation into planets [15]. However this would depend on how close or far apart the binary members are.
The first sensible place to look for planets are stars close to our sun that are F, G, or K class and are single stars of high metallicity. This gives the greatest chance of finding solar systems like our own.
Methods of Detection of Exosolar Planets
There are two ways to detect planets around other stars. This is by direct observation of the light reflected from the planet or by indirect observations of the planet�s influence on the host star. Planets can be indirectly detected by Astrometry, Radial Velocity, Transits, Gravitational Microlensing and Interferometry.
Direct Observation of Exoplanets
Direct imaging of planets is extremely difficult due to the differences in brightness of the parent star and its planet and the light scatter from the dust that surrounds the star.
For example, Jupiter is only one billionth the brightness of the sun. To distinguish Jupiter from the sun if observing from Alpha Centauri, a 40 metre telescope would be required to resolve Jupiter from the sun [12].
The advantages of direct imaging would be that the spectrum of the light from the planet could be analysed. This would tell us what chemical constituents would be in the atmosphere of the planet by observing the dark absorption lines. This could hint on if there was life on the planet. Detecting the absorption bands of oxygen would be a tell tale sign of life.
Planets could be observed directly by blocking out the light from the parent star. This can be done using the nulling method or the coronagraph method. The nulling method uses an interferometer. It interferes with the light from the star cancelling it out. Infrared light would then be looked for which could be the tell tale sign of a planet. The planet star contrast is weaker at infra red wavelengths. The coronagraph blocks out the star except for its corona and any orbiting planets [6].
There are two plans to be able to view planets directly. They are The Stellar Interferometry Mission and the Terrestrial Planet Finder Mission. The aim is to use multiple telescopes in space. The resolution required to see planets is achieved by having a very wide baseline between telescopes. Earth size planets up to 50 light years away could be observed using 25 40 metre telescopes several kilometres apart in space [12].
Astrometry
All stars in the galaxy orbit around the centre of the galaxy. If we look long enough at a star we will notice it change position given enough time to look. This motion is called the proper motion of the star. Stars that are closer to us will appear to have faster proper motions than stars that are further away. For example Barnards star is 6 light years from our position and moves at 10 arc seconds per year.
A planet orbiting around a star will cause the star to wobble in its proper motion as viewed from Earth using the background stars as reference points. This is the result of the star planet system orbiting around a common centre of gravity. Observing the wobble over time a sinusoidal motion is observed. Observing from Earth the size or amplitude of the wobble depends on the mass ratio of the star to the planet, how far away the star is from Earth, how far the planet is from the star, and if the wobble motion is perpendicular to our line of sight. This technique is known as Astrometry or the Wobble Method [6].
Due to the huge difference in masses between a star and its planet(s), the wobble is very small. The level of precision required to measure this wobble was on the edge of technology. A precision of 0.5 milliarcseconds would be required to detect the wobble of the sun due to the influence of Jupiter from 30 parsecs distance. Ground based telescopes can achieve a few milliarcseconds [46].
A disadvantage is that it can take many years of study to confirm a wobble is caused by a planet especially if its orbital period is a number of years. The period from one peak in the sinusoidal curve to the next is equivalent to the period of the planet. Several periods would have to be observed before a planet can be confirmed. Another disadvantage is that the host star needs to be relatively close for us to observe the wobble. The further away a star is the smaller the wobble is and the wobble is lost in the noise of the measurements.
Advantages of this method over others is that the orbit of the planet does not have to be edge on like the transit method and it gives an accurate value for the mass of the planet unlike the radial velocity method which gives an estimate of the lower mass. The inclination of the orbit can also be calculated. The technique will make it easier to detect planets far from the star as the shift will be greater. This is the opposite of the radial velocity method which detects high mass planets at close positions to the host star.
Astrometry is the oldest technique used for detecting exoplanets. It was first used by Kaj Strand of Sproul Observatory who claimed to have detected a wobble in the motion of 61 Cygni. However the results are treated with scepticism and the �planet� has not been confirmed. Sarah Lippincott announced her findings of a wobble of Lalande 21185 in 1960 from the same observatory. The wobble was said to have been caused by a planet in orbit of 10 Jupiter masses [45].
Barnards star was the next candidate for this technique as it has the largest proper motion of any star (10.3 arcseconds per year [34]) and it is relatively close to the Earth (6 light years). In the early 1960s an astronomer named Peter van de Kemp published results from the study of 24 000 photographic plates taken of Barnards star over a period of 46 years. He detected a periodic wobbling motion of the star in its proper motion. He deduced this to be an unseen planet about 1.6 times the mass of Jupiter revolving about Barnards Star every 24 years with a highly elliptical orbit. He later refined this to 1.7 Jupiter masses and a 25 year orbit. Later he published another paper reporting there were 2 planets in a circular orbit. One planet revolved around every 12 years and the other every 26 years. This was updated to 0.4 and 1 Jupiter masses and orbital periods of 22 and 11.5 years. He decided to take more photographic plates and in 1982 reported that the masses were 0.7 and 0.5 Jupiter masses with periods of 12 and 20 years. The masses were 1.1 and 0.8 Jupiter masses. However Gatewood and Eichhorn in 1973 challenged his results and found no evidence of a wobble. Herschey later examined Peter van de Kemp�s plates and noticed that other stars on the plates wobbled as well to the same extent as Barnards star. The wobble of Barnards Star was actually an error in the telescope used to take the pictures. As yet there are no confirmation of van de Kemps Planets [35,36].
In 1996, Gatewood detected Lalande 21185 to wobble. This star is 8 light years away and is a red dwarf M2 type. It is the 4th closest star system to Earth. Gatewood attributed this to an unseen planet. Data was collected from photographic plates take between1930 to 1984. The planet lies within 2.2 AU of the star, is 0.9 Jupiter masses, and has a period of 5.8 years with a circular orbit. There maybe 2 other planets much further out. The degree of wobble is 0.002 arcseconds [26].
Epsilon Eridani was also studied by Peter van de Kemp. He detected a wobble in its proper motion as well and put this down to a planet. Further studies using radial velocity and IRAS suggest there is a dust cloud around the star and perhaps a planet of 1 to 5 Jupiter mass. This planet has not been confirmed [35,37].
The existence of these planets has not been confirmed by other detection methods.
High precision astrometry can be achieved by long baseline optical interferometry at the Palomar interferometer. This could detect planets of 0.5 Jupiter masses. It also has the advantage of calculating the inclination of planets around other stars unlike the radial velocity method. Also it can detect planets that are widely separated from the star unlike the radial velocity method [27].
Interferometry is also planned at the Keck observatory. This will produce precisions of 10s of microarcseconds on stars up to 25 parsecs away. This technique works by having a very large baseline between two instruments on the ground. The baseline needs to be larger than the distance between the beams from two instruments at the top of the atmosphere. This is generally more than 10 km. The position of the two instruments needs to be more than 10km apart to achieve greater precision and accuracy. The use of phase referencing takes out the interference from Earth�s turbulent atmosphere.
The future of astrometry for detecting planets lies in observations from above the Earths atmosphere. There are two projects under plan NASA's Space Interferometry Mission (SIM), scheduled for 2009, will be able to make angle measurements of single stars as accurate as 2 micro arcseconds. The European Space Agency's Gaia mission, currently planned for 2010, will make wide-angle observations of hundreds of millions of stars at an accuracy of around 10 micro arcseconds.
Radial Velocity Technique
Also known as the Doppler spectroscopy method [6] involves detecting small changes in the light received from a star. The dark absorption spectral lines in the stars spectrum change their wavelengths in a periodic fashion [10] when the star is revolving around a common centre of gravity influenced by other objects in the same system. If the plane of the star and the orbiting object are edge on to our point of view, the star will periodically move away, stop, then move back towards us, then stop, the move back away from us. The spectrum of the star is red shifted when it moves away and blue shifted as it moves closer. We can measure the radial velocity over time and plot it on a graph similar to that in Figure 2.
Figure 2. Observation of the radial velocity of 51 Pegasi [11].
A sinusoidal curve results. The amplitude of the curve is dependant on the gravitational influence of the body or bodies in orbit about the host star. It depends on the mass of the object and how far away it is from the host star. The objects in the system may be a companion star, a brown dwarf, or a planet.
The period between each peak is the orbital period of the planet.
Using Keplers 3rd law the distance from the host star can be calculated.
r3 = GMstar P2 / 4?2
where
r is the semimajor axis.
G is the gravitational constant.
Mstar is the mass of the host star.
P is the period of the planet
The mass of the star can be estimated from its position on the HR diagram and by knowing its luminosity assuming it is a main sequence star.
v = (2 ?G/P)1/3 mp sin i/ Mstar 2/3 .
where
v = the amplitude or radial velocity of the planet.
mp = the mass of the planet.
i = the inclination of the planet
Thus the mass of the planet can be calculated if the inclination of the planet is known. If this is not known then �i� is assumed to be edge on to our viewing point. This gives us an estimation of the minimum mass [23].
The effect is very small when a planet is in orbit around the star as the orbits of stars are quite slow. For an observer analysing the suns spectrum, the spectral line shift caused by Jupiter would be 2.6 x 10-5nm or 1 part in 25 million [10]. This is the result of the sun moving around its orbit of a common centre of gravity at 12.5metres per second. Saturns impact on the sun is 2.7 metres per second.
The extrasolar planet search at Haute � Provence Observatory has precisions of 8 metres per second (ms-1) by the Elodie spectrograph and 5 metres per second by Coralie detector at the Geneva Observatory. This is good enough to detect Jupiter size planets. The detector at Lick observatory has a precision of 3ms-1 using an iodine absorption cell [14]. This is accurate enough to detect Saturn size planets. To detect an Earth size planet around a G type star a precision of 0.1ms-1 is required [24]. An Earth size planet around a M5 type star with a closer orbit than 1 AU in order for liquid water to exist, the radial velocity to detect would be 0.63ms-1 [24].
The easiest planets to detect using the radial velocity method are Jupiter size planets close to their host stars as these create the largest amplitudes in the radial velocity curve. In our model of solar systems, Jovian planets form far from a star where temperatures are low enough to allow the build up of hydrogen and helium to form large planets. These planets cannot be formed close to a star where it is just too hot to allow the build up of a hydrogen gas envelope. However using the radial velocity method, this is exactly what is observed. Most of the planets discovered are in orbits very close to the star, even well inside the orbit of where Mercury�s orbit would be. Some planets have highly eccentric orbits. These planets do not fit into our model of how a solar system forms.
The first planet detected by this method was around a star named 51 Pegasi (type G5V) lying 48 light years away from Earth (reference [13] quotes 42 lightyears and a type G2V star). This was discovered by Mayor and Queloz of the Geneva Observatory in Switzerland in 1995 using the Elodie spectrograph [10]. The planet has a mass slightly less than half that of Jupiter but orbits 51 Pegasi at a distance of 0.051AU. Mercury�s distance from the sun is 0.4AU. It revolves around the star in just 4.2 days! How can this be? Is the method of detection and calculation of orbits incorrect or is our model of the way planets form around stars incorrect? The planet this close to the star would be a 1000 degrees C. It would be a molten ball of rock and iron and probably about the size of 7 earths [13]. It would have retained most of its atmosphere due to the high gravity of the planet [14].
These measurements were later confirmed independently by Marcy and Butler at Lick Observatory [14]. There is also evidence for another planet much further out but this has not been confirmed.
Variations in orbital velocity can also be caused by pulsation of the star and by spot rotation [14]. In the case of 51 Pegasi, this has been ruled out. However the star has been monitored for transits, none have been found. David Gray of the University of Western Ontario suggested that the data was flawed and the planet does not exist. He claims that the absorption bands are not moving but changing shape. He claimed that this was due to the star pulsating in a complex way. His paper however was never published and he retracted his findings later when better instrumentation proved that the absorption bands were moving [17].
The second planet to be discovered by the radial velocity method was 70 Virginis. This planet orbits in an eccentric orbit, which takes 116 days and has a mass of 9 Jupiters (ref [19] quotes 6.6 Jupiters). There are speculations that this massive planet maybe a brown dwarf [15].
Figure 3. Radial Velocity Curve for 70 Virginis [16]
The orbit takes the planet from 0.17 AU to 1.8 AU. [19]
Another planet discovered was found orbiting around a sun like star 16 Cygni B from Lick Observatory in 1996 [22]. The variation in radial velocity is 43.9ms-1. This planet has a mass of 1.7 Jupiters, but its orbit varies from 0.6AU to 2.7AU. 30% of stars discovered using the radial velocity method have highly eccentric orbits. The planets orbit ranges from 0.56 AU to 2.8 AU. Like the planet orbiting 51 Pegasi, it is thought that the planet originally formed in a circular orbit. The star is in fact a binary system. This may have caused the orbit of the planet to change into an eccentric one. The discoverers in their paper have not discounted that the planet may be a low mass brown dwarf [22]. This is the first planet found in a binary system [21]. 16 Cygni A would appear twice the brightness of our moon as viewed from the planet of 16 Cygni B. The two stars are about 840AU apart.
Figure 4. Radial Velocity Curve for 16 Cygni B [22].
Note the saw tooth like pattern. This is interpreted by the planet having a high eccentricity. A circular orbit would give a sinusoidal curve.
Figure 5. Radial Velocity Curve for 16 Cygni A [22].
This does not have the saw tooth curve which rules out the possibility of a systematic error in the data.
It is possible that these planets formed according to our current theory of stellar and planetary formation. The Jovian type planets may have formed far out from the star originally. The planets may have spiralled inward after their formation. If a large amount of gas and dust still existed in a disk around the star after the planets formed, the interactions may cause this spiralling in of the planets. Interactions between planets may cause the highly eccentric orbits we see [10]. Another possibility is that the system is affected by nearby stars in their formation in the same interstellar cloud. If the star is much more massive the winds produced by the star may have affected the orbits of the planets.
A disadvantage with the radial velocity method is that it can only give an estimation of the minimum mass of the planet. This is because the plane of the orbits relative to our view is unknown. If the plane is edge on to our view the mass estimated would be close. But if the orbit is inclined to our view the mass would be much higher. The planets could well be Brown Dwarfs. Brown dwarfs form as stars do from the contraction of the gas from the solar nebula. There masses are not sufficient enough for fusion of hydrogen into helium to start. Brown dwarfs may have masses more than 13 Jupiters.
Planets have been found that behave in the way we expect. An example is the planet orbiting 47 Ursae Majoris. It was discovered by the Lick oberservatory in 1996. The planets orbits at 2.1AU and its orbit is circular. Its mass is 2.4 Jupiters. It is in the region where it may be possible to have liquid water [13].
Figure 6. Radial Velocity Curve for 47 Ursae Majoris [16]
Transit method
When a planet in orbit around another star moves across the face of the star as viewed from Earth, we see a temporary small drop in brightness. The larger the planet is the larger the brightness dip. The brightness will drop by an amount proportional to the relative areas of the 2 objects. An earth sun system would have a 10-4 drop in brightness [24].
Figure 7. Figure based on one by Hans Deeg, from 'Transits of Extrasolar Planets' [42]
Disturbances on the star itself may cause fluctuations in brightness such as sunspots. So the star would have to be monitored for some time to show that the fluctuation is periodical as the planet revolves around the star. If the planet is like the Earth, we would have to wait for a year for a repeat performance. If the planet is close to the star like the planets discovered to date, the transit would repeat after a few days.
Binary stars would be a good place to start searching for planets as we would know whether the system is edge on to our point of view by the binary stars eclipsing. Any planets in the system would likely be in the same plane. Unfortunately binary stars may not form planetary systems. The best binaries to look at would be red dwarf stars as a greater percentage of light from the star would be eclipsed by a transiting planet. The best chance to find a planet with this method is to analyse as many stars as possible by looking to the centre of the galaxy. Detecting lower mass planets in these systems would be easier than detecting them around sun like stars.
HD 209458 is a star that has a planet orbiting around it and was detected by the radial velocity method by Marcy, Butler, Vogt in 1999. Measurements showed that the mass was 0.63 Jupiters and has a period of 3.523 days. This planet has subsequently been detected by the transit method by Henry and was the first planet outside of our solar system. The transit time match that predicted by the radial velocity method and it also predicted the actual time of the transit! The drop of magnitude of the star was 0.017 magnitudes. This is independent confirmation that exosolar planets exist and that the radial velocity method does detect real planets. The transit method can tell us the size of the star by knowing the drop in magnitudes and the size of the star by its spectral class. Along with the mass known from the radial velocity method and also knowing that it is edge onto us the lower mass calculated by the radial velocity method is its actual mass, the density of the planet can then be calculated. This planet has a lower density than Jupiter as its mass is less but its size is larger. This fits the theory that gaseous planets close to their star are bloated [41].
Figure 8. [40] Radial velocity curve of HD 209458. The data around 0.5 is due to the rotation of the star, as the planet blocks of the approaching and receding limbs of the star.
Figure 9. Hubble Space Telescope version of the HD209458 tranist light curve.
[39]
Marcy has discovered many planets by radial velocity measurements and they are examined by Henry to find the transit. The others detected before hand did not show any transits, but this is because the system is not edge onto us. There is a 10% chance that the �hot Jupiters� discovered by Marcy will have transits.
Since then elements have been detected in the atmosphere of the planet by looking at the absorption spectrum of HD209458 in transit and out of transit. The difference in the spectrums is due to the planet. Sodium, oxygen and carbon have been found. Hydrogen has also been detected coming from a comet like tail behind the planet. The planet is believed to be slowly losing its atmosphere as it boils off because of its close proximity to the star [43].
The Ogle survey, which was originally intended to search for microlensing events, has found 137 planetary transit candidates from 155,000 stars observed. 3 stars have been confirmed to have planets to date. A Jupiter size planet would block about 1% of the light as it would cover 1/100 of the disc of a sun like star. The survey is looking at a brightness dip of a few percent [1].
Under development is a project named Kepler. This will be a space based observatory. It will focus on 100,000 stars in a star field for 4 years. As the chances of observing a transit of a planet across the face of a star are very small, looking at 100, 000 stars raises the chances of observing many planetary transits. The sensitivity will be able to detect Earth size planets [45] and even Mercury Size Planets [50]. Its main mission is to detect Earth size planets in the habitable zone around stars in our neighbourhood similar to our sun. The observatory is to be launched in October 2007.
Microlensing
A star can act as a lens brightening and focusing a more distant star behind it. The lens star focuses the light from the source star by a factor of 2 to 5 times. The magnification varies smoothly and predictably as the lens star moves across the line of sight of the source star. A planet around the lens star may increase the brightness to up to 100 times in a few hours. The star � planet acts as a double lens. It has the ability to detect low mass planets.
Microlensing surveys are finding dozens of star � star microlensing events per year and it is possible that a planet could be discovered this way in the near future. The planets would need to be 3 to 6 AU from the lens star to be detected. This is called the lensing zone. There is an 18% chance of detecting a Jupiter size planet around a star during a lensing event. By extending these surveys 100s of planets could be discovered of all sizes in the next decade. The highest probability of finding a lensing event is to look towards the centre of the galaxy [24].
Figure 10. Pictorial demonstration on how microlensing works. [24]
.
Figure 11. The affect a planet has as a double lens. [29]
A planet has been discovered by this method by the OGLE and MOA teams in July 2003. The microlensing event named OGLE 2003-BLG-235/MOA 2003-BLG-53 is caused by a 1.5 Jupiter mass planet orbiting a star at 3AU. [31] Below is a chart of the light curve.
Figure 12. Light curve of OGLE 2003-BLG-235/MOA 2003-BLG-53 in different time scales: [31]
The first planet discovered by microlensing is MACHO-97-BLG-41 in June 1997. This is the first planet to be discovered in a binary system. The 2 stars are separated by a distance of 1.8 AU. One star is a K type star while the other is a M type star. The planet has a mass of 3 Jupiters and orbits at a distance of 7AU [32].
Macho �98-BLG-35 was the next event to find a planet through microlensing. This planet has a mass in between that of Earth and Neptune.
Pulsar Planets
The first planets around a pulsar was detected in 1991 by Wolsczcan and confirmed in 1994 using the Arecibo radio telescope. They are detected by measuring the variations in the pulsar speed of millisecond pulsars, which can be interpreted as the gravitational effects of planets. Millisecond pulsars are very stable and have no variation in their pulsar timings. The glitches that are observed from young pulsars resulting from neutron star seismology are not observed in millisecond pulsars, therefore the explanation for theriodical change in timings is due to planets in orbit around the pulsar.
Three planets were detected orbiting PSR 1257+12. One planet is the size of the moon while the other 3 and 4.3 times the size of the Earth [13, 53]. It is thought they were produced from the supernova and not from the original stellar nebular (Memnonides Scenario). Planets that formed in this process would have been destroyed by the supernova. However reference [53] postulates that the planets did form from the protoplanetary disk as at least two of the planets are in the same plane and the spacings between the three planets are very similar to that of the inner planets of our solar system. This discovery is the only evidence we have of Earth size planets to date.
Another pulsar PSR 0329+54 is thought to have a planet with two Earth masses orbiting every 16.9 years.
Planets around Pre Main Sequence Stars
Evidence of planetary formation can be seen by observing the dust disks of new born stars at infrared wavlengths. The planets cannot be seen directly however we can observe their effects on the dusty disks that surround the star.
Beta Pictoris is an A5V type star surrounded by a thin disk of cloud and dust. This disk extends to around 100 to 500 AU. The system is edge onto our view and it appears very similar to our model of solar system formation. Spectroscopic studies show evidence of cometary like clouds occulating the star. It is believed planets may have already formed around Beta Pictoris [13]. Observations from the Hubble Space Telescope (HST) show the disk to be warped. This may be caused by the gravitational affects from a Jupiter size planet.
The disk was initially discovered by IRAS in the mid 1980s. Observations have found a gap 5 billion Km from the star. This is thought to be a lane cleared by planet formation [20], possibly 2 planets [35].
The Spitzer Space Telescope recently discovered a hole in a disk around the star CoKu Tau 4. It is hypothesized it is caused by a new planet that has acreted from the planetesimals in that region and thus leaving a hole where it has formed. The planet is probably less than 1 million years old, therefore showing that planet formation is still an active process in the Milky Way Galaxy [48]. Spitzer has also peered through the RCW 49 nebula to discover over 300 new protostars. Two observed so far have protoplanetary disks and it is expected that all 300 will be the same [49].
Properties of the Planetary Systems Found to Date
The solar system we live in is what we imagine other planetary systems to be. There will be some small rocky planets in the inner solar system and some massive gas giants further out from the star. They will all have roughly circular orbits, travel in the same direction and in roughly the same plane. Much further out there are countless small icy bodies and comets.
As described above we have found evidence of protoplanetary disks around newborn stars that support the theory of how our solar system was born.
The planets so far discovered are but anything like this. Butler and Marcy refer to 3 types of exosolar planets. 1: 51 Peg type, 2: massive eccentric, 3: pseudo jovian [22].
1: 51 Peg type. These are massive planets with a minimum size of Jupiter. The planets orbit very close to the host star. They have orbital periods of days. Also known as �Very Hot Jupiters� because of their mass and high surface temperature. They are very rare, one for every 2500 to 7000 stars [1,3].
2: Massive Eccentric. Have a minimum mass of 6 Jupiters and have eccentric orbits 0.3 to 0.4 and are 0.4 to 0.5 AU from the host star. It is thought these maybe brown dwarfs at the lowest end of their mass range. Another explanation is that they were in circular orbits while the disk of the protostar existed. But as the disk dissipated the orbits of the planets interacted with each other becoming chaotic. Some of the planets collide and combine giving the higher mass planets observed with eccentric orbits [22].
3: Are found more than 2 AU from the parent star and are 1 to 2 Jupiter masses. There orbits have low eccentricity.
The massive planets discovered thus far were probably born from the solar system model formation theory we have today. Jupiter formed from a massive rocky icy core about 5AU from the star. The planet experienced runaway growth adding an envelope of hydrogen and helium from the solar nebula. In turn it accumulated more planetesimals within its range until the planetisimals were depleted [22]. The planet carves out a groove in the disk. The inner disk loses energy by friction. This causes the planet to migrate inward from the affect of tidal interactions with the dusty disk of the protostar. The planet loses its angular momentum to the disk and spirals inward. The planet stops its migration close to the star as it interacts with the spin of the star.
Others have proposed variations to the current theory of star system formation. Paul Artymnwicz and Patrick Cassen proposed that the gravitational forces exerted by the protoplanets create an alternating spiral density wave analgous to that of spiral arms in galaxies.The waves exert forces back on the planets being formed and draw them into a circular motion around the star. Over millions of years the planets can wander into eccentric orbits.
Another theory is that if the giant planets in our solar system Jupiter, Saturn, Uranus, and Neptune all became larger, they would have exerted more gravity onto each other and affect their orbits and would eventually intersect. This would explain the eccentric orbits we have seen for some of the exoplanets.
It has been observed that planetary systems are more common in stars that have higher percentages of metalicity or iron abundance as shown in the graph below. This is hardly surprising since the cores of planets are made from iron. Planets would not form if the original gas clod the system formed from was devoid from elements heavier than hydrogen and helium. By analysing the spectrum of stars, the iron abundance can be determined and therefore the percentage of stars that have planets �Fp�
Figure 13. [9].
Near and Long-term Prospects for Finding Terrestrial-like Planets
Butler, Marcy, Vogt and Fischer did a study on the numbers of planets at different masses.
Figure 14. P.Butler et al [9]
Extrapolating the graph back to Earth masses, the number of Earth size planets should be very large. Detecting them is another matter.
Improving detection techniques of higher precision in radial velocity measurements would lead to the detection of Earth like planets around other stars. As mentioned above, a radial velocity of 0.63 ms-1 would be required to detect an Earth size planet around an M5 star. 0.1 ms-1 would be required to detect Earth size planets around G type stars like our sun.
Using a 1m telescope in space for 6 years looking at 5000 stars simultaneously may produce in the order of 10s of earth size planets using the Transit Method [24].
Planet Search Project (MPS) is searching for Jupiter to Earth size planets using the microlensing technique. This technique is probably the best way to detect an earth size planet around another star. MPS is searching the Galactic Bulge from ground based telescopes in South Africa, Australia, and South America. [29] Microlensing has the potential to find one Earth size planet every year [30].
A project known as �PLANET� or Probing Lensing Anomalies Network is to search for Earth size planets in a similar way as above.
Under development is a project named Kepler. This will be a space based observatory. It will focus on 100,000 stars in a star field. As the chances of observing a transit of a planet across the face of a star are very small, looking at 100, 000 stars raises the chances of observing many planetary transits. The sensitivity will be able to detect Earth size planets [45].
Terrestrial Planet Finder (TPF) is a planned mission that involves two telescopes. One is a coronagraph operating at visible wavelengths and the other is a large baseline interferometer operating at infrared wavelengths. TPFs mission is to detect and characterise Earth size planets in the habitable zone around other stars within 45 lightyears of the Earth [51]. The two telescopes are to be launched in 2014 and 2020.
The coronagraph will be 3 to 4 times larger than the Hubble Space Telescope. It will be able to reduce the light from the star to a billionth and therefore hopefully see the fainter planets.
The Interferometer will consist of many telescopes fixed to a structure or flying separately to give a larger baseline. The interferometer will be able to reduce the infra red light from a star to a millionth. Planets will be more visible in infra red than visible light, so planets can be observed using the nulling effect on the star.
The Space Interferometry Mission (SIM) is being designed to be able to detect Earth size planets around nearby stars. It will use the astrometric technique to measure the wobbles of stars down to one milliarcsecond against the background stars. This will be achieved using an optical interferometer [52].
Conclusion.
Planet hunters have used astrometry, radial velocity technique, transit technique, gravitational microlensing, and pulsar timings to detect planets around other stars. To date over 100 planets have been confirmed. The first exoplanets to be discovered were the pulsar planets by measuring the periodical variations in pulsar timings. These planets are similar to what is found in our inner solar system, being Earth size and similar spacings between planets. The majority of planets have been discovered by the radial velocity technique, however this method is biased towards finding Jupiter mass planets orbiting very close to their host star. Some of these planets have been confirmed by observing the transit across the face of the host star. These planets do not fit into our model of the solar system, though theories show that these planets migrated close to the star from where they formed further out from the star.
Emerging techniques are transits by planets and microlensing techniques which have the potential to find Earth size planets in the near future. Projects in the pipeline for detecting Earth size planets lie in building observatories in space. These include Kepler, SIM, and TPF missions. It is a matter of time before Earth like planets will be discovered routinely around other stars. With this knowledge the variable in the Drake Equation (Fp) the fraction of stars that contain planets can be more closely estimated.
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Appendix 1. Exoplanets Discovered.
Technique Star Star type No of Planets Planet size Planet Mass(Jupiter masses) Planet 1 dist(AU) Planet distance from Earth (light years) Planet 1 period ref
Transit OGLE-TR-113 F 1 1.1J 1.35 0.0228 6000 1.43 days 1
Transit OGLE-TR-132 K 1 1.15J 1 0.0306 1200 1.69 days 1
Transit OGLE-TR-56 G 1 1.23J 1.45 0.0225 1500pc 1.21 days 5
Radial Velocity Pegasi 50 1 - 2 0.5 0.05 65 4.2 days 8
Doppler shift Gliese 876 1 1.6 0.2 15 61 days 8
Beta pictoris 8
Radial Velocity 47 Ursae majoris G0V 1 3 to 4 2 47 (44 [13]) 3 years 8, 13
Radial Velocity 70 virginis G2.5 - 5V 1 7.44 0.48 78 116d 8, 13, 18, 19
Radial Velocity 51 Pegasi G2-5V 1 0.468 +- 0.007 0.052 42 - 48 4.23077 +-0.00005d 11,13, 14
Pulsar Psr 1257+12 Pulsar 4 0.02E, 4.3E, 3.9E, 100E 0.19, 0.36, 0.46, 40 25.2, 66.5, 98.2, 170Y 13,28
Pulsar PSR 0329+54 Pulsar 1 2E 16.9 y 13
Pulsar PSR B1620-26 Pulsar 1 2.5 23 100y 28
Radial Velocity 55 Cancer 3 0.84, 0.21, 4.05 0.11, 0.24, 5.9 14.65, 44.28, 5360 16,28
Radial Velocity Tau Bootis F7V 1 3.87 0.0462 15.6pc 3.3128 16,28
Radial Velocity Upsilon Andromedae 3 0.69, 1.19, 3.75 0.059, 0.829, 2.53 4.617, 241.5, 1284 16,28
HD 114762 10 16
Radial Velocity 16 Cygni B G3V 1 1.69 0.56 � 2.8e 56 2.2y 16, 21
Radial Velocity HD210277 G0 1 1.29 1.12e 1.18y
Astrometry Lalande 21185 M2V 3 0.9, 1.6, 1 2.2, 11, ? 8.25 5.8y,30y, ? 25
Radial Velocity HD 73256 G8 1 1.85 0.037 2.55 28
Radial Velocity HD 83443 K0V 1 0.41 0.04 43.5pc 2.99 28
Radial Velocity HD 46375 K1IV 1 0.25 0.041 3.024 28
Radial Velocity HD 179949 F8V 1 0.84 0.045 27pc 3.093 28
Radial Velocity HD 187123 G5 1 0.52 0.042 3.097 28
HD 330075 1 0.76 0.043 3.369 28
Radial Velocity BD-10_3166 1 0.48 0.046 3.487 28
Radial Velocity HD 75289 1 0.42 0.046 3.51 28
Radial Velocity, Transit HD 209458 G0V 1 1.43 0.69 0.045 150 3.52 28, 39
Radial Velocity HD 76700 1 0.197 0.049 3.971 28
Radial Velocity HD 49674 1 0.12 0.0568 4.948 28
Radial Velocity HD 68988 1 1.9 0.071 6.276 28
Radial Velocity HD 168746 1 0.23 0.065 6.403 28
Radial Velocity HD 217107 1 1.28 0.07 7.11 28
Radial Velocity HD 162020 1 13.75 0.072 8.42 28
Radial Velocity HD 130322 1 1.08 0.088 10.724 28
Radial Velocity HD 108147 1 0.41 0.104 10.901 28
Radial Velocity HD 38529 2 0.78, 12.7 0.129, 3.68 14.309, 2174 28
Radial Velocity GL 86 1 4 0.11 15.78 28
Radial Velocity HD 195019 1 3.43 0.14 18.3 28
Radial Velocity HD 6434 1 0.48 0.15 22.09 28
Radial Velocity HD 192263 1 0.72 0.15 24.34 28
Radial Velocity GLIESE 876 2 0.56,1.98 0.13, 0.21 30.1, 61.02 28
Radial Velocity RHO CRB 1 1.04 0.22 39.845 28
Radial Velocity HD 74156 2 1.86, 6.17 0.294, 3.40 51.643, 2025 28
Radial Velocity HD 168443 2 7.7, 16.9 0.29, 2.85 58.116, 1739.5 28
Radial Velocity HD 3651 1 0.2 0.32 62.23 28
Radial Velocity HD 121504 1 0.89 0.32 64.6 28
Radial Velocity HD 178911 B 1 6.292 0.32 71.487 28
Radial Velocity HD 16141 1 0.23 0.35 75.56 28
Radial Velocity HD 114762 1 11 0.3 84.03 28
Radial Velocity HD 80606 1 3.41 0.439 111.8 28
Radial Velocity HD 216770 1 0.65 0.46 118.45 28
Radial Velocity HD 52265 1 1.13 0.49 118.96 28
Radial Velocity GJ 3021 1 3.21 0.49 133.82 28
Radial Velocity HD 37124 2 0.75, 1.2 0.54, 2.5 152.4, 1495 28
Radial Velocity HD 219449 1 2.9 0.3 182 28
Radial Velocity HD 73526 1 3 0.66 190.5 28
Radial Velocity HD 104985 1 6.3 0.78 198.2 28
Radial Velocity HD 82943 2 0.88, 1.63 0.73, 1.16 221.6, 444.6 28
Radial Velocity HD 169830 2 2.88, 4.04 0.81, 3.6 225.62, 2102 28
Radial Velocity HD 8574 1 2.23 0.76 228.8 28
Radial Velocity HD 89744 1 7.2 0.88 256 28
Radial Velocity HD 134987 1 1.58 0.78 260 28
Radial Velocity HD 40979 1 3.32 0.811 267.2 28
Radial Velocity HD 12661 2 2.30, 1.57 0.83, 2.56 236.6, 1444.5 28
Radial Velocity HD 150706 1 1 0.82 264.9 28
Radial Velocity HD 59686 1 6.5 0.8 303 28
Radial Velocity HR 810 1 2.26 0.925 320.1 28
Radial Velocity HD 142 1 1.36 0.98 338 28
Radial Velocity HD 92788 1 3.8 0.94 340 28
Radial Velocity HD 28185 1 5.6 1.0 385 28
Radial Velocity HD 142415 1 1.62 1.05 386.3 28
Radial Velocity HD 177830 1 1.28 1.00 391 28
Radial Velocity HD 108874 1 1.65 1.07 401 28
Radial Velocity HD 4203 1 1.65 1.09 400.9 28
Radial Velocity HD 128311 1 2.63 1.06 414 28
Radial Velocity HD 27442 1 1.28 1.18 423.8 28
Radial Velocity HD 210277 1 1.28 1.097 437 28
Radial Velocity HD 19994 1 2 1.3 454 28
Radial Velocity HD 20367 1 1.07 1.25 500 28
Radial Velocity HD 114783 1 0.9 1.20 501 28
Radial Velocity HD 147513 1 1 1.26 540 28
Radial Velocity HIP 75458 1 8.64 1.34 550.7 28
Radial Velocity HD 222582 1 5.11 1.35 572 28
Radial Velocity HD 65216 1 1.21 1.37 613 28
Radial Velocity HD 160691 2 1.7, 1 1.5, 2.3 638, 1300 28
Radial Velocity HD 141937 1 9.7 1.52 653 28
Radial Velocity HD 41004A 1 2.3 1.31 655 28
Radial Velocity HD 47536 1 4.96-9.67 1.61-2.25 712 28
Radial Velocity HD 23079 1 2.61 1.65 738 28
Radial Velocity HD 4208 1 0.80 1.67 812 28
Radial Velocity HD 114386 1 0.99 1.62 872 28
Radial Velocity GAMMA CEPHEI K2 1 1.59 2.03 45 903 28, 38
Radial Velocity HD 213240 1 4.5 2.03 951 28
Radial Velocity HD 10647 1 0.91 2.10 1040 28
Radial Velocity HD 10697 1 6.12 2.13 1078 28
Radial Velocity HD 190228 1 4.99 2.31 1127 28
Radial Velocity HD 114729 1 0.82 2.08 1131 28
Radial Velocity HD 111232 1 6.8 1.97 1143 28
Radial Velocity HD 2039 1 4.85 2.19 1193 28
Radial Velocity HD 136118 1 11.9 2.335 1210 28
Radial Velocity HD 50554 1 4.9 2.38 1279 28
Radial Velocity HD 196050 1 3 2.5 1289 28
Radial Velocity HD 216437 G4IV-V 1 2.1 2.7 26.5 pc 1294 28
Radial Velocity HD 216435 G0V 1 1.49 2.7 33.3 pc 1443 28
Radial Velocity HD 106252 G0 1 6.81 2.61 37.4 pc 1500 28
Radial Velocity HD 23596 F8 1 7.19 2.72 52 pc 1558 28
Radial Velocity 14 HER K0V 1 4.74 2.80 18.1 pc 1796 28
Microlensing OGLE-235/MOA-53 1 1.5 � 2.5 2.8-3 ? 28
Radial Velocity HD 39091 G1IV 1 10.35 3.29 20.6 pc 2063.8 28
Radial Velocity HD 72659 G0V 1 2.55 3.24 51.4pc 2185 28
Radial Velocity HD 70642 G5V 1 2 3.3 29 pc 2231 28
Radial Velocity HD 33636 G0V 1 9.28 3.56 28.7 2447 28
Astrometry, Radial Velocity EPSILON ERIDANI 2 0.86, 0.1 3.3, 40 2502, 280Y 28
Radial Velocity HD 30177 G8V 1 9.17 3.86 55 pc 2819.6 28
Radial Velocity GI 777A 1 1.33 4.8 2902 28
S ORIO 70 Free 1 3 N/A 440 pc N/A 28