Sara Levin Saglind
Jennifer Björklund
Teacher: Anders Västerberg

EXOPLANETS

Exoplanets introduction:

The term exoplanets defines every planet that revolves around another star than our Sun.
In approximately ten years 135 exoplanets have been discovered.
The research of the exoplanets is a brand new field of astronomy, that requires the most recent techniques and instruments, which are most of the time even not efficient enough to obtain perfect data.

How exoplanets are formed

Since the formations of the planets in the universe pretty much are the same, we will describe shortly how our solar system was formed.
After the Big Bang, our galaxy was only a gigantic cloud made of stuff and gases, mostly hydrogen and helium.
The cloud which formed our solar system contracted because of a supernova that exploded nearby and disturbed the balance in it, and at the same time enriched the cloud with some heavy elements.
When the cloud contracted, the sun was born. (Because of all the heat and matter on the same place).
The matter in the cloud then started to whirl around the centre, which became a star, and took the shape of a flat disk. The matter in that disc then gathered until the original planets arose.
So what scientists do in the search for en exoplanet is to look for discs round stars, then they look for planets in these discs.
So if there's life on one planet of nine, in our solar- system, it's definitely not impossible for other planets in the universe to also have life.
The planets in our solar system follow the three Kepler laws. The extrasolar planets also follow these laws. Read more about the laws later.


Life on exoplanets

A question the mankind for a long time have been asking is if there is life on other worlds than Earth. If there is life anywhere else, what kind of life is there? Humankind, plants, animals? Or just small life forms such as bacteria?
This field has become a very interesting and popular field to work with. So with different technologies and methods, scientists are trying to find planets, exoplanets, with life on them. The goal for the astronomers is to find earth-like planets, because they think that the chance for life to exist on the planet is bigger then.
Earth-like planets do they call planets which reminds of the earth; hard surfaces with mountains and valleys, but they could also have craters and/or volcanoes. Two examples of earth-like planets are Mars and Venus.
But astronomers have only found gas giants, so they haven't found any earth-like planets yet.
The technique develops all the time, but with the technique we have today it's nearly impossible to find any earth-like planets.
The Scientist centrum Heureka made a time path, where 180 metres corresponded the age of the Earth, 4½ billion years. The life on Earth came as soon as there were oceans on Earth, 3.6- 3.9 billion years ago, when 140 metres are left of the time path. The last 20 metres on the time path correspond the time when fossils from small cells are known. The whole time the human being have had writings, for five thousand years, only correspond one fifth of a millimetre on the time path (whish correspond a fairly thin line of a pencil).
In front of us there is at least 40 metres of the time path, may be a few hundred metres, before life on earth is possible.
If there is life on another planet, is the starting point for WHERE the other planets time paths begins most likely a different one than Earth's.
And the evolution certainly doesn't run parallel with the one on Earth.
So the chance is biggest to find life forms like amoebas or small organisms- not humans or humankind beings.

Shortly about exo-Neptune and exo-Uranus

Of the extrasolar planets found in 2004, most significant were the "exo- Neptunes". These objects are far smaller than any previously detected planet around a sun-like star. The find of the exo-Neptunes bring the astronomers one step closer to find earth-like planets.
Another new planet was discovered around 55 Cancri, already known to have three other planets. This makes the first four-planet system identified so far. The new planet orbits its sun in 2.81 days, lies just 0.038 AU away from its star, and has nearly the same mass as Neptune, or about 18 times of Earth.
A team led by Michael Mayor of Switzerland's Geneva Observatory, reported the least massive exoplanet yet found; a 14-Earth-mass "exo-Uranus". The planet orbits a Sun-like star about 50 light-years away from Earth, in 9.6 days at a distance of 0.09 AU.

Life on moons

Astronomers started to speculate about life on exoplanet' s moons in the year 1997. Astronomers had only detected gas-giants by then, and earth-like life can not exists on that kinds of planets, but gas-giants can have very big moons of stone, and life can exist on them, if there is the right environment. The scientists took help from the data the Galileo-probe had collected when it explored the moons around Jupiter. The astronomers searched for earth-like life, and therefore they wanted to find what life needs on earth to survive. They came to the conclusion that to find earth-like life there must exist oxygen, hydrogen, carbon and nitrogen, but also smaller quantities of sulphur and phosphorus.
The moons of the exoplanets also have to be in the right distance from its sun. in other words; the exoplanets have to orbiting the star not too close and not far away.
Also, the orbits can not be too stretched, then it can be both too cold and too hot, and life can't adjust to the climate. The moons also need to consist of stone; it has to be solid ground.
The tide-effect on moons around very big planets, are so strong that the rotation have been bound. This is one of the problems for life, because the sun - light and heat - then reach the sides of the moon with long intervals. One day and one night on such a moon, is the same as the time of orbit. For example: Callisto, one of Jupiter's largest moons, has got a time of orbit at seventeen earth days. With this long days and nights the temperature different make it hard for life to exist.
The moon has also got to be at least one eight of earth, or it can't be able to keep an atmosphere in place, says the astronomer Darren Williams. It's also important that the moon got a magnetic field that protects the moon from particle-radiation.
Darren William says that if the star 47 in the Plough, that has got a gas-giant, has also got a moon, there is a small possibility that the moon has got conditions for life.

History of detection

In 1981, when the telescopes at La Silla in Chile were new, a Schweitzer team was about to use the star Beta Pictoris as a standard-light (the La Silla-telescopes were built by ESO on the mountain La Silla in Chile. La Silla is 2400 meters high.). But the team discovered that the brightness of the star was fluctuating with two percent at a ten days period. The observations were stopped.
Two years later, in 1983, a disc of dust was discovered around the star. This disc is seen from its edge, and astronomers have been able to take photographs of it trough telescopes.
The first disc like this was discovered around Vega, and it was detected with an infrared satellite, this too was discovered in 1983.
Those discs of dust and gas are supporting the theory about how our own solar system came into existence and then also how other suns and planets, exoplanets, are formed from discs of dust and gas.
Later, and with more observations, astronomers could do some conclusions about the disc; it's about five hundred AU in diameter (the orbit of Neptune is about thirty AU). While we see the disc obliquely from the edge, it blocks some of the light from the star. But the brightness showed variation, and that's because of that there was a whole in the disc, and in that whole there was a planet. This explains why the brightness was increasing and decreasing. The whole appeared because of that the planet draws the dust in the disc into itself and in that way grown. There is an other theory and it says that there are not one big planet but many small "unfinished" ones that are growing into planets.


The picture above shows how the discs can appear.
Some astronomers have lately been interested in Beta Pictoris. Three astronomers have, using the Hubble Space Telescope, created a theory that this star isn't a Normal finished star but a young star that has only existed about twelve million years (our sun is 4 billions years old. In October 1995 the first exoplanet was discovered. It is orbiting the star 51 Pegasi. The planet was discovered by the two astronomers Michel Mayor and Didier Queloz. They used radial velocity measurement to detect it. The astronomers know the star's spectrum and discovered some displacement in the spectral lines, in other words the light that the star's element is spreading. This light has its own wavelengths and the displacements in these wavelengths indicate that the lightsource is moving along the line of sight. And this indicated a planet (see radial velocity measurement). When the displacement was studied the astronomers could decide the mass of the planet (about one half of Jupiter's mass) and how far from 51 Pegasi the planet was orbiting (eight million kilometres, about five percent of how far the earth is orbiting our sun). This is close enough for the rotation to be bound, that means that the planet is always turning the same side towards its sun, like our moon towards the earth.
Most astronomers agree that the scientific data are correct, but some are still doubtful, this is because the "planet" are so big but still so close to the star. The astronomers were searching for variations in the radial velocity, and the fact that the planet was orbiting the star so close made it easier for the astronomers to detect it, while the time of the orbiting are then shorter, witch make the variations appear more often. In our solar system the sun's variations in radial velocity is a period of eleven years. The period of 51 Pegasi is only four days.
The picture above shows the spectral lines
In the beginning of 1996 two more exoplanets were discovered. Those were gas giants and were detected with radial velocity measurements by two astronomers from USA. One of the planets was as big as nine Jupiter masses, with an orbit on 116 earth days. This planet was located in the constellation Virgo. The other one was smaller, tree Jupiter masses. The time it takes for the planet to complete an orbit is 1100 earth days. This planet was located in Ursa Major. The two astronomers believed there was a possibility for life.
This statement was mostly made on speculations. The facts that supported this were partly the temperature on the planets. On the planet in the Virgo, the astronomers had with the help of the distance from the planet to its sun and the period of revolution, counted the degrees to be 85 ºC on the surface. That temperature is enough for water to exist as liquid, and that makes it possible to find complicated organic organisms. There is also highly likely that the planets got one or more moons, witch may be as big as the planet Mars. Even if there didn't exist any life on these planets, the discovery was still interesting, and it was followed by the development of new equipments in the searching for more exoplanets. NASA started a project that was named Origin, witch was meant to be developing telescopes and interferometers.

In 1997 about ten exoplanets were discovered and more were detected during the following year. The majority of the detected planets were at least the size of Jupiter, partly because of the sensitivity of the instruments; bigger planets are affecting their sun more than smaller ones, when they are detected with radial velocity measurements. Most of the planets were also orbiting their sun closer than the gas-giants in our solar system. Those are easier to detect, because when a planet is orbiting its star closer, the time of orbit is shorter, and that means that the observations doesn't need to take so long like it would if the planets were orbiting their sun at the same distance as Jupiter, witch has a time of orbit at twelve years.
Astronomers are not searching for life on the gas-giants, but there is likely that they have moons. In our solar system, all of them have got more than one moon. In the beginning of 1997, the searching for life on moons started. The equipment were and are still not enough to detect moons around exoplanets. But the speculations around this subject are growing. Darren Williams and his science-team talked about what was needed for life to be able to exist on a moon. They based their statements on earth-like life. It's for example important that the gas-giant and its moon are at the right distance from their sun.
Two planets of the ten detected was found to have the right temperature; not to far away from the star, then it's too cold, and not to close, that makes it to hot. The science-team was using the data from the Galileo-probe, witch was exploring the moons of Jupiter. And by that, they concluded that life also, of course, needed oxygen and a couple of other elements that is needed for life on earth.

In 1998 astronomers had detected thirteen stars with planets orbiting them. The equipment the astronomers used to detect exoplanets with variations in radial velocity had been developed, and they could measure as small variations as tree meter per second.
Before 1998, astronomers had only detected exoplanets orbiting stars in single-star-systems. But this year, astronomers detected for the first time incomplete planet systems, made of gas and dust discs, around the stars in a double-star-system.

In year 2002 the 100th discovery of a planet orbiting an other star than our sun was made.
This year, 2005, astronomers have detected about 135 exoplanets and expecting to find more of them during the year. Telescope that are meant to take the first pictures of exoplanets are under construction, and the search for life on other solar systems than our own are preceding.

Astrometic observations

The stars appear to slowly move over the sky, partly because of their own movements, and partly because of our galaxy's rotation. A star without a planet moves in a straight line, but a star with one or more planets will move in a wave-like kind of way. This because of that the star also moves around its own and its planet's common centre of gravity. While the movement are very slow and very small, it takes a long time to go through with an observation like this.

We have only got this information confirmed by one source, so we don't know how trustworthy this is. But it didn't sound too unbelievable, so we decided do add this to the facts anyway.

Direct detection

Long we have been searching for life in universe, but we have not yet succeeded. Many scientists presume that if we ever will find any life, it will be on a planet, a planet not too unlike earth. Nor have astronomers detected any planet like that, they are too small for the instrument to detect. But astronomers have discovered planets, so called extrasolar planets, or exoplanets.
Now, in 2005, scientists have detected a great number of exoplanets. Detection is made in different ways. It is very hard to detect a planet visually. Depending on different things; the visual light from the stars is so much brighter than the reflecting light from the planet, a billion times stronger. (You can compare that with trying to take a photograph of candlelight in the light of a searchlight).
KI, the Keck interferometer, searches for planets orbiting other stars than our sun. KI is a project that connects the two 10 meters Keck telescope on a high mountain on Hawaii. Instead of using visual wavelengths it uses infrared wavelengths, by that the planets light increases in relation to its stars brightness. The brightness of the star is then only a million times stronger than the planets. Keck interferometer will also search for planets in other wavelengths, this to detect faint light from planets orbiting much brighter stars. The planets they are hoping to find is so called "Hot Jupiters", planets with a mass of Jupiter, and that lies close enough to a star so that the surface becomes hot enough for the KI to sense.
What the Keck interferometer does is called direct detection, partly (like we wrote) of Hot Jupiter's but also of brown dwarfs.
To be able detect an exoplanet with direct detection, the planet has to be smaller than thirteen Jupiter masses, this to have "the right" to be a planet; if the celestial body are bigger than that but smaller than seventy-five Jupiter masses it's a brown dwarf. The planets can not be too small; if they are, it's impossible for the instrument to detect them.
Scientists have not yet managed to take a picture of an extra solar planet, but they think that they will be able to do that sometime around the year 2007. They think they will have good enough instruments by then. The instruments are now under construction. Then you can take pictures of already known exoplanets.
Like we said, it's hard to detect a planet orbiting another star. It is, however, easier to detect discs of gas with direct detection, than to detect the planets. These discs of gas may create solar systems, like we believe the solar nebula once did. Astronomers are not yet sure if all the discs becomes complete planetary systems, while it takes such a long time to develop a planetary system from a disc of gas and dust, much more time than mankind have existed.


The Keck Observatory

Transits

Planetary transits means that a planet passes on our side of its sun. In that way astronomers can discover exoplanets, while they dim the light from the star. The stars brightness faint a little. It can be very hard to detect a planetary transit, while the star shines very bright and the planet is very small compared to the star. Then, at the transit, the brightness doesn't decrease with more than some thousandths. These small variation in brightness doesn't necessary indicate a planet, the star may just be pulsating.
A planetary transit even happens in our solar system, the inner planets (Mercury and Venus) passes between the earth and the sun. In 2004 was the last Venus transit. It became quite famous, while no now living person had ever witnesses one before, because the one before this one occurred 1882. The next Venus transit won't be until 2012.
Since only about seven percent of al the stars in the galaxy got one or more planets, the planets are hard to detect by observations by magnitude changes. Especially while the changes for examples are only one percent for a planet with Jupiter-size, and only 0.1 per thousand by a planet that's about the same size as earth. A part of all the planet transits has because of that, been detected with so- called variations in radial velocity; one example is the planet HD209458b, Witch is orbiting a star 145 light years away in Pegasus. A planet that has been detected with the transit method is OGLE-TR-56b, who has also been confirmed by radial velocity measurements.
The probability that a planet transit takesplace if the star got a planet isn't so big either; 0.5 % if the planet orbiting the star at a range at one au. Cause the planet needs to be in the right plane so that it can pass in front of the star, from earth's point of view.
Different planet transits lasts different times depending among other things on the distance between the planet and its star. The Venus transit lasted about six hours.
To make sure a planet exist by using the transit method, you have to observe the same star a long time. And while the brightness faint insignificantly the planet has to be pretty big. You can therefore not detect earth-like planets with this method, at least not with today's technology. Astronomers have to observe a long time to make sure the brightness reduce regularly, so that it doesn't depend on something else than a planet. We also have to (like we wrote above) be in the right plane, or there is nothing to dim the light from the star.

Radial velocity measurement

The one method that has so far proven to be the best in search after extrasolar planets is the radial velocity measurement.
A star's radial velocity is the star's velocity towards or away from us. This you cannot see with the eye, while you do not have deep seeing in space. In other words, it's the movement along the line of sight by an astronomical object that's called radial velocity. Radial velocity movements appear when a star has one or more planets in orbit, and those affects the star with their gravity so that the star moves slightly back and forth. A solar system always got a common centre of gravity, and it is this one that the star is moving around. If the planet's mass is big enough, it will create a measurable variation in the star's movement along the line of sight. If the planet has the same mass as Jupiter, the radial velocity may be some ten metres per second. Our sun's maximal radial velocity is thirteen meter per second, because of Jupiter.
Radial velocity measurements are made with the help of redshift, witch is like the Doppler effect but for light instead of sound. According to this physical law wavelength increases if the object (the star) is moving toward the observer (the earth), and decreases if the star is moving away from the earth. It's like the wavelength is stretching out when the star is moving away and is pressed together when the star is moving against us.
It's fully possible to use this when to measure the radial velocity, if you know the star's real wavelength, the so-called position of rest wavelength. In other words; if you know the star's colour, you can by changes in the colour (wavelength) find out the radial velocity by the star, and in that way receive facts about the planet or planets, if they exists, for example the mass, according to the size of the variations. And by the time of the period of the variations astronomers can decide the distance to the sun, and in that way also the heat of the planet.

Detection of HD202206c

This planet was detected in November last year. The planet is orbiting the star HD202206, witch is located 150 light years away in Capricornus. A brown dwarf also orbits this star.
It can be seen with the naked eye. Its magnitude is 8.8 and its mass is 1.15 sun masses.
To detect the planet, astronomers used radial velocity measurements. The planet is a gas giant and is 2.44 Jupiter masses. The time it takes for the planet to orbit the star is about 1383 days and the average distance between the star and the planet is 2.55 AU.
The planet HD202206c is the 135:th now known exoplanet around a normal star.


Shortly about brown dwarfs

The definition of a brown dwarf is that it is smaller than a star but bigger than a planet, between thirteen and seventy-five Jupiter masses. The brown dwarfs never gets hot enough so that the combustion of hydrogen can start. Therefore it only shines with gravitational energy. Because of this low brightness, they are kind of hard to detect and observe.

Shortly about Keplers laws

Keplers laws consists of three laws.
1. A line from the sun to the planet sweeps over surfaces that have the same size during the same time in space. (Look at the picture below) The surface between A and B have the same size as C and D.
2. Each planet moves in an elliptical orbit with the Sun at one focal point of the ellipse.
3. This law says that there is a connection between the planets period of revolution and the distance to the sun. The period of revolution squared is proportional to the distance cubed.

P 2 = a³
P= the planets period of revolution (in years)
a= mean distance to the sun (in AU)

The picture above shows the two first laws.

Fact about exoplanets:

  • First discovery in 1995.
  • 135 discovered exoplanets round 103 stars.
  • The masses of the exoplanets are between 0.1 to 13 Jupiter masses.
  • Periods of revolution are between 2.5 days and nights till 15 years.
  • Approximately 7% of all the stars have a planet (an exoplanet).

    Sources:

    Web pages:
    http://www.pp.htv.fi/jwestman/astro/exoplanet95.html
    http://www.pp.htv.fi/jwestman/astro/exo9802.html
    http://www.astro.su.se/sas/popast/Tidigare_nr/3-2001/exoplaneter.html
    http://www.all2know.com/sv/wikipedia/e/ex/exoplanet.html
    http://planetquest.jpl.nasa.gov/index.html
    http://planetquest.jpl.nasa.gov/Keck/keck_science.cfm
    http://hem.passagen.se/hlesjo43/kepler.htm
    http://home.swipnet.se/astronomisida/extra.htm

    Books:

    "National encyclopaedia" on CD from the year 1997.
    Astronomy (magazine) January 2005 vol.33 page 39.
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