Exoplanets

This page was created as report for Catch a Star project about exoplanets.
by Peter Greskovic and Peter Rudy

Abstract

Our report contains definition of planet according to the International Astronomical Union and explanation of that definition in brief language.

It also contains history of ideas and opinions on exoplanets focused more on key events and people in past centuries than on the recent history of discoveries.

Following this there is list methods that we use to detect exoplanets. Each of this methods is shortly explained in brief english.

There is also list of the most important programmes that use this methods and a very short statistic about discovered exoplanets and assumtions that we can made based on it.

Introduction


Upsilon Andromedae system by Lynette Cook (extrasolar.spaceart.org).
Image credit: Lynette Cook.

Exoplanets

  • what are they?
  • how do we know they are there?
  • what are they like?
  • who were the first people to think about them?
These are only some of the questions that we tried to answer in our report.

Definition

  What are exoplanets? We could simply say: extrasolar planets are planets orbiting around other stars. Well... what is so difficult about that?
Maybe you noticed in evening news or curiosity corner of your newspaper titles like this: "The tenth planet of Solar System finaly found!" or even eleventh or twelveth...
   On the other hand, if you read popular scienctific magazines, you could have found out, that our Solar system has only eight planets. Who is right and why there was such confusion about that?
   Planets are not the only objects that orbit our Sun. There are milions of smaller objects like asteroids and comets there. The difference between asteroid and planet is not very sharp. One planet - Pluto - is so small, that it is comparable in size to the bigges asteroids in so called Kuiper's belt that orbit Sun quite close to Pluto. Therefore some astronomers classified the biggest of them as planets. But when more and more of them was discovered, there would be "too many" planets in our solar system. Other astronomers decided to deny planetary status even to Pluto, because it is so small.
   And there was exactly oposite problem with exoplanets. All the biggest planets of our Solar system have chemical composition similar to stars. How big can a planet be not being a star then?
   When you look for a definition of planet in older astronomical encyclopedia you will find something like this:

"Planets are cold bodies orbiting stars that don't emit radiation themselves."

   But is that realy true? Some planets in our solar system radiate more energy than they get from the sun. The question is: how much energy can a planet emit still being considered a planet. There were extremly small stars observed in universe called brown dwarfts.
   The main difference between planets and stars is in the way, they produce their energy. Only stars can produce energy burning hydrogen to helium in termonuclear fusion. And only body that is massive enough does have sufficient pressure and temperature for this kind of fusion inside it. The the limiting mass for termonuclear fusion is about 13 times mass of the Jupiter for objects of simmilar chemical composition as Sun. It was matter of prestige to be one of the first discoverers of exoplanet for astronomers that found very small objects orbiting other stars. That's why IAU decided to issue a statement of the definition of a "planet." In short this definition can be written as follows:

"A ``planet'' is an object that has a mass between that of Pluto and the Deuterium-burning threshold and that forms in orbit around an object that can generate energy by nuclear reactions."
G.Marcy, R.P.Butler

Sub-brown dwarfts objects that are free floating interstelar space are not considered planets. [1]

History

   People were looking for the other worlds for very long time. When you look at history of astronomy you must notice one significant fact: people still considered themselves center of the Universe. First they lived in the middle of land surrounded by sea, everything was placed on flat surface on top(!) of some animal. Then they lived on top of round Earth and everything, Sun, Moon, planets and stars, was revolving around them. When this idea came to obvious contradiction with observation data, people found themselves orbiting the new center of Universe - the Sun. Then somebody proved, that Sun was just one of stars and later it became clear, that all the sars that you can see on the sky belong to big gathering of stars - Galaxy. Of course, Sun was still in the center of the Universe. Only in past century people were brave enough to admit, there is no center in Universe.
   However, search for extrasolar planets is something special. It went against this efforts.
   To find out an answer for the last of the questions promoted in introduction we have to look as far back in the time as 2400 years ago. Anicent greek philosophers argued about pros and cons of existence of other worlds. The most significant group amongst defenders of existence of the other worlds was group of atomists like Leucippus, Democritus and Epicurus.

"There are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number, as was already proven, (...) there nowhere exists an obstacle to the infinite number of worlds."
Epicurus(~341-270 B.C.)

"In some worlds there is no Sun and Moon, in others they are larger than in our world, and in others more numerous. In some parts there are more worlds, in others fewer (...); in some parts they are arising, in others failing. There are some worlds devoid of living creatures or plants or any moisture."
Democritus(~460-370 B.C.)

Maybe they didn't really understand, how their own world was organized, but at least they belived, it wasn't unique. Unfortunately, the mainstream of scientific knowledge was governed by the ideas of another anicent philosopher:

"There cannot be more worlds than one."
Aristotle(384-322 B.C.)

   Shortly after Nicolaus Copernicus introduced his heliocentric theory, some people stated existence of infinite number of Suns with planetary systems. The best known of them was italian philosopher Giordano Bruno:

It was in times of unparalleled renewal, times of William Shakesperare and religious fervor across Europe... A man of passionate utterings and full of controversial ideas like Giordano Bruno didn't have chance to survive inquisition court. To the execution fire he was brought by his book "De l'Infinito, Universo e Mondi (On the Infinite Universe and Worlds)". One day later people found this inscriptin on the square where he was burned: "We will revenge for Giordano Bruno!" It was written with a piece of red chalk...

In fact we don't know exactly why Giordano Bruno was executed. But he was almost right about the exoplanets.
   The first documented scienctific attemtp to discover exoplanets was made by Christian Huygens in the late seventeenth century. Not even all planets of our Solar system were known then. Uranus was discovered in 1781, Neptune in 1846 and Pluto only in 1930. [2]

Methods

   If distances in universe were measured in kilometers, then distance to the nearest star after the Sun, Proxima Centauri would be 40680272546473441 km. Long number, isn't it? Distances in universe aren't measured in kilometers. In scienctific literature they are often expressed in parsecs.
   A parsec is distance at which radius of Earth's orbit around the Sun would be visible at angle of just 1 arcsecond. This is diameter of 2 euro coin more than 5 kilometers away from you. Some of the stars that we know that they have exoplanets are as far as 1500 pc away from us ( OGLE-TR-10 [3]).
   If extrasolar planets were as bright as our biggest planet Jupiter (if they were so close), at the distance they would be approximately 3,500,000,000,000,000,000,000 times fainter. Brightenss decreases with square of the distance.
How do we know there are any exoplanets? How can we be so sure that there is something so small, so faint, so incredibly far away? We have a few metods for finding exoplanets. The most successfull of them is currently Doppler shift method.

The doppler shift method


Fig 1.: Example of the spectrum.
Fig 2.: Explanation of the doppler shift
measuring.
   From your lessons of physics you may remember that planets like Jupiter or Satrn orbit our Sun. But it's not comple true. To understand why, you can simply imagine a pair of children holding their hands and running around. They both "orbit" center of their common mass. If a child plays the same game with his father, you notice that father is much closer to the center of mass and radius of his "orbit" is much smaller. This is the same case as with planets orbiting stars. They also attract each other by gravitational force. However, our Sun weights more than 1000 times more than our biggest planet, therefore the diameter of Sun's orbit is 1000 times smaller than diameter of Jupiter's orbit.
   A planet orbiting around star causes the star to 'wobble' around the center of mass. If we see orbit of planet nearly from the edge, then we can observe star as it moves towards us or away from us depending on which part of orbit it is. We can measure this movement very precisely using Doppler spectroscopy. Limiting speed that we can measure is 3 m/s. This method is also reffered to as radial velocity method.
   This principle is based on the fact, that wavelength of light from a source that is moving towards us shifts to shorter values (blue) and wavelength of light from a source that is moving away shifts to bigger values (red). Because star emits light in all wavelengths and even in infrared and ultraviolet, we see ultraviolet light becoming violet during blue shift and infrared becoming red durnig red shift. We vitually don't see any difference in change of colour of star. To measure the real change in wavelength we must select one color of precisely known wavelength and then compare it with its actual wavelength. We know, that star emits or rather doesn't emit light of specific wavelengths. We see this colors in spectrum of the star like narrow lines. When we measure wavelength of this lines periodicaly, we may get value of doppler shift and determine speed of the star againts us. If we find star moving in predicted manner, we can determine it has exoplanet orbiting around it.
   With this method we can find big planets orbiting very close to their stars (therefore orbiting them very fast). But main disadvantage of this method is relativly big uncertainity in determination of mass of the planet. It is because observed speed also depends on angle of orbital plane to the line of sight.

The astrometric method


Fig. 3.: Explanation of the astrometric
method.
   Using astrometric method we can find those exoplanets that orbit their stars in plane that is almost normal to the direction from us to the stars. This method allows to find more massive planets that orbit their stars further and their orbital period is longer.
   It is based on periodic measurment of position of star on the sky. If the star orbits center of mass with its exoplanet, it reveals its existence by very small displacement on the sky.
   Peter van de Kampf tried to confirm exoplanets orbiting Barnard's star using this method already in 1982. This planet was not widely accepted. One planet confirmed by this method was discovered by HST and space mission called SIM will use this method.

Pulsar timing


Fig 4.: A pulsar.
Fig 5.: Change in period may indicate
orbiting planet.
   The first method that realy confirmed existence of exoplanet was based on change of timing of pulsar. Pulsars are special cases of neutron stars: extremly dense stars, a tombstones of very masive stars that eneded their lives in supernova explosion. Thanks to the law of conservation of angular momentum slowly rotating star speeds up its rotation many times when it colapses to pulsar. If there is hotspot on the surface or magnetic and rotational axis are not aligned, Earth may be hit by a beacon of radio waves once per rotation. As there isn't virtually anything to slowdown the speed of rotation, the period is very regular.
   When there are planets orbiting in magnetic field of neutron star, very small changes in period of rotation can betray existence of them. Planets of earth size can be detected like this. It is belived that it's unlikely for the planets to survive an explosion of supernova and such planets were formed only after that. This shows us how common phenomenon planet formation probably is. Alhough planet was suspected to orbit pulsar already in 1978, the first widely accepted exoplanet was discovered by this method by Wolszczan and Frail in 1994.

Direct imaging


Fig. 6.: Proposed TPF mission.
   Although it may seem unbelivable, using some techniques direct imaging of planets would be possible. This techniques use earth's atmosphere or multiple telescopes to perform so called interferometry. Interferometers combine light waves from multiple sources so that they can obtain extremly high angular resolution. Multiple mirrors placed in some distance from each other work as one big telescope. In addition, interferometer can be adjusted so that it blocks light from the exact center of an image. This way bright star is not visible but everything else around is.
   There were no planets found using this method yet, but it is important direction for develpment. Space misions to search for exoplanets planed by major space agencies will use this method.

Gravitational lensing


Fig 7.: Gravitational lens.
Fig 8.: Lensing by a planet.
   Each of above mentioned methods have its pros and cons. All of them can find planets only in very limited distance. But there is one method that can find even planets very, very far away. There were planets in distance of 30,000 ly found using gravitational lensing.
   Albert Einstein revolutionizet our understanding of space and time in the beginnig of the last century. In his general relativistic theory he explained gravitational force as curvature of the spacetime. Every object that has some mass, including exoplanets, curve spacetime around it. It means that it changes our definition of what is straight. For light that travels only in straight lines, can curved spacetime around planet work like optical lens. When distant planet moves right in front of background star we can observe a gain in brightness for that star for short period of time.
   This technique requires a very precise photometry of big number of stars. There are ongoing projects that use it and they already have first results.

Transit method

   Also the simpliest method of finding exoplanets uses photometry. When planet orbits its star so that when you are looking from the Earth it transits in front of the star, you can observe, that the star is a little bit dimmer during the transit. For those who participated in Venus Transit 2004 project this situation may be familiar.

Fig. 9.: Transit method.
   When you can observe transit of exoplanet, you can very precisely determine angle of its orbital plane. Also in combination with other methods (particulary spectroscopy) you can determine its mass, size, and sometimes even chemical composure of its atmosphere.
   For some planets and stars this method is very simply to perform and even amateur astronomers with appropriate equipement reported successfull observations. In colaboration with local astronomical observatory we are trying to preform this kind of observation too. Today we are waiting ready with ST-8 CCD camera mounted on 40 cm cassegarin telescope already for almost two months. It is quite frustrating that there wasn't single clear night during this time. Now we understand why the big astronomical observatories are build only on sites with more than 360 clear nights per year. Although our observations cannot be made part of our Catch a Star report, we don't surrender and will observe the HD209458 star and its planet as soon as possible.

Searches

   There are many ongoing searches for exoplanets. We will name here the most important or the most interesting of them. See http://www.obspm.fr/encycl/searches.html for more detailed list:
Anglo-Australian Planet Search Program tries to search for changes in radial velocity of stars indicating orbiting planets around them.
The Darwin project was proposed by ESA to look for chemical signatures of life on earth-like extrasolar planets. It will use interferometry for direct planet imaging as well as
TPF (Terrestrial Planet Finder) that is similar project by NASA.
Keck Interferometer is ground based interferometry project that will be able to detect exoplanets. MPS - Microlensing Planet Search Project is project that uses gravitational microlensing to find exoplanets.
STARE (STellar Astrophysic & Research on Exoplanets) project tries to find extrasolar planets using last mentioned photometry method [2]. Also the NASA space project
Kepler will use transit method to find terestrial planets orbiting other stars.
Of course, there are other projects that are not carried out specificaly to look for exoplanets but can help finding them (OGLE).

Summary

   Today we know total of 135 exoplanets, some of them in multiple planetary systems (14) [3]. Some of the exoplanets orbit pulsars, the other ones stars of main sequence (normal stars). From total number of discoveries we can assume that at least 5% of all main sequence stars have a giant planets orbiting them in distance 4-5 AU (4-5 times the distance Earth-Sun) [6]. There were planets discovered around dwarfts and some of them are on very eliptic orbits. This data made our understanding of planetary systmes formation more clear and are important in considering of possibility of extraterestrial life. Maybe one day extrasolar planets reaserch will give the answer to the famous question: "Are we alone in the universe?"

Fig. 10: Mass distribution of known exoplanets.

Fig. 11: Semi-major axis of the known exoplanets.

Bibliography

[1] Definition of exoplanet according to the IAU.
[2] STARE project
[3] Extrasolar planets encyclopedia
[4] Planetquest - the search for another world
[5] M. A. C. Peryman, Extra-solar planets, Rep. Prog. Phys, 2000, Vol. 63, 1209-1272
[6] Jean Shneider, The Study of Extrasolar Planets, C.R. Acad. Sci. Paris t. 327, Serie IIb, n.6, p. 621 1999
[7] Stephen Kane, AS3012: Exoplanetary science

List of figures

   We are not original authors of graphics in this document. This graphics may be subject to copyright. See original sources of the graphics for details:
Graphic in introduction - (c) 1999 Lynette Cook, all rights reserved, taken from [2];
Fig. 1. - (c) Nick Strobel 1998-2000, taken from www.astronomynotes.com;
Fig. 2. - taken from www.utsc.utoronto.ca/~shaver/stars.htm;
Fig. 3. - (c) D. Kolinsky, [2];
Fig. 4.,5. - taken from www.astro.psu.edu/users/alex/pulsar_planets_text.html;
Fig. 6. - Courtesy NASA/JPL-Caltech, taken from planetquest.jpl.nasa.gov/TPF/tpf_index.html;
Fig. 7. - (c) Jim Brau, taken from physics.uoregon.edu/~jimbrau/;
Fig. 8. - (c) Sun Hong Rhie, taken from www.nd.edu/~srhie/MPS/;
Fig. 9. - taken from [2];
Fig. 10.,11. - taken from [7];