The Discovery of Planets beyond the Solar System Luis A Aguilar Instituto de Astronomía, UNAM México
First of all, What is a planet?
Contrary to what you may have thought, this is something difficult to answer Have you ever wondered, what exactly is a planet?
When we think about planets, what comes to mind are the planets of the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto
You may think that a planet is a heavenly body that doesn t shine with its own light and goes around a star but asteroids are not luminous and go around the Sun!
What is then a planet?
What is then a planet? You may be surprised to know that not even astronomers are sure
Let s see if we can define what is a planet First of all, a planet is not a star
Let s see if we can define what is a planet First of all, a planet is not a star The fundamental difference between a star and a planet is its size, or more precisely, its mass The more massive a heavenly body, the greater the pressure and temperature at its center In a star it gets so hot, that thermonuclear explosions get started, and these produce so much energy, that a star shines with its own light The minimum stellar mass is 008 of the Sun s mass
Let s see if we can define what is a planet This makes a star shine with its own light, while a planet can only reflect the light it receives from a star
Let s see if we can define what is a planet However, there are so called brown dwarfs, which are failed stars that couldn t get thermonuclear reactions started A brown dwarf is less massive than a star, but more massive than a planet A brown dwarf shines but not as much as a star To be a brown dwarf an object must have a mass at least 10 times Jupiter s mass
Let s see if we can define what is a planet So, the largest mass that a planet can have is about 10 times Jupiter s mass! and what is the smallest mass?
Let s see if we can define what is a planet Here the mass is important to determine the shape
Let s see if we can define what is a planet Here the mass is important to determine the shape In a planet, the force of gravity is strong enough to squeeze it into a round shape
Let s see if we can define what is a planet Here the mass is important to determine the shape In a planet, the force of gravity is strong enough to squeeze it into a round shape In a smaller object, like an asteroid, the force of gravity can t overcome the stiffness of rocks, and so it remains irregularly shaped
Let s see if we can define what is a planet Here the mass is important to determine the shape In a planet, the force of gravity is strong enough to squeeze it into a round shape In a smaller object, like an asteroid, the force of gravity can t overcome the stiffness of rocks, and so it remains irregularly shaped
Let s see if we can define what is a planet But there are spherical objects in the Solar System that we don t call planets Some, like the moon, don t go around the Sun, but around a planet But others, like the recently discovered, Sedna and Quaoar, go around the Sun The smallest mass of a planet is not well defined, for some astronomers it is Pluto s mass, which is a hundred thousand times smaller than Jupiter s
A planet is then: A heavenly body with a mass between a hundred-thousandth and ten times that of Jupiter It does not shine with its own light, but with reflected light It travels around a star It is spherical To call it a planet, an object must satisfy all these conditions
How is a planet?
How is a planet? Mercury Venus Earth Mars Pluto In the Solar System there are two different types of planets: On one side, we have the terrestrial planets: these are small, made of rock and with a metallic core Except for Pluto, which is the smallest and most distant, they move close to the Sun
How is a planet? Saturn Uranus Neptune Jupiter In the Solar System there are two different types of planets: We also have the gaseous giants: these are big, made of gas and light elements All are far from the Sun and spin very fast
How is a planet? Orbits Except for Pluto, all planets follow almost circular paths around the Sun and lie on the same plane
How is a planet? Sizes Orbits Compared to Earth, their diameters span from a fifth for Pluto, to eleven times for Jupiter
How is a planet? Sizes Orbits Distances Mercury goes around the Sun 3 times closer than Earth, while Pluto is 40 times farther away
Are there planets going around other stars?
For a long time, philosophers, writers and scientists, have speculated about the existence of worlds beyond the Solar System
For a long time, philosophers, writers and scientists, have speculated about the existence of worlds beyond the Solar System And populated them with a zoo of fantastic creatures
However, astronomers studying the formation of the Solar System, had many reasons to believe that there ought to be many other planets We think that when a star is born, a gaseous disk surrounds it Dust condenses in the disk, and through sticky collisions, it forms gravel, pebbles, rocks and so on, all the way to planetesimals and planets, At the end, planet formation shuts down when the bright light from the infant star dissipates the gaseous disk
However, astronomers studying the formation of the Solar System, had many reasons to believe that there ought to be many other planets This process of planet formation can be studied using computer simulations
However, astronomers studying the formation of the Solar System, had many reasons to believe that there ought to be many other planets In fact, astronomers have recently seen disks of gas and dust surrounding young stars It is thought that planets are being formed in them
But finding a planet beyond out planetary system is an extremely hard task
Why is so difficult to find extrasolar planets?
Why is so difficult to find extrasolar planets? One reason is the vast distances involved Earth is only 8 lightminutes away from the Sun, but the closest star is 4 light-years away At this distance, it is very, very hard to see a planet
Why is so difficult to find extrasolar planets? Another reason is because planets don t shine with their own light, this makes them very faint compared to their stars Have you noticed how the same flashlight looks bright at night and dim during the day?
How to find then an extrasolar planet?
How to find then an extrasolar planet? When two stars move around each other, they move equally
How to find then an extrasolar planet? But when a planet moves around a star, the star moves very little, because the planet is a lot less massive
How to find then an extrasolar planet? The tiny stellar motion can be measured
How to find then an extrasolar planet? The tiny stellar motion can be measured Using what is called the Doppler effect When a car approaches, we hear a high pitch sound, and when it recedes, the sound turns to a lower pitch Sound is a wave, when the car is approaching us, the wave is compressed, when the car recedes, it is stretched A short wave is a high pitch tone, a long wave is a low pitch tone
How to find then an extrasolar planet? The tiny stellar motion can be measured Using what is called the Doppler effect The same happens with light, here short waves correspond to blue light, while long waves are red light This can be used to detect the motion of a star
How to find then an extrasolar planet? The tiny stellar motion can be measured Using what is called the Doppler effect The variation in color is very, very small It is equivalent to a highway patrol officer detecting a difference between a car moving at 60 mph and another moving at 6005 mph, a difference of about 80 yards per hour!
How to find then an extrasolar planet? Another way to discover planets is by occultation
How to find then an extrasolar planet? Another way to discover planets is by occultation When going around a star, a planet may pass in front of its star, as seen from Earth When this occurs, the brightness of the star will appear to diminish a little bit
How to find then an extrasolar planet? Another way to discover planets is by occultation The size of the effect depends on the relative sizes of the planets and the star Since planets are very small compared to their stars, the effect will be very little
How to find then an extrasolar planet? Another way to discover planets is by occultation The size of the effect depends on the relative sizes of the planets and the star Since planets are very small compared to their stars, the effect will be very little For Jupiter passing in front of the Sun, the dimming is only 3% This is like distinguishing between a 97 watt and a 100 watt light bulbs For the Earth the effect is much smaller
Discovering extrasolar planets
Discovering extrasolar planets Despite all these difficulties, on October 5, 1995, two French astronomers: Michel Mayor and Didier Queloz, From the Observatoire de Haute Provence, announced to the world the discovery of the first extrasolar planet
Discovering extrasolar planets Despite all these difficulties, on October 5, 1995, two French astronomers: Michel Mayor and Didier Queloz, From the Observatoire de Haute Provence, announced to the world the discovery of the first extrasolar planet It was a planet with half the mass of Jupiter, going around the star 51-Pegasi, a star like the Sun 50 light-years away The change in the star s velocity was only 55 yards/s
Discovering extrasolar planets Despite all these difficulties, on October 5, 1995, two French astronomers: Michel Mayor and Didier Queloz, From the Observatoire de Haute Provence, announced to the world the discovery of the first extrasolar planet Just a week later, the American astronomer Geoff Marcy confirmed this discovery from Lick observatory After centuries of search, the race to find the first extrasolar planet had a photo finish! But the new planet took everybody by surprise: it went around its star in only 4 days, moving at one twentieth of the Earth-Sun distance
Discovering extrasolar planets And then the avalanche of discoveries began
Discovering extrasolar planets And then the avalanche of discoveries began In January of 1996, Marcy discovered the second extrasolar planet It went around the star 70-Virginis, 57 light-years away The new planet was a giant 7 times more massive than Jupiter, moving around its star in 116 days at half the Earth-Sun distance
Discovering extrasolar planets And then the avalanche of discoveries began The following month, a third planet was discovered It went around the star 47-Ursa Majoris, 45 light-years away The new planet had a mass in between the previous two: 25 times the mass of Jupiter The size of its orbit was larger: 2 time the Earth-Sun distance, going around its star in 3 years
Discovering extrasolar planets And then the avalanche of discoveries began In August of the same year, additional planets are discovered around 55-Canceri, Tau-Bootis and Upsilon-Andromedae In 1996 a total of 6 planets are discovered One in 1997 Eight in 1998 Thirteen in 1999 And 28 in 2000! Up to April 16th, 2005, 154 extrasolar planets have been discovered
What have we learnt about extrasolar planets?
What have we learnt about extrasolar planets? All discoveries have been in stars near the Sun But this is just because the closer the star, the easier to detect a planet
What have we learnt about extrasolar planets? With a pair of exceptions, all discovered planets move very close to their stars This is due to the Doppler effect used in most discoveries: the star motion is larger when the planet going around is closer
What have we learnt about extrasolar planets? The masses span a range from one half to ten times Jupiter s mass, right at the limit were brown dwarfs begin
What have we learnt about extrasolar planets? The Doppler effect technique can not discover planets like those in the Solar System, at least not yet, because these are either too small or move to far away from the Sun
What have we learnt about extrasolar planets? However, in 1999 the discovery of a planet around the star HD-209458 was announced The importance of this discovery lies in that it was the first planet discovered using the occultation technique This opens up new possibilities for discovery
What have we learnt about extrasolar planets? What have we learnt about their orbits? Contrary to expectations, many planets follow orbits that are quite elongated This is still a big mystery
What have we learnt about extrasolar planets? Something important to us is the extent of the habitable zone, this is the region around a star were water can exist in liquid form on a planet s surface This is the region to look for life Unfortunately, most of planets found to date lie outside this zone
The diversity of discovered planets was a big surprise to Astronomers There is a planet so small, that its mass is only 02 times that of Jupiter There is one so big, that is 11 times more massive than Jupiter There is a planet going around its star in only 3 days, there is another that takes 45 years The planet closest to its star moves at only 004 of the Earth-Sun distance The planet moving the farthest away is at 28 times the Earth- Sun distance from its star In the Solar System, all planets move in near-circular orbits (except Pluto); among the discovered extrasolar planets, almost a third move in very elongated orbits Planets have been found even around pulsars, which are the remains of massive stars that exploded as Supernovae There may be enough planets in our Galaxy to give one to each person on Earth!
What to expect next? The direct exploration of planets in the Solar System will continue Landing of the probe Huygens in Titan, one of Saturn s satellites The Cassini spacecraft is at upper right (December 2005)
What to expect next? The direct exploration of planets in the Solar System will continue Kepler will search for extrasolar planets using the occultation technique Around 2007, NASA will launch Kepler, a satellite that will detect minute variations in the brightness of nearby stars It will search for planets passing in front of their stars
What to expect next? The direct exploration of planets in the Solar System will continue SIM will use sophisticated interferometric techniques to try to get direct images of extrasolar planets Around 2007, NASA will launch Kepler, a satellite that will detect minute variations in the brightness of nearby stars It will search for planets passing in front of their stars Even more ambitious is the SIM mission planed for 2009 Its goal is to get direct images of extrasolar planets
What to expect next? Pioneer and Voyager, the first emissaries of mankind, are leaving the Solar System We live in extraordinary times
I hope you have enjoyed this talk and learnt something new The End