# Elements of the Solar System, Exploring Extrosolar Planets and Evolution of Planetary Systems FIZ463, İTÜ.

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Elements of the Solar System, Exploring Extrosolar Planets and Evolution of Planetary Systems
FIZ463, İTÜ

The Astronomical Unit (AU)
The appropriate length unit for studying the Solar System is AU AU is the average distance between the Sun and the Earth 1 AU = 150 Million km=8 light minutes

1-The Solar System

Not only the Sun and the Planets
Planets (terrestrials and Jovians) Moons of the planets Meteorites Astroid belts Comets Oort Cloud Kuiper Belt Interplanetary dust

Mass Distribution Sun: 99.85% Planets: 0.135% Comets: 0.01% ?
Satellites: % Minor Planets % ? Meteoroids: % ? Interplanetary Medium: % ? Simply: Sun 99.9 % & 0.1% Jupiter

The Nine Planets Mercury Venus Earth Mars Jupiter Saturn Uranus
Neptune Pluto(?) MNEMONIC: My Very Educated Mother Just Sent Us Nine Pizzas

Imagening the distances
Imagine the Solar System being a soccer ground (about 100 m long). The Sun would be a glaring orange in the centre. Pluto would encircle the sun at the edge of the soccer ground, having the size of a dust particle. The Earth would be 1,30m away from the “orange“, having the size of a sesame seed.

Bode’s Relation a simple rule that gives the distances of the planets from the Sun where N=0, 3, 6, 12, 24…for Mercury, Venus, Earth, Mars, etc.

Mercury 0 (0+4)/10 = 0.4 AU AU Venus 3 (3+4)/10 = 0.7 AU AU Earth 6 (6+4)/10 = 1.0 AU AU Mars 12 (12+4)/10 = 1.6 AU AU ____ 24 (24+4)/10 = 2.8 AU _______ Ceres AU Jupiter 48 (48+4)/10 = 5.2 AU 5.2 AU Saturn 96 (96+4)/10 = 10.0 AU 9.5 AU Uranus 192 (192+4)/10 = 19.6 AU 19.2 AU Neptune ? ? AU Pluto 384 (384+4)/10 = 38.8 AU 39.5 AU

What does Bode’s Law tell us?
Bode's Law predicted that there should be a planet between the orbits of Mars and Jupiter. The "missing planet" turned out to be the asteroid belt.

Obliquity of the Planets

The orbit of the planets lie on a plane (except for the Pluto’s)

Terrestrial Planets The inner four planets at the center of the solar system: Mercury, Venus, Earth, Mars They all are small, rocky, rotate slow, they have small number of moons. Metal cores.

Jovian Planets Outer planets of the Solar System
Jupiter, Saturn, Uranus & Neptun They are made of gas/liquid/ice No solid surface Small solid core (rock) They have rings Large number of moons

Terrestrial and Jovian Planets

Interiors of Jovian Planets: cross-cuts

Interiors of Jovian Planets: cross-cuts
Saumon & Guillot (2004)

Gas giant planets: Jupiter & Saturn
Dominant composition: Hydrogen + Helium, like the sun Surface clouds: ammonia ice, water ice.... Deep in interior: liquid metallic hydrogen Even deeper: rocky core of ~ M These are model results which depend on equation of state of hydrogen For Saturn this is certain (unless models are wrong) For Jupiter the uncertainty includes Mcore=0

Ice giant planets: Uranus & Neptune
Dominant composition: Water + Ammonia + Methane ices Only atmosphere contains H, He (in total only minor) Uranus: 25% Iron + Silicates 60% Methane + Water + Ammonia 15% Hydrogen + Helium Neptune: 20% Iron + Silicates 70% Methane + Water + Ammonia 10% Hydrogen + Helium

Thermal emission of Jupiter and Saturn
Jupiter and Saturn emit more radiation than they receive from the sun. They are not massive enough for nuclear burning (need at least 13 Mjup) Kelvin-Helmholz cooling time scale much shorter than current age (at least for Saturn) Possible solution: Helium slowly sediments to center, releases gravitational energy

Why U+N ice, J+S hydrogen?
Theory: All four formed at similar location, first forming a rock+ice core by accumulating icy bodies Somehow U + N were moved outward and did not accrete much gas anymore J + S remained and accreted large quantities of hydrogen gas

Summary - What do the inner planets look like?
They are all… rocky and small! No or few moons No rings

Summary - The Jovian Planets
They are all… gaseous and BIG! Rings Many moons

Quantitative Planetary Facts

What are Moons? Moons are like little planets that encircle the real planets. Usually, they are much smaller than planets. Planets can have no moons (like Mercury and Venus), one moon (like Earth) or up to a very large number of moons (e.g. >63 for Jupiter). Mars (2), Saturn (>34), Uranus (>27), Neptun (>13), Pluto (1)

Asteroids Small bodies planetoid, minor planet
Their mass is not sufficient to make them spherical Many of the asteroids are part of the asteroid belt between Mars and Jupiter. Believed to be left over from the early evolution of the solar nebula. Largest object Ceres is about 1000 km accross

Asteroid Belt The doughnut-shaped concentration of asteroids orbiting the Sun between Mars and Jupiter More that asteroids Total mass, a few 1024 g, is 1/30 of the Moon. if the estimated total mass of all asteroids was gathered into a single object, this object would be less than 1,500 kilometers across

The Origin of the Asteroid Belt
The asteroid belt may be material that never coalesced into a planet, perhaps because its mass was too small; the total mass of all the asteroids is only a small fraction of that of our Moon. A less satisfactory explanation of the origin of the asteroid belt is that it may have once been a planet that was fragmented by a collision with a huge comet.

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Kirkwood Gaps If you plot the radius of the orbits of the asteroids you do not get a smooth `bell-curve' shape. There are concentric gaps in the asteroid belt known as Kirkwood gaps. These gaps are orbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulled into Jupiter). For example, an orbit in which an asteroid orbited the Sun exactly three times for each Jovian orbit would experience great gravitational forces each orbit, and would soon be pulled out of that orbit. There is a gap at 3.28 AU (which corresponds to 1/2 of Jupiter's period), another at 2.50 AU (which corresponds to 1/3 of Jupiter's period), etc. The Kirkwood gaps are named for Daniel Kirkwood who discovered them in 1866. This is an example of resonance. This resonance phenomenon has Jupiter passing by any asteroid in the Kirkwood gaps every two or three asteroid years, depending on which gap. The repeated tugging induces an asteroid into larger, longer orbits closer to Jupiter. Eventually, however, an asteroid's resonance with Jupiter disappears as its orbit increases.

Comets a white dust tail and a blue gas (ion) tail.
A comet consists of a tiny nucleus with diameter less than 10 km. The nucleus is made up of frozen gases and dust. Eccentric orbit around the Sun. Most comets spend most of their time at vast distances from the Sun. When they approach the Sun, some gases will be vaporized and an extended coma will then be produced (of size km). The tail can be up to 1AU long. Orbits of a comet may be open or close. A comet with an open orbit will only visit the Sun once. However, a comet with a closed orbit (actually it is elliptical) will visit the Sun again and again. Perhaps, the most famous one is the Comet Halley, it has a closed orbit with a period of 76 years.

Comet Tails When a comet moves close to the Sun, the solar wind (charged particles ejected from the Sun) and the Sun's radiation pressure push the dust and gases of the comets away, this will result in a beautiful long tail. From this, we know why the comet tail is always pointing away from the Sun. The dust trail is made of particles that are the size of sand grains and pebbles. They are large enough that they are not affected much by the Sun's light and solar wind. The gas tail, on the other hand, is made of grains the size of cigarette-smoke particles. These grains are blown out of the dust coma near the comet nucleus by the Sun's light.

Comet Orbits

Meteoroids, Meteors and Meteorites
When asteroids collide with one another they can produce small fragments known as meteoroids. If a meteoroid enters the atmosphere of the Earth, it glows due to heat generated by friction. These are called meteors. If the rock survives the trip through the atmosphere and strikes the surface of the Earth, the remnant is called a meteorite. Only 2 documented cases in which a person is hit by a meteorite.

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Two documented Cases Annie Hodges of Sylacauga, Alabamawas napping on her couch on November 30, 1954 when an eight-pound meteorite crashed through the roof. It bounced off a large console radio and hit her in the arm and then in the leg, leaving her bruised but okay. On the afternooon of June 21, 1994, Jose Martin and his wife, Vicenta Cors, were driving in Spain from Madrid to Marbella. As they zoomed past the town of Getafe, a three-pound meteorite smashed through their windshield on the driver’s side, ricocheted off the dashboard, and bent the steering wheel, breaking the little finger on Martin’s right hand. It then flew between the couple’s heads and landed on the back seat. Other than the broken little finger, they were okay.

Meteor Shower Comets exposed to the heat of the inner solar system slowly disintegrate This is another source of meteoritic material When the Earth passes through the debris left in a comet’s orbit, the result is a metor shower of micrometeorites.

Perseid Meteor Shower Usually the best meteor shower of the year.
It starts in August 10 and peaks the following 2 days Specs of rock that have broken off the comet Swift-Tuttle. August 10, 1998

November 13, 1833

Kuiper Belt & Oort Cloud
Kuiper Belt is a "junkyard" of countless icy bodies left over from the solar system's formation. Kuiper Belt is shaped like a disk. The Kuiper Belt extends from inside Pluto's orbit to the edge of the solar system. Kuiper Belt was discovered in 1992 There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU. The Oort Cloud, which is much further (50000 AU), is a vast spherical shell of billions of comets.

Kuiper Belt & Oort Cloud

Kuiper Belt & Oort Cloud

When is a planet not a planet?
Recently, the International Astronomical Union (IAU) had a fierce to try to iron out the definition of a planet. They decided that a planet: Is in orbit around the Sun. Has sufficient mass for their self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. Has cleared the neighbourhood around its orbit. Objects that pass the first two tests, but fail the third, and which are not themselves satellites of other planets, are now called dwarf planets.

Quaoar and Sedna: new planets?
Quaoar is a Kuiper belt object discovered by Trujillo and Brown in 2002 with the Palomar Telescope. It orbits outside Pluto and was the largest Solar System object discovered since Pluto in Its diameter is about 1300km (half the size of Pluto), and it is on a very circular orbit currently one billion miles outside Pluto. Sedna is a similar object that is even further away, and takes over 10,000 years to orbit the Sun. It was discovered in 2004 by the same astronomers.

2003UB313, aka Xena Xena and its moon Gabrielle, imaged by the Keck telescope. In 2003, a Kuiper-belt object was found which is bigger than Pluto. It even has its own moon! Its orbital period is 560 years on a highly-inclined orbit. Although colloquially known as Xena, it is called 2003UB313 until an official name is decided.

2-Formation of the Solar System

How was the Solar System Formed? A viable theory for the formation of the solar system must be based on physical principles (conservation of energy, momentum, the law of gravity, the law of motions, etc.), able to explain all (at least most) the observable facts with reasonable accuracy, and able to explain other planetary systems. How do we go about finding the answers? Observe: looking for clues Guess: come up with some explanations Test it: see if our guess explains everything (or most of it) Try again: if it doesn’t quite work, go back to step 2.

Planetary Nebula or Close Encounter?
Historically, two hypothesis were put forward to explain the formation of the solar system…. Gravitational Collapse of Planetary Nebula (Latin for “cloud”) Solar system formed form gravitational collapse of an interstellar cloud or gas Close Encounter (of the Sun with another star) Planets are formed from debris pulled out of the Sun during a close encounter with another star. But, it cannot account for The angular momentum distribution in the solar system, Probability for such encounter is small in our neighborhood…

Common Characteristics and Exceptions of the Solar System
We need to be able to explain all these!

Common Characteristics and Exceptions

The Nebular Theory* of Solar System Formation
Interstellar Cloud (Nebula) *It is also called the ‘Protoplanet Theory’. Protoplanetary Disk Protosun Gravitational Collapse Metal, Rocks Condensation (gas to solid) Sun Gases, Ice Heating  Fusion Terrestrial Planets Accretion Nebular Capture Jovian Planets Asteroids Leftover Materials Comets Leftover Materials

Gravitational Collapse
A Pictorial History Gravitational Collapse Interplanetary Cloud Condensation Accretion Nebular Capture

Pre-main Sequence Evolution
Cloud collapse 104 yr Planetary system + debris disk 109 yr 105 yr 100 AU 107 yr Tstar (K) Lstar Main sequence 8,000 5,000 10 1 2,000 Protostar+ primordial disk Planet building

The Interstellar Clouds
The primordial gas after the Big Bang has very low heavy metal content. The interstellar clouds that the solar system was built from gas that has gone through several star-gas-star cycles.

Collapse of the Solar Nebula
Gravitational Collapse Denser region in a interstellar cloud, maybe compressed by shock waves from an exploding supernova, triggers the gravitational collapse. Heating  Prototsun  Sun In-falling materials loses gravitational potential energy, which were converted into kinetic energy. The dense materials collides with each other, causing the gas to heat up. Once the temperature and density gets high enough for nuclear fusion to start, a star is born. Spinning  Smoothing of the random motions Conservation of angular momentum causes the in-falling material to spin faster and faster as they get closer to the center of the collapsing cloud.  demonstration Flattening  Protoplanetary disk. Check out the animation in the e-book! The solar nebular flattened into a flat disk. Collision between clumps of material turns the random, chaotic motion into a orderly rotating disk. This process explains the orderly motion of most of the solar system objects!

Condensation of the Solar Nebula
Composition of the Solar Nebula As the protoplanetary disk cools, materials in the disk condensate into planetesimals The solar nebular contains 98% Hydrogen and Helium (produced in the Big Bang), and 2% everything else (heavy elements, fusion products inside the stars). Local thermal environment (Temperature) determines what kind of material condensates. Water and most hydrogen compounds have low sublimation temperature, and cannot exist near the Sun. They exist far away from the Sun. Metals and rocks have high sublimation temperature, and can form near the Sun. Frost line lies between the orbit of Mars and Jupiter.

The Four Phases of Matter
There are in fact more than three phases of matter. Plasma – when the temperature is very high, high energy collision between atoms will knock the electrons lose, and they are not bounded to the atoms anymore… Core and corona of the Sun and stars Surface of the Sun and stars Surface of Earth White dwarfs, CMB

Transition Between Phases
Liquidation Evaporation Solid Liquid Gas Solidification Condensation Condensation Sublimation: atoms or molecules escape into the gas phase from a solid.

Initially, small dust and ice particles in the early solar nebula collided, sticking electrostatically. As this accretion process continues, gravity plays a greater role in forming these planetesimals. These can be as large as asteroids. Within a few million years, some of these planetesimals have grown to hundreds of kilometers and are nearly spherical as a result of their self gravitation. They start to affect the orbits of nearby planetesimals, increasing the number of collisions.

Accretion: Formation of the Terrestrial Planets
Accretion The process by which small ‘seeds’ grew into planets. Near the Sun, where temperature is high, only metals and rocks can condense. The small pieces of metals and rocks (the planetesimals) collide and stick together to form larger piece of planetesimals. Small pieces of planetesimals can have any kind of shape. Larger pieces of planetesimals are spherical due to gravity. Only small planets can be formed due to limited supply of material (~0.6% of the total materials in the solar nebula). Gravity of the small terrestrial planets is too weak to capture large amount of gas. The gas near the Sun were blown away by solar wind. Click it!

Solar Winds Solar wind is the constant outflow of gas from the Sun…
Evidences of Solar Wind Tails of Comet always point away from the Sun, indicative of the existence of solar wind. SOHO (SOlar and Heliospheric Observatory) C2 and C3 movies. Effects of Solar Wind on Planet Formation At certain stage of the planet forming process, Solar winds blow away the gases in the planetary nebula, ending the formation of the planets.

Nebula Capture: Formation of the Jovian Planets
In the regions beyond the frost line, there are abundant supply of solid materials (ice), which quickly grow in size by accretion. The large planetesimals attract materials around them gravitationally, forming the jovian planets in a process similar to the gravitational collapse of the solar nebula (heating, spinning, flattening) to form a small accretion disk. Abundant supply of gases allows for the creation of large planets. However, the jovian planets were not massive enough to trigger nuclear fusion at their core.

The Results of Selective Condensation…
Not much light gases were available for the formation of planets near the Sun, but small amount of metals and rocks are available: The planets close to the Sun are small and rocky… There are abundant supply of light gases farther out… The planets far away from the Sun are big and composed of gases of hydrogen components… These processes can explain the two types of major planets, their size differences, locations, and composition.

Origin of Comets and Asteroids
Rocky leftover planetesimals of the inner solar system. Most of the asteroids are concentrated in the asteroid belt between the orbit of Mars and Jupiter. Jupiter’s strong gravity might have disturbed the formation of a terrestrial planet here. Jupiter also affects the orbit of these asteroids and sent them flying out of the solar system, or sent them into a collision cause with other planets. Comets Icy leftover planetesimals of the outer solar system. Comets in between Jupiter and Neptune were ‘bullied’ away from this region, either collide with the big planets, or been sent out to the Kuiper belt or the Oort cloud. Comets beyond the orbit of Neptune have time to grow larger, and stay in stable orbit. Pluto may be (the biggest) one of them.

Explaining the Exceptions: Impact and Capture
Heavy Bombardment There were many impact events during the early stage of the solar system formation process, when there were still many planetesimals floating around. Evidences of Impact Comet Shoemaker’s collision with Jupiter Surface of the Moon and Mercury, More in Chapter 7… Effects of Impact Tilt of the rotation axis of planets (Venus, Uranus) Creation of satellites (May be our moon) Exchange of materials (Where did the water on Earth come from if most of the gases were blown away by solar wind after Earth was formed?) Catastrophes (Where did all the dinosaurs go?)

Where did the moons come from?
Giant Impact Our moon may have been formed in a giant impact between the Earth and a large planetesimal… Captured Moons Phobos & Deimos of Mars may be captured asteroids. Triton orbits in a direction opposite to Neptune’s rotation Capture of Comet Shoemaker by Jupiter

The Age of the Solar System
Through radioactive dating, the age of the solar system is determined as 4.6 billion years… Potassium-40 (an isotope of Potassium [K19]) decays to Argon-40 by electron capture, turning a proton in its nuclei into neutron (thus changing its chemical properties)… Potassium-40 exists naturally Argon is an inert gas that never combine with anything, and did not condense in the solar nebula… By determining the relative amount of Potassium-40 to Argon-40 trapped in rock, we can determine the age of rock, assuming that there were no Argon-40 initially…

Formation of the Solar System
Formed Gigayears ago (=age of oldest known solids in solar system) Mars formed about 13 Megayears later Earth formed 30 to 40 Megayear later Leading theory for formation of the moon is that about 100 Myr after the birth of the solar system Earth was hit by a Mars-size object. The heavy cores of both objects formed the new Earth and the light silicate crusts formed the moon. Jovian planets (Jupiter, Saturn, Uranus, Neptune) must have formed in less than 10 Myrs (life time of gaseous protoplanetary disks)

For every 1.25 billion years, half of the Potassium-40 decay and turn into Argon-40… 1.25 billion years is called the half-life of Potassium-40.

The Formation Of Solar System: Simulations
Simulations from Check them out! History of the Solar System, Part 1 History of the Solar System, Part 2 Orbit in the Solar System, Part 4 History of the Solar System, Part 3

Do we Have a Viable Theory?
YES! We can explain most of the properties of the solar system, including the exceptions. We used only good physics. Testing Our Theory against other solar system Can we find protoplanetary disks (before planets were formed)? Can we find other solar system? If we do find other solar system, does our theory explain the other solar system?

Evidences Of Protoplanetary Disks
Do we have any evidence of the existence of planetary nebulae outside of the solar system? Evidences Of Protoplanetary Disks We now have many observational evidences of the existence of the protoplanetary Disks. Hubble Space Telescope image of the dust disk surrounding Beta Pictoris Each disk-shaped “blob” is a disk of material orbiting a star…

Origin of the Solar System: Key Concepts
How the Solar System formed: (1) A cloud of gas & dust contracted to form a disk-shaped solar nebula. (2) The solar nebula condensed to form small planetesimals. (3) The planetesimals collided to form larger planets. When the Solar System formed: (4) Radioactive age-dating indicates the Solar System is 4.56 billion years old.

Clues to how the Solar System formed: How things move (dynamics)
All planets revolve in the same direction. Most planets rotate in the same direction. Planetary orbits are in nearly the same plane.

(1) A cloud of gas and dust contracted to form a disk-shaped nebula.
The Solar System started as a large, low-density cloud of dusty gas. Such gas clouds can be seen in our Milky Way and other galaxies today.

The flat, rapidly rotating cloud of gas and dust was the solar nebula.
The central dense clump was the protosun. Similar flat, rotating clouds are seen around protostars in the Orion Nebula.

The contraction of the solar nebula made it spin faster and heat up
The contraction of the solar nebula made it spin faster and heat up. (Compressed gas gets hotter.) Temperature of solar nebula: > 2000 Kelvin near Sun; < 50 Kelvin far from Sun.

(2) The solar nebula condensed to form small planetesimals.
Approximate condensation temperatures: Kelvin: metal (iron, nickel) Kelvin: rock (silicates) Kelvin: ice (water, ammonia, methane) Inner solar system: over 200 Kelvin, only metal and rock condense. Outer solar system: under 200 Kelvin, ice condenses as well.

As the solar nebula cooled, material condensed to form planetesimals a few km across.
Inner Solar System: Metal and rock = solid planetesimals Water, ammonia, methane = gas. Outer Solar System: Metal and rock = solid planetesimals Water, ammonia, methane = solid, too. Hydrogen and helium and gaseous everywhere.

(3) The planetesimals collided to form larger planets.
Planetesimals attracted each other gravitationally. Planetesimals collided with each other to form Moon-sized protoplanets.

Protoplanets collided with each other (and with planetesimals) to form planets.
Inner Solar System: Smaller planets, made of rock and metal. Outer Solar System: Larger planets, made of rock, metal and ice. In addition, outer planets are massive enough to attract and retain H and He.

Collisions between protoplanets were not gentle!
Venus was knocked “upside-down”, Uranus and Pluto “sideways”. Not every planetesimal was incorporated into a planet. Comets = leftover icy planetesimals. Asteroids = leftover rocky and metallic planetesimals.

How does this “nebular theory” explain the current state of the Solar System?
Solar System is disk-shaped: It formed from a flat solar nebula. Planets revolve in the same direction: They formed from rotating nebula. Terrestrial planets are rock and metal: They formed in hot inner region. Jovian planets include ice, H, He: They formed in cool outer region.

More Protoplanetary Disks
MAUNA KEA, Hawaii (August 12, 2004) The sharpest image ever taken of a dust disk around another star has revealed structures in the disk which are signs of unseen planets. Dr. Michael Liu, an astronomer at the University of Hawaii's Institute for Astronomy, has acquired high resolution images of the nearby star AU Microscopii (AU Mic) using the Keck Telescope, the world's largest infrared telescope. At a distance of only 33 light years, AU Mic is the nearest star possessing a visible disk of dust. Such disks are believed to be the birthplaces of planets.

3-Extra-solar Planets

Do you believe solar systems like our own are common or rare among sun-like stars in the disk of the Milky Way galaxy? Why? We expect to find planetary systems around other systems because of the Copernican Principle.

Are there more planets in the Universe?
Yes, there are other planets, so-called extra-solar planets (around stars other than the Sun). But it is very difficult to spot them, since they are far far away. Recall that a planet is much smaller than a star. How can planets of other stars be spotted then?

Planets of other stars There are three main ways that astronomers search for these planets: Doppler method Transit method Gravitational (micro)lensing

Doppler Method If you observe a star very accurately
The planet will pull the star into a small circle about the center of mutual mass, called the system barycenter. On the sky, the star will move from side to side. If you observe a star very accurately with Doppler instruments, you may be able to measure a slight “wobble“ around the center of mass. This can indicate a planet.

Radial Motion of Stars due to Planets

Astrometrically (via a positional “wobble”)
Spectroscopically (via blueshifts and redshifts of absorption lines)

Astrometric (Wobble) Detections
If a star’s position on the sky (proper motion) wobbles with time, it could be due to an unseen companion. Only Jupiter-mass planets have enough mass to be detected in this way.

First Success 1995

Transiting Planets If you can observe many stars, you may sometimes see one get slightly fainter for a little while. This happens if a planet passes between us and the star – like a mini-eclipse.

Transiting Extra-solar Planets

Gravitational Lensing Detections
If a star/planet moves exactly in front of a background star, the brightness of the background star can be greatly magnified by the gravitational lens effect.

Detection via microlensing
OGLE-2003-BLG-235 Foreground faint (invisible) star passes across background faint (invisible) star. Gravity of foreground star amplifies background star. Brightening of background star. If planet is present around foreground star, AND one is lucky that it also passes background star: one sees ‘blip’ in the signal.

Detection via microlensing
OGLE-2003-BLG-235

Extrasolar planets to date
First extrasolar planet was discovered around a neutron star in 1991 First extrasolar planet orbiting a normal star was found in 1995 by Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland orbiting the star 51 Pegasi More than 200 planets have been discovered see It is estimated that there are at least 20 billion planetary systems in our Galaxy.

What has been found? We have abundant indirect evidence of the existence of extrasolar planets!

More Known Planets

What’s wrong with this picture?
These are all Jupiter-sized planets orbiting very close to the star! Our Jupiter is way out here, 4.5 AU…

Selection Effect Actually, our methods of detecting extra-solar planets can find only massive planets that are close to the stars. So it is not surprising that all we have found are such planets. But we still need one explanation...

But, why are these large planets so close to the stars?
According to our planetary nebular theory, large planets can only be formed far away from the host star, behind the frost line, where there are abundant quantities of gases…So, why do we see these large planets so close to the stars? Possible Explanation Maybe these planets were formed far away from the stars as our planetary nebular theory predicts. But for some reason (say friction between the planets and the dense planetary gas) caused the planets to lose their orbital angular momentum and migrate toward the stars. (Planetary migration is an active research field)

Eccentricity of Planets
From: Review by G. Marcy Ringberg 2004

Is The Nebular Theory OK?
We have evidences for the existence of protoplanetary disks! We have found many extrasolar planets…by indirect methods. We have not found any solar system like ours! All the extrasolar planets we found so far are large, Jupiter-sized (or larger) planets. All these planets are located very close to the host star, inconsistent with the nebular theory. Why we don’t find any solar system like ours? May be we just haven’t found them yet! Possible Explanation  Detection Limit Larger planets at close distance to the host stars produce larger Doppler effect and intensity drop…Smaller planets far away from the star produce much smaller effect, and are more difficult to detect.

Summary We have a viable theory to explain the formation of our solar system. We have evidences that planetary nebulae exist in other star systems. However, we have not found a solar system similar to ours outside of our own. Extrasolar planets we found so far do not agree with our theory – The physics of our theory is fundamentally correct, but details of the model may need adjustment…