The Solar System. Interplanetary Matter and the Birth of the Planets. Chapter 4. Reading assignment: Chapter 4

Transcription

1 The Solar System Interplanetary Matter and the Birth of the Planets Chapter 4 Reading assignment: Chapter 4

2 The structure of the Solar System Planets, satellites (moons), dwarf planets, asteroids, comets and Kuiper belt objects are part of the solar system Kuiper belt A region of the solar system located between AU from the Sun Bodies in the Kuiper belt are composed of ices, mainly methane, ammonia and water ices. Pluto, Eris, Makemake and Haumea are examples of Kuiper belt objects

3 The structure of the inner Solar System

4 Comets Astronomer Fred Whipple named them Dirty snowballs they are composed of dust and rock in methane, ammonia and water ices The light we received from a comet comes from two sources: reflected light from the Sun and light emitted from the gas released by the comet, being ionized and excited by UV emission from the Sun. Parts of a comet: Nucleus Coma Hydrogen envelope Ion and dust tails The nucleus is the solid part of the comet. The size of the nucleus is just a few km in diameter. It is too small to be resolved with telescopes. Its diameter is beyond the resolution of a telescope The nucleus has been imaged by spacecraft only Halley s Comet in 1986 The coma can measure up to 100,000 km in diameter

5 The orbits of most comet take them far beyond the orbit of Pluto Long period comets take up to 1 million years to orbit the Sun! These comets originate in the Oort cloud. The orientation of their orbits are random respect to the plane of the ecliptic Short period comets orbit the Sun in 200 years or less (e.g. Halley s comet) The orientation of their orbits are close to the ecliptic plane Short period comets may have originated in the Kuiper belt Kuiper belt comets get kicked into an eccentric orbit, bringing them into the inner part of the solar system The Oort cloud: a reservoir of millions or perhaps billions of comets and icy type of objects located at a distance of about 50,000 to 200,000 AU from the Sun

6 The development of the gas and dust tails as the comet approaches the Sun When the comet begin approaching the Sun, the ices sublimate and release gas and dust from the nucleus The dust forms the dust tail which point in the general direction of the trajectory of the comet. The gas points in direction opposite to the Sun and forms the gas or ion tail

7 Comet Hale-Bopp (1997) dust and gas tails

8 A recent comet: Comet McNaught 2007

9 Images of comet C/2012 S1 ISON It reached the closest distance from the Sun, about 1 solar diameter on November 28, 2013 (Thanksgiving day)

10 An animation of comet ISON it is pass close to the Sun on Nov.-Dec (Images SOHO Spacecraft)

11 The nucleus of a comet The first image of the nucleus of comet Halley was obtained by the Giotto spacecraft in 1986 at a distance of about 400 km. Notice the jets coming out of the nucleus Diagram of Halley s comet nucleus showing its size and structure

12 Images of comet 67P (C-G) (Churyumov-Gerasimenko) taken by the Rosetta spacecraft in A comparison of the size of comet 67P (C-G) with downtown Los Angeles

13 Recent images of comet 67P C-G taken by the Rosetta spacecraft in 2015 The spacecraft crash-landed on the surface of the comet on Sept. 30, 2016 Image taken from a distance of 150 km showing the jets An outburst of activity Details of the surface showing Sinkholes. Sinkholes form from the collapse of material after ices sublimated

14 Meteoroid, Meteor and Meteorite Meteoroid interplanetary rocky material smaller than 100m (down to grain size). It is called a meteor when it enter and burns in the Earth s atmosphere If some material survives the entry and makes it to the ground, it is called a meteorite Castor and Pollux (The Twins, Gemini constellation) Meteor radiant (Geminids meteor shower) Meteor showers Most meteor showers are the result of the Earth passing through the orbit of a comet which has left debris along its path. They take the name of the constellation were the radiant seems to be located Some 2016 Meteor showers: Perseids ( 109P/Swift-Tuttle) - August 12 about 90/hour Orionids (Halley s comet) Oct 21 (after midnight in the Eastern sky, only 20/hour) Leonids (Comet Tempel_Tuttle) Nov 17 (after midnight in the Eastern sky, only 20/hour) Geminids (Phaethon object) December 14 Probably one of the best for 2017, predicted about 120 meteor/hour)

15 Types of meteorites Stony. This are the most common meteorites Iron-Nickel. Easy to find using metal detectors An iron-nickel meteorite is a fragment of a larger body. The body was large enough to generate internal heat and went through the differentiation process. Heavier material (iron, nickel) sunk to the core. Collisions broke the large body, exposed the central part (core) from which the smaller pieces came from. Stony-Iron

16 A stony meteorite Usually covered by a dark crust, created by the melting of the surface during the entry through the atmosphere

17 A iron-nickel meteorite

18 The Windmanstatten pattern in the etched slice of an iron-nickel meteorite

19 An example of slice of a pallasite meteorite (olivine crystals)

20 Some meteorites can be large The Hoba iron-nickel meteorite, in Namibia, Africa. Estimated mass about 60 tons. Composition, 80% Fe and 16% Ni

21 Known meteorite impact craters sites

22 Meteoroids Most meteoroids are rocky,. A small fraction are mainly iron and nickel Some contain carbonaceous material - rich in organic material Meteoroids are old billion years based on carbon dating. Most were formed when the solar system formed The Barringer Crater. A meteor crater near Winslow, AZ Formed from the impact of a 50 m asteroid weighing 200,000 tons! Diameter of crater 1.2 km Age, around 25,000 years

23 Asteroids - rocks with sizes greater than 100m across Most asteroids are in orbit around the Sun in what it is called the Asteroid belt between the orbits of Mars and Jupiter About 2000 asteroids have orbits that cross Earth s path. Called NEO, Near Earth Objects. Some of these may come at distances < 0.05 AU from the Earth. The are called PHA s (Potentially Hazardous Asteroids) Some of the more recent collisions Extinction of the Dinosaurs A km size asteroid that collided about 65 millions years ago in Chicxulub, in the Yucatan peninsula (Mexico). It left a crater about 180 Km diameter A layer of clay enriched in iridium found in many part of the world fits the age of the impact. Iridium is found in meteorites and asteroids The cloud of dust may have shrouded the planet for a few years, extinguishing the Sun s rays, killing plants and disrupting the food chain The Tunguska event in 1908 in Siberia is one of the most recent. The body (30m) exploded several km above the surface. It did not create a crater, just a depression. The most recent: The Chelyabink event. Fell in Russia on February 15, Size was about m Check the spaceweather.com website for a list of asteroids (called PHAs) coming close to the Earth!

24 Asteroids range in size from 100m to ~1000km They are composed of carbon, iron and other rocky material. The Asteroid belt is a group of asteriods that appear to have never joined to make a planet (as opposed to having once been a planet that was later destroyed). Some of the evidence is this: The total mass of all the asteroids is too small to be a planet They have different chemical compositions The reason for that is the presence of Jupiter... Jupiter may have prevented the formation of a body at the distance from the Sun where the Asteroid belt is located. The orbital period of those small bodies may have a resonance with the orbital period of Jupiter. There are gaps in the asteroid belt at several distances from the Sun where the orbital period is a fraction of Jupiter orbital period (1/3, 2/5, 1/2 )

25 Images of asteroids (All the images were taken by spacecrafts)

26 More images of several asteroids taken by spacecrafts The image of Vesta (Diameter 525 km, second largest asteroid) was taken by the Dawn spacecraft mission in Vesta has a rocky composition Comparison of sizes of different asteroids

27 The best images of the dwarf planet (asteroid) Ceres Ceres diameter is 950 km. Until it was reclassified as a dwarf planet it was considered the largest asteroid. Composition: rock and ices Best image by Hubble Space Telescope A better image taken by the Dawn spacecraft (2015)

28 The best high resolution picture released from the bright spot in Ceres (Dawn spacecraft). (The Dawn spacecraft is in orbit around Ceres)

29 Ceres bright spot, a close-up. Image taken by the Dawn spacecraft in February of 2016 from a distance of 385 km The bright spot is located in the center of the Occator crater. The close-up shows a central dome crossed by linear features and fractures The bright area may be a crust of salt marking the location of a salty ocean under the surface that broke through the central part of the crater. The low density of Ceres suggest the presence of a substantial amount of water

30 4.3 The Formation of the Solar System

31 Formation of the Solar System Any theory to describe the formation of our Solar System must be consistent with these facts: 1. Each planet is isolated in space. 2. The orbits are nearly circular. 3. The orbits of the planets all lie in roughly the same plane.

32 6. The direction of the planetary satellites orbits is the same as that of the planet s rotation. 4. The direction the planets orbit around the Sun is the same as the Sun s rotation on its axis (Counterclockwise as viewed from Earth north pole). 5. The direction most planets rotate on their axes is the same as that for the Sun.

33 7. The terrestrial and Jovian planets have different characteristics 8. Asteroids are different from both types of planets. 9. The Kuiper belt is a collections of asteroid-size bodies orbiting the Sun beyond the orbit of Neptune. Their composition is mainly ices 10. The Oort cloud comets are primitive, icy fragments. They do not orbit the Sun in the plane of the ecliptic. The Oort cloud is located around 50,000 AU

34 But there are some exceptions Venus rotates backwards (Rotational axis tilted close to 179 degrees) Uranus rotates on its side (Rotational axis tilted close to 98 degrees) Most small satellites or moons do not share the orbital plane of the planet Earth is the only terrestrial planet with a large satellite or moon Pluto has an inclined orbit respect to the orbital plane of the planets

35 A model for the formation of the solar system has to account for: Different composition of planets (rocky, gaseous, icy) Existence of many asteroids and comets

36 The nebular hypothesis (and the rejected collision theory) The idea that the solar system was born from the collapse of a cloud of dust and gas for proposed by Immanuel Kant (1755) and by Pierre Simon Laplace (40 years later). During the first part of the 20 th century, some proposed that the solar system was the result of a near collision of the Sun with another star. Planets formed from debris of the collision. But we know now that collision (or near collisions) between two stars are very, very rare. Considering that collision are rare, the proposed idea of the collision may explain a unique event on how our planetary system formed but not how other planetary systems formed. During the rest of the 20 th century, new ideas and theories about the formation of stars (and possible planets) made this collision theory obsolete and was discarded In 1994, the first exoplanet, 51 Pegasi was discovered (Exoplanets: planets orbiting other stars) Many more planets have been found so far in the solar neighborhood ( close to 1000 confirmed and more than 2000 possible ones). It is clear now that formation of planets is not a rare event. Any theory about the formation of planetary system must explain the formation of planets, not as a single, unique and rare event but more like a common event in a galaxy

37 Nebular Theory for Solar System formation Our Sun and the planets originated from the collapse of an interstellar cloud of dust and gas (nebula) Normally the gas and dust does not collapse by itself. But a pressure wave generated from a supernova explosion or a density wave in the galaxy may compress the cloud and trigger the collapse. After that, the cloud begin contracting under its own gravity; it develops a disk. The Sun (or a star) is formed at the center. The cloud starts to spin and the smaller it contracts, the faster it spins. The reason for that is: Conservation of angular momentum The cloud forms a flattened disk (solar nebula). Why they form a flattened disk? Centrifugal forces perpendicular to the rotational axis provide a push outward that resist the contraction. The forces in the direction of the rotational axis pointing away from the center are small. The gravitational forces along the rotational axis are not opposed by any other forces

38 How a flattened protoplanetary disk forms In addition to the forces involved (gravitational and centrifugal forces), collisions of particles that crosses the disk will bring those particles to an orbit contained in the plane of the disk Slow rotation Gravitational force Gravitational force Centrifugal force Fast rotation

39 Conservation of Angular Momentum Angular momentum mass velocity radius Conservation of angular momentum in a skater: The rotational speed increases when she bring her arms inside

40 Angular Momentum Objects rotating around a point have angular momentum. Consider a simple case, a small sphere orbiting a larger mass L = m x v x r L :angular momentum of the small sphere m: mass of small sphere v: velocity of the small sphere r :separation between the small sphere and the larger object Conservation of angular momentum if r changes, v must change (ice skaters) But the value of L remains constant Example: If r decreases to one half, v must increase by a factor of two to keep the value of L constant L = m x v x r L = m x 2v x r/2

41 We ve seen these disks around other young stars! A classic example is the star Beta Pictoris and its protoplanetary disk Beta Pictoris is about 50 light-years away. The disk is about 1000 AU across. The star is about 100 millions years old. It is going through the same process that the Sun went 4.6 billions years ago

42 Condensation Theory for Planet Formation The gas in the flattened nebula would never eventually clump together to form planets. However, the dust grains that are part of the cloud provide a way to clump the material together and form nucleus of condensation. Dust grains are just a few micrometer in size but they are the key for the process of condensing into bigger clumps Interstellar dust (grain-size particles) lies between stars These dust grains form from the material ejected from stars at the end of the life of the stars. When a supernova explode, it will contaminate the interstellar medium with heavy element from which the grains form. These dust grains form condensation nuclei - other atoms attach to them to start the collapsing process to which form smaller bodies (planetesimals) and finally the planets in the gas cloud. Dark cloud of dust Barnard 86

43 What happened next.. Solar nebula contracts and flattens into a disk. Condensation nuclei form clumps that grow into moon-size planetesimals. Planetesimals collide, stick together and grow. Growing planetesimals form the planets over about 100 million years. The more massive proto-planets are also able to sweep up large amounts of gas to become the Jovian planets. Solar wind (Or stellar wind in general) from the star formation blows out the rest of the gas.

44 An artist s impression of a young star and its protoplanetary disk in the process of forming planets The young Sun gas/dust nebula solid planetesimals

45 More Evidence Beyond our Solar System Early stages of a planetary system formation can be imaged directly Dust disks have large surface area and radiate effectively in the infrared Hubble image of a young solar system. Young star clearing part of the gas A recently released ALMA image (Radio wavelengths) of protoplanetary disk in HL Tauri, 450 ly away, about 1 Myear old Thick disk

46 What creates a difference between inner and outer planets in the solar system? The answer: TEMPERTURE! Rocky inner planets: The type of the material that condensed out of the nebular cloud at these higher temperatures was metallic and rocky in nature. Gaseous, Bigger outer planets: Both rock and gas could condense out of the cloud at lower temperatures where these planets formed. But gas such as H, He, water, methane and ammonia also condense at the low temperatures Why are they gaseous? gas and ices are present at that distance Why are they bigger? - accretion onto the planet starts sooner because they are further from the Sun, less affected by solar wind

47 Extrasolar Planets or Exoplanets (Planets orbiting around other stars) Over 1779 exoplanets have been confirmed as detected. They are in orbit around more that 1102 planetary systems. All these systems are in nearby stars, around the solar neighborhood. There are 460 multiple planetary (Two or more planets) systems confirmed There are different methods to detect exoplanets. The two methods that are discovering more exoplanets are: Observing the star s wobble (Doppler shift) due to gravitational attraction of the orbiting planet(s). Observing the transit of a planet in front of the star 51 Pegasi - the first detection in 1994 (using the Doppler shift method) of an extrasolar planet A sketch of 3 planets orbiting Upsilon Andromedae! (The star have actually 4 planets) For more about exoplanets, check on Planetquest, the search for another Earth: planetquest.jpl.nasa.gov

48 An animation of a planet orbiting a star The planet and the star orbit around the common center of mass (+) Observing the Doppler shift of the absorption (or emission) lines in the spectrum of a star will show a periodic variation in the radial velocity of the star

49 Detecting planets using the Doppler shift of the star

50 Detecting exoplanets using the Doppler shift method The star 51 Pegasi has one planet orbiting the star. We see a periodic variation in the radial velocity of the star The star Upsilon Andromedae has 4 planets. The plot clearly show two periodicities in the radial velocity of the star. Other periodic variation caused by the other two planets are smaller and are mounted on top of the two periodicities

51 Detecting exoplanets using the transit method The light from the star is decreased by a small amount when the planet is transiting in front of the star. The decrease is small, about 1/10,000 of the light of the star. Measuring the change in the light curve of the star over time, reveals the presence of an exoplanet The Kepler spacecraft is using the transit method.

52 The Kepler spacecraft The spacecraft was launched on March 7 th, 2009 It is in orbit around the Sun It has detected 961 exoplanets 3,845 exoplanets candidates Instrumentation: It has a 0.95 meter diameter telescope with a photometer It is monitoring 145,000 stars in the Cygnus and Lyrae constellations

53 Field of view of Kepler and location of exoplanets candidates The region of the sky being monitored by Kepler is between the Deneb and Vega stars in the Summer Triangle (formed by Vega, Altair and Deneb stars)

54 Latest announcement of planets detected by Kepler (March 2014)

55 Naming exoplanets Exoplanets are named after the star they orbit. The name of the planet is the name of the star plus a lower case letter starting with b for the first planet, c for the second planet and so on. Example: The star Kepler 47 has two planets, one planet is named Kepler 47b the other Kepler 47c

56 Examples of light curve of some of the exoplanets detected by Kepler If the size of the planet is large with respect to the size of the star, the dip in the light curve is larger

57 A few notes regarding detection of exoplanets Most of the exoplanets detected are relatively large, about Jupiter s mass or a few times the Jupiter s mass and reside close to the parent star. These planets are easier to detect. Because of their bigger mass and close distance to the star, they produce larger radial velocity changes in the star which are reflected in a large Doppler shift of the star. Because they are close to the star, they have short orbital periods. It is easier to observe an occultation; the occultation occur more often. The light curve can be obtained in a short time. The detection of bigger planets is a selection effect. Small planets and farther away from the star may be there but are more difficult to detect because they produce a small Doppler shift or their orbital period is long or the dip in the light of the star is small. A few planets close or a little larger than the Earth have been detected. Planets with the same mass or close to Earth mass have already been detected Detections of Earth mass planets in the habitable zone is a high priority in the search for planets. The habitable zone is the region around a star where water can exist in liquid form

58 The location of the Habitable Zone A plot of distance from a star as function of star mass (or temperature) where water can exist in liquid state Notice that the scales of the plot are logarithmic

59 A few important notes regarding detection of planets, the location of the habitable zone and the possibility of life on planets in that zone One condition for a planet to be able to harbor life is to be located in the habitable zone. But there are other important conditions. Stars of higher temperature radiate more energy (F = T 4 ). The habitable zone must be farther from the star. Planet located a large distance are more difficult to discover, they have long orbital periods. The habitable zone in stars of lower mass (lower T) may be closer. Planets located in the habitable zone may have shorter orbital periods, They are easier to detect Star of larger mass evolve much faster, life may not have enough time to start or evolve. Stars of low mass evolve much slower. The life time of those stars may be long for life to start and evolve. Higher temperature stars radiate more UV, not good for sustaining life. Planets can be located in the habitable zone but its atmosphere may have evolved (Example: Venus). They may have high temperature. The composition of the atmosphere may contain gases that are not good for sustaining life. Others planets located in the habitable zone may not have the protection of a magnetosphere.

60 The Kepler 62 system has 5 planets, two of them in the habitable zone Comparison of the habitable zone in the Solar System and in the Kepler 62 planetary system

61 Kepler 47, planets around a binary star Comparison of Solar System and Kepler 47 habitable zones One of Kepler 47 planets reside in the habitable zone

62 Potentially habitable exoplanets (As of Nov. 2012)

63 An example of an image of an exoplanet in the star Fomalhaut (Hubble Space Telescope)

64 Is it possible to detect the composition of gases (or chemical elements) present in the atmosphere of an exoplanet? The light from the star is transmitted through the atmosphere of a planet during the primary eclipse. It can be analyzed in a spectrograph to reveal the presence of elements and molecules in the atmosphere of the exoplanet Sodium (Na), potassium (K), methane have been detected in the atmosphere of exoplanets Planet atmosphere

65 Latest news regarding detection of exoplanets In October 2012, it was reported the discovery of an exoplanet in Alpha Centauri (binary star), the closest star to the solar system, 4.3 ly away The planet has a mass of 1.2 Earth mass, orbital period 3.2 days. The distance from the star is 0.04 AU. Estimated temperature is high, 1,500 K ( For comparison, Venus 735 K) The detection need to be confirmed. If it can be confirmed, it will be the closest exoplanet to the solar system.

66 A summary on the latest news regarding detection of exoplanets Exoplanets residing in the habitable zone have been discovered Exoplanets have been detected around binary stars. The binary star Kepler 47 has two planets orbiting around it. One of them in the habitable zone Detection of exoplanets with mass close to Earth mass One example is the planet discovered around Alpha Centauri with 1.2 mass of the Earth. But it distance is too close to the star (0.04 AU) and the temperature is too high (1,500 K) Kepler 186f was announced in It has a 1.2 Earth s mass, located in the habitable zone, in orbit around a red dwarf star. Kepler 452b was announced in It has a 1.63 Earth s radius, located in the habitable zone, in orbit around a G2 star.

67 A note regarding the structure of other planetary systems Several of the planets found so far are large planets, their size and mass are around or bigger than Jupiter. Some of them are gaseous planets, with very low density. Some densities as low as 500 kg/m³ (Water density 1000 kg/m³) They are located close to the star, at a distance less than the distance of Mercury from the Sun. Their orbital periods are in the range of a few days. If they are massive, they are called hot Jupiters This is an unusual configuration is we compare with the Solar system. According with simulations, these planets may not have been born at that close distance from the star. They were formed at a larger distance and later they migrated inward due to interaction with the material in the proto planetary disk