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Module 2 - Planetary Formation and Differentiation

The Solar Nebula

The most widely accepted model for the formation of the Solar System is the nebular hypothesis, which holds that the Solar System formed from nebular material. In short, a supernova explosion at 4.55 billion years ago formed a nebula (an immense cloud of gas and dust in space; also called a “molecular cloud”) and the formation of the Sun and of the Solar System is the product of the condensation and gravitational collapse of the solar nebula. A Sun-like star usually takes ~1 million years to form from a nebula. The planetary system evolves over the next 10 to 100 million years.

Please watch this video for a good summary of the nebular hypothesis. More details follow in the text below.

The nebula that became our solar system began as a large irregularly shaped mass of gas and dust in space. The nebula was gravitationally unstable. Within the nebula the pressure of the gases act outwards to cause it to expand while gravitational forces (forces that pull bodies towards each other) act to cause the nebula to collapse onto itself.
The force of gravity prevailed over gas pressure and the nebula collapsed and began to spin.
As the diameter of the nebula was reduced with collapse, the rate of spin increased. An analogy is a spinning figure skater who draws her arms toward her center to increase rotation speed. (This is called conservation of angular momentum. Don't worry about the details behind the physics.)
Due to the interaction of the pressure and gravitational forces, as the nebula was spinning it became flatter and formed a broad disk as the nebula continued to collapse.
As the density of the centre of the disk increased along with its temperature, the core of the nebula became the protosun. Collapse of the protosun is often accompanied by jets of dense gas, called molecular outflow, that emanate along the rotational axis of the disk . The initial collapse to a protosun the size of the one that formed our Sun takes around 100,000 years.

Within the cloud swirling eddies developed drawing matter towards their centres to form the protoplanets.

As the protosun became even hotter gases were driven off the inner region of the Solar System. The protoplanets became solid planets and continued their orbit, governed by the initial spin of the swirling nebula.

The temperature gradient within the solar nebula influenced the formation and distribution of the planets.

The very high temperatures toward the centre of the solar nebula caused only the most refractory materials to condense upon initial cooling. (Refractory refers to difficulty of melting.)
The very high temperatures toward the centre of the nebula also meant that the innermost planet Mercury is particularly volatile-poor (e.g., low water and carbon dioxide) and richer in more refractory phases, such as iron. Mercury therefore has a higher density than the other planets when compression is taken into account.
Gas giants are thought to form beyond the frost line, which is a distance in the solar nebula from the central protostar where it is cold enough for volatile compounds such as water, ammonia, methane, and carbon dioxide to condense into solid ice grains. The presence of these ices in the gas giants causes them to have low overall densities.

Protoplanetary disks have been observed around young stars by the Hubble Space Telescope.

The Nebular Hypothesis is attractive because it explains many features of the Solar System. For example, the orbits of the planets lie in a plane with the sun at its center. This plane is called the "orbital" plane or "ecliptic" plane and it is also the plane of the early disk-shaped nebula.
The Nebular Hypothesis also explains why the planets mostly rotate in the same direction and their axes of rotation are nearly perpendicular to the orbital plane. This direction of rotation was inherited from the direction of spin of the eddies in the spinning nebula that formed the protoplanets.

Venus and Uranus do not rotate in the same direction as the other planets. Venus’ rotational axis is at right angles to the plane of the planets (the ecliptic plane) but it rotates in the opposite direction compared to the other planets. Uranus rotates about an axis that is almost parallel to the plane of the planets. Modern thinking is that the rotations of both planets were affected by major collisions with other bodies very early in their history.

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Molecular outflow from an otherwise hidden newborn star This image from NASA's Spitzer Space Telescope transforms a dark cloud into a silky translucent veil, revealing the molecular outflow from an otherwise hidden newborn star. Using near-infrared light, Spitzer pierces through the dark cloud to detect the embedded outflow in an object called HH 46/47. Herbig-Haro (HH) objects are bright, nebulous regions of gas and dust that are usually buried within dark clouds. They are formed when supersonic gas ejected from a forming protostar, or embryonic star, interacts with the surrounding interstellar medium. These young stars are often detected only in the infrared. The Spitzer image was obtained with the infrared array camera. Emission at 3.6 microns is shown as blue, emission from 4.5 and 5.8 microns has been combined as green, and 8.0 micron emission is depicted as red. HH 46/47 is a striking example of a low mass protostar ejecting a jet and creating a bipolar, or two-sided, outflow. The central protostar lies inside a dark cloud (known as a 'Bok globule') which is illuminated by the nearby Gum Nebula. Located at a distance of 1140 light-years and found in the constellation Vela, the protostar is hidden from view in the visible-light image (inset). With Spitzer, the star and its dazzling jets of molecular gas appear with clarity. The 8-micron channel of the infrared array camera is sensitive to emission from polycyclic aromatic hydrocarbons. These organic molecules, comprised of carbon and hydrogen, are excited by the surrounding radiation field and become luminescent, accounting for the reddish cloud. Note that the boundary layer of the 8-micron emission corresponds to the lower right edge of the dark cloud in the visible-light picture. Outflows are fascinating objects, since they characterize one of the most energetic phases of the formation of low-mass stars (like our Sun). The jets arising from these protostars can reach sizes of trillions of miles and velocities of hundreds of thousands miles per hour. Outflows are clear evidence of the presence of a process that creates supersonic beams of gas. This mechanism is tightly bound to the presence of circumstellar discs which surround the young stars. Such discs are likely to contain the materials from which planetary systems form. Our Sun probably underwent a similar process some 4.5 billion years ago. Hence the interest in understanding how quickly and efficiently this mass accretion and loss process takes place in protostars. Image credit: NASA/JPL-Caltech/A. Noriega-Crespo (SSC/Caltech), H. Kline (JPL), Digital Sky Survey
Figure skater demonstrating conservation of angular momentum
Nebula flattens with time and becomes a disk
A stable planetary system
Side view of planetary disk
Hubble Confirms Abundance of Protoplanetary Disks around Newborn Stars *Description*: A Hubble Space Telescope view of a small portion of the Orion Nebula reveals five young stars. Four of the stars are surrounded by gas and dust trapped as the stars formed, but were left in orbit about the star. These are possibly protoplanetary disks, or "proplyds," that might evolve on to agglomerate planets. The proplyds which are closest to the hottest stars of the parent star cluster are seen as bright objects, while the object farthest from the hottest stars is seen as a dark object. The field of view is only 0.14 light-years across. The Orion Nebula star-birth region is 1,500 light-years away, in the direction of the constellation Orion the Hunter. The image was taken on 29 December 1993 with the HST's Wide Field and Planetary Camera 2. Technical facts about this news release: Back to entire collection [ http://hubblesite.org/newscenter/archive/releases/index/561/ ] Next release [ http://hubblesite.org/newscenter/archive/releases/1994/23/ ] Previous release [ http://hubblesite.org/newscenter/archive/releases/1994/26/ ] *News Release Number:*: STScI-1994-24b
Competing forces in the nebula
Nebula collapse and spin
Protoplanets form in disk
Debris disks around young stars RELEASE 14-114 http://www.nasa.gov/press/2014/april/astronomical-forensics-uncover-planetary-disks-in-nasas-hubble-archive Astronomical Forensics Uncover Planetary Disks in NASA's Hubble Archive Debris disks around young stars The two images at top reveal debris disks around young stars uncovered in archival images taken by NASA’s Hubble Space Telescope. The illustration beneath each image depicts the orientation of the debris disks. Image Credit: NASA/ESA, R. Soummer, Ann Feild (STScI) Astronomers using NASA's Hubble Space Telescope have applied a new image processing technique to obtain near-infrared scattered light photos of five disks observed around young stars in the Mikulski Archive for Space Telescopes database. These disks are telltale evidence for newly formed planets. If astronomers initially miss something in their review of data, they can make new discoveries by revisiting earlier data with new image processing techniques, thanks to the wealth of information stored in the Hubble data archive. This is what Rémi Soummer, of the Space Telescope Science Institute (STScI) in Baltimore, Md., and his team recently did while on a hunt for hidden Hubble treasures. The stars in question initially were targeted with Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) based on unusual heat signatures obtained from NASA's Spitzer Space Telescope and the Infrared Astronomical Satellite that flew in 1983. The previous data provided interesting clues that dusty disks could exist around these stars. Small dust particles in the disks might scatter light and therefore make the disks visible. But when Hubble first viewed the stars between 1999 and 2006, no disks were detected in the NICMOS pictures. Recently, with improvements in image processing, including algorithms used for face-recognition software, Soummer and his team reanalyzed the archived images. This time, they could unequivocally see the debris disks and even determine their shapes. The NICMOS instrument, which began collecting data in 1997, has been so cutting-edge that ground-based technology only now is beginning to match its power. Because Hubble has been in operation for almost 24 years, it provides a long baseline of high-quality archival observations. "Now, with such new technologies in image processing, we can go back to the archive and conduct research more precisely than previously possible with NICMOS data," said Dean Hines of STScI. "These findings increase the number of debris disks seen in scattered light from 18 to 23. By significantly adding to the known population and by showing the variety of shapes in these new disks, Hubble can help astronomers learn more about how planetary systems form and evolve," said Soummer. The dust in the disks is hypothesized to be produced by collisions between small planetary bodies such as asteroids. The debris disks are composed of dust particles formed from these grinding collisions. The tiniest particles are constantly blown outward by radiation pressure from the star. This means they must be replenished continuously though more collisions. This game of bumper cars was common in the solar system 4.5 billion years ago. Earth's moon and the satellite system around Pluto are considered to be collisional byproducts. "One star that is particularly interesting is HD 141943," said Christine Chen, debris disk expert and team member. "It is an exact twin of our sun during the epoch of terrestrial planet formation in our own solar system." Hubble found the star exhibits an asymmetrical, edge-on disk. This asymmetry could be evidence the disk is being gravitationally sculpted by the tug of one or more unseen planets. "Being able to see these disks now also has let us plan further observations to study them in even more detail using other Hubble instruments and large telescopes on the ground," added Marshall Perrin of STScI. "We also are working to implement the same techniques as a standard processing method for NASA's upcoming James Webb Space Telescope," said STScI teammate Laurent Pueyo. "These disks will also be prime targets for the Webb telescope." Soummer's team has just begun its work. They next will search for structures in the disks that suggest the presence of planets. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. STScI in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.
Nebular hypothesis theory