The Birth of the Solar System

Phil Plait, Bad Astronomy Blog

Discover Magazine

 

Much like all stars, the Sun was formed by the collapse of a dust and gas cloud called a nebula. Some perturbation causes the nebula to start rotating, and in order for the nebula to keep from dissipating, the cloud must start to collapse and shrink in size. This is a law of physics called conservation of angular momentum.

A good example of the conservation of angular momentum is to think of a figure skater. When the skaters spin on their skates with their arms outstretched, the skaters spin slowly. As they bring their arms in closer to their bodies, the rate of spin increases. To keep the same angular momentum, the angular speed must increase as the outmost portion of the rotation decreases.

Angular Momentum

The University of Reading

L = R*ω
where

• L is the angular mometum, which is the same for the left and right pictures above
• R is the distance from the center of spin to the outermost portion of the body spinning (this is shown as I above with is actually the moment of Inertia around the center of spin)
• ω is the angular speed, i.e. how fast the angle is changing over time.

When a nebula does this, not only does the cloud shrink in radius, but the center becomes more dense. As the density increases, so does the temperature. When the temperature reaches about 15 million Kelvin, it is hot enough that the protons in the center can overcome their natural repulsion due to electromagentic forces to fuse together. This is the beginning of the proton-proton chain which was discussed way in the beginning. As the temperature begins to rise and fusion takes place, the radiation pressure from the core of the protostar stops the collapse of the cloud, putting the star in hydrostatic equilibrium. The radiation pressure is enough to push back against the gravitational force trying to squash the star.

Astronomy Online

The rest of the cloud is now free to consolidate into planetessimals, freeing up the area around their orbits. The inner part of the solar nebula contained mostly refractory elements, leading up to more dense planetessimals and the outer part contained a lot more volatile elements, leading to planetessimals that were both rocky and icy. As the planetessimals starting colliding with each other, they stuck together, melting, and allowing the heaviest elements to sink to the center of the bodies. As more and more planetessimals collided, the bodies grew bigger and bigger, getting to a point where the bodies were able to form spherical shapes, and leading to the planets we see today.

Near Earth Asteroids

A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt.

 

Besides the asteroid belt and the Greek and Trojan Asteroids, there are also asteroids that come close to Earth. These asteroids are called near-Earth asteroids, or near-Earth objects (NEOs). You have probably have heard of these asteroids on the news, telling us a new one has been discovered and it will make a near approach to Earth sometime in the future.

What actually defines what we mean by a near-Earth asteroid? By definition, a near-Earth asteroid is one that can come within 0.3 AU of Earth. We use 0.3 AU as a baseline, because at closest approach between Earth and Venus, the planets are only 0.27 AU apart (when Venus is at Aphelion of 0.728 AU and Earth is at perihelion of 0.983 AU). This number is rounded up to 0.3 AU to make it easier to define NEOs.

As mentioned in a previous post, asteroids are numbered sequentially, i.e. in what order it was discovered. Typically, near-Earth asteroids are not numbered until they have been observed at opposition at least twice. Recall, opposition is when an object (in this case, an asteroid) is 180° away from the Sun in the sky.

An asteroid (brown circle) at opposition

 

It should be noted that although these asteroids do come close to the Earth, they do not necessarily cross the orbit of the Earth and are not potentially harmful to the well being of the inhabitants or our planet.

 

There are three types of near-Earth asteroids which will all be discussed in their own blog posts. We refer to these asteroids as Aten, Amor, and Apollo asteroids, with each group named after the first asteroid discovered to fall into that group.

• Aten asteroids are asteroids that have semi-major axes smaller than one AU
• Amor asteroids are asteroids that have semi-major axes larger than one AU
• Apollo asteroids are asteroids that have semi-major axes around one AU.

Greek and Trojan Asteroids

Jupiter has a two groups of asteroids lagging behind it in its orbit and leading it as it goes around the Sun. These asteroids are called Trojan asteroids and the two groups are broken down into the two camps during the Trojan War; the Greek camp which is the leading group and the Trojan camp which are the ones lagging behind.

Both of these groups are situated at Lagrangian points which are gravitational balanced points in a three-body system. The Greek asteroids are located at L4 in the Sun-Jupiter system and the Trojan asteroids are at L5.



From our post about Lagrangian points, we know that the Greeks are at 60° ahead of Jupiter in its orbit and the Trojans are 60° behind. Where did these asteroids come from?

 

The leading theory is that these asteroids are remnants of the formation of the Solar System and in as they moved in space, the asteroids got caught in the L4 or L5 Lagrangian points, which we learned is stable. Once they got to those points, they were there permanently.

 

One last thing about Trojan and Greek asteroids is that all the ones that have been discovered are named after Greek or Trojan heroes from the Trojan war.

Asteroids

Asteroids are believed to be remnants from the formation of the Solar System. The majority of these objects are found in the space between Mars and Jupiter called the asteroid belt.



As seen in the above image, there are also a few asteroids that are 60° ahead of Jupiter called "Greek" asteroids and some that are 60° behind Jupiter called "Trojan" asteroids. Not seen, but will be discussed are the three-A asteroid groups: Amor, Apollo, and Aten. These are near-Earth asteroids that are a concern for scientists and are searched for and catalogued by astronomers.

In our previous post about Phobos and Deimos, we learned a little about C-type and D-type asteroids. There are two other main types (and no, they are not A-type and B-type) called S-type and M-type. As mentioned before, C-type asteroids (and to an extent, D-type) have strong carbon lines in their spectra. S-type asteroid are stony asteroids, made of silicates, with densities similar to Earth's density, and make up about 17% of all asteroids. M-type asteroids are metallic, with the most common metal found in them iron and some having nickel. This is one reason why mining asteroids might be a lucrative business in the future, when travelling easily from Earth to the outer reaches of the Solar System will be achieved.

253 Mathilde - C-type Asteroid

NASA

15 Eunomia - S-type Asteroid

Astronomical Institute of the Charles University

16 Psyche - M-type Asteroid

Astronomical Institute of the Charles University

 



 Asteroids, generally, come in many sizes, with the largest being 1 Ceres at 950 km in diameter. However, most asteroids are only a few miles in size and are irregularly shaped. It is typically believed that asteroids are planetessimals (baby planets) that were not able to accrete into a planet because of the presence of Jupiter. Measurements of the mass of the asteroid belt show that there is only enough mass there to create a planet with the quarter of the size of a planet.

Asteroids have two parts to their names, a number designation and a proper name. The first few asteroids were just given names, but as more and more were discovered, astronomers started placing a number in front to give an indication of the sequence of discovery. So 1 Ceres was the first asteroid discoved and 253 Mathilde was the 253rd. As of 2013, there are several hundred thousand asteroids named with more than a million probably out there. Many of those asteroids are in the asteroid belt, but as mentioned above, there are thousands leading and trailing Jupiter in its orbit and thousands that are near Earth's orbit. We will learn more about Amor, Aten, and Apollo asteroids in a future post.

Most asteroids orbit independently around the Sun, but there are some that mutually orbit each other.  The most well known pair is Ida and Dactyl, where Dactyl was the first satellite discovered around an asteroid.  The Solar System is a strange place.

The Satellites of Mars

Mars has two satellites, Phobos and Deimos. They are named after creatures summoned by Ares (the Greek equivalent of the Roman war god Mars) in the Iliad and their names mean Fear (Phobos - where the word Phobia comes from) and Fright (Deimos). Unlike the Moon, these satellites did not form in the same location in the solar nebula as Mars but rather formed elsewhere (likely, the asteroid belt between Mars and Jupiter) and wandered too close to Mars and were captured by its gravity.

AreaVoices.com

 

Phobos is an 11-km diameter, irregularly shaped object that only orbits 6000 km from the surface of Mars. Compare that to the Moon, which orbits 384,400 km from the surface of the Earth. If the Moon orbited only 6000 km from the Earth, not only would our tides be much higher (see post on tides) but the Moon would appear 64 times bigger in the sky making it about 32° across. At that apparent diameter, it would fill up a sixth of the sky!  Because Phobos is so close to Mars, it only takes about seven and a half hours to orbit Mars, which means that it crosses the Martian sky twice in one Martian day, taking only about four and a quarter hours to cross the sky.  It also orbits retrograde around Mars, meaning that it rises in the west and sets in the east.

 

Deimos is smaller than Phobos, being only 6.2 km in diameter, but orbits much farther away, at 23,500 km from the Martian surface.  At that distance, Deimos takes about 30.3 hours to complete one orbit around Mars, or about a Martian day and a quarter.

 

Both Phobos and Deimos were discovered in August of 1877 by Asaph Hall at the United States Naval Observatory in Washington, D.C. (Quick fact - the USNO is home to the official Master Clock for the US and is also the official residence of the Vice President.)  Despite being smaller, Deimos was actually discovered first on August 12th and Phobos was discovered on August 18th.  The names were suggested by Henry Madan from the Iliad.

 

The reason why Phobos and Deimos are believed to be captured asteroids is because they are similar in composition, albedo, densities of C- or D-type asteroids.

• C-type asteroids (carbonaceous asteroids)
• The most common type of asteroids (make up about 75% of all asteroids
• They have a low albedo which means they do not reflect a lot of light, almost appearing black
• Their compositions are similar to the early solar nebula except for the lack of volatile elements (gases, water, etc) but do contain hydrated minerals (water-containing minerals)
• D-type asteroids
• They have a lower albedo than C-type asteroids
• Their spectra are the strongest toward the red end of the electromagnetic spectrum
• They contain organic, carbon, and anhydrous (lacking-water) silicates
• However, they may have water ice cores