Astronomy: Chapters 5 & 6

About how long after the universe came into existence did our solar system form?

10 billion years

How many planets are known to exist in the solar system?

8: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, & Neptune

Hydrogen and helium together account for what percentage of the total mass of all the matter in the universe?

98%

Comets are typically

chunks of ice that begin to vaporize if they pass close to the Sun.

The word "albedo" refers to the

amount of light reflected by a planet or other object.

The most abundant material in the universe is

hydrogen.

Meteroid

Space debris smaller than 10 m & composed primarily of rock & metal.

Microlensing

(Gravitational microlensing) Matter warps its surrounding space, causing, among other effects, passing light to change direction. A star with an exoplanet, passing between Earth & a distant star, can focus the light from the distant star, causing it to appear to change brightness. This change occurs twice, once as the distant star's light is focused toward us by the closer star & again, to a larger extent, when the light is focused toward us by the exoplanet & the star together.

How many stars are there in the solar system?

1, the Sun

Comet

All debris that is a roughly equal mix of ice & rock. More precisely, the solid body of ice & rock that exists even when a comet has no tail is called a comet nucleus.

Which planets have 1 or more satellites (or moons) orbiting them?

All except the inner 2 planets

Were the Sun & its planets among the 1st generation of objects created in the universe?

All matter & energy were created by the Big Bang. However, all the elements in the solar system besides hydrogen, helium, & lithium were created in previous generations of stars. The solar system formed billions of years after the Big Bang occurred from the debris expelled by stars that preceded it.

Metal

All of the elements in the universe other than hydrogen & helium. The difference in chemical composition between the gas & stars in the early universe (lacking metals) & the metal-rich solar system & other planetary systems today shows that these systems were not among the 1st generation of stars. The solar system didn't form as a direct result of the Big Bang.

Trans-Neptunian Objects (TNOs)

All the objects that orbit the Sun farther than Neptune. All Kuiper Belt & Oort Cloud bodies are TNOs, because they orbit farther from the Sun than the outermost planet.

What typical shape(s) do moons have, & why?

Although some moons are spherical, most look roughly like potatoes. Those that are spherical are held together by the force of gravity, which pulls down high regions. Those that are potato-shaped are held together by the electromagnetic interaction between atoms, just like rocks. These latter moons are too small to be reshaped by gravity.

Describe how exoplanets are discovered by measuring stellar radial velocities.

As a planet moves toward or away from us, its star moves in the opposite direction (radial motion). The spectroscopy is used to measure the Doppler shift of the star's spectrum, which reveals the effects of the unseen planet.

What created most of the craters in the solar system? What else could create craters? (Hint: Think of craters on Earth.)

Asteroids, Comets, & Meteors

Terrestrial Planet

Because Mercury, Venus, & Mars have similar densities on Earth, they are composed of similar chemicals. Consequently, all 4 bodies are called terrestrial planets. They are relatively small, have high average densities, & are composed primarily of rock & metal.

Orbital Inclination

Because the planets formed in a disk, they should all orbit in more or less the same plane as Earth (that is, nearly in the plane of the ecliptic). Observations bear out this conclusion. The angles of the orbital planes of the other planets with respect to the ecliptic are called their orbital inclinations, & all are 7° or less.

Planet

Celestial body that (1) orbits the Sun or another star; (2) has enough mass so that its own gravitational attraction causes it to be essentially spherical, rather than being a large, irregular rock in space; & (3) has enough celestial attraction to clear its neighborhood of other orbiting debris. Planets "clear their neighborhoods" by either pulling surrounding debris onto themselves or flinging debris far away from their orbits. There are 8 known planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, & Neptune.

Accretion

Coming together of smaller pieces of matter to form larger one, of dust particles continued as they spiraled inward. Like static cling between clothes, accretion of dust-sized particles is due to the attraction of their charged particles, protons & electrons, while accretion of much larger pieces of debris is primarily due to their mutual gravitational attraction. Through accretion, the dust grew into pieces of debris meters across.

Protosun

Deep inside the nebula, gravitational attraction caused gas and dust particles to fall rapidly toward its center. The cloud's gravitational potential energy was being converted into kinetic energy. As a result of the innumerable collisions there, the density, pressure, and temperature at the center of the nebula began to increase, producing a concentration of matter called the protosun. As the protosun continued to contract (become denser) & to increase in mass, atoms within it collided with increasing speed and frequency. This activity created growing pressure inside the protosun, which in turn caused its contraction to slow dramatically. Such collisions also created heat, causing the protosun's temperature to soar.

Dense Core

Each collapsing fragment destined to become a star (or pair or stars orbiting each other called a binary star system, or even 3 stars orbiting together called a trinary or ternary star system) & possibly planets is called a dense core. Just as smoke created in a fireplace swirls as it rises, interstellar clouds are comprised of swirling masses of gas & dust. As a result, many dense cores are rotating as they collapse.

How long has Earth existed, & how do we know this?

Earth formed along with the rest of the solar system, about 4.6 billion years ago. The age is determined from the amount of radioactive decay that has occurred on Earth.

Pluto is most similar in composition to which of the following objects?

Eris

Name 1 dwarf planet (other than Pluto) & state where it is located.

Eris, which is located in the Kuiper Belt.

Protoplanetary Disk

Formed by gas & dust swirling faster & faster as it condenses. While most are relatively uniform, some have unusual features, including ripples, analogous to water waves, running waves, running around them, or spiral waves, similar to the shape of a spiral galaxy. These spirals are believed to either result from the gravitational pull of protoplanets there or from some external gravitational disturbance of the disk. In the former case, we may be seeing the disks shortly before the planets start to form there.

Albedo

Fraction of incoming light returning directly into space is called a body's albedo. An object that scatters no light has an albedo of 0:0; for example, powdered charcoal has an albedo of nearly 0.0. An object that scatters all of the light that strikes it (high-quality mirror comes close) has an albedo of 1. Multiplying the albedo of an object by 100 gives the percentage of light directly scattered off that body.

Hot Jupiter

Gas giant in a small orbit that is the easiest class of exoplanets to detect using the radial velocity method.

Average Density

How much mass the object has in a unit of volume. Total Mass/Total Volume

Crater

Impacting bodies larger than about 0.5 m across create scars, called craters. Most of the craters on our planet's surface have been erased, but we can still see numerous ones on our Moon, as well as on nearly all the other bodies with solid surfaces in the solar system. Venus is the exception, as it is periodically resurfaced.

Super-Earth

In 2005, Doppler measurements were used to detect a terrestrial (rocky) exoplanet labeled Gliese 876 d orbiting star Gliese 876. This exoplanet has 6.8 times Earth's mass &, because it is less than twice Earth's radius, it is classified as a super-Earth. Super-Earths have been discovered orbiting about 60% of the Sublime stars near us.

Which giant planet formed 1st?

Jupiter

Which is the largest planet in our solar system?

Jupiter

Which planet of our solar system has the highest mass?

Jupiter

Asteroid Belt

Major reservoir of leftover rock & metal debris remaining in the inner solar system, the asteroid belt, located between Mars & Jupiter. Orbiting in this region are millions of planetesimals & smaller pieces of debris. The total mass of debris in the asteroid belt is about 5% of our Moon's mass, much less than the mass of a planet. There were bodies in the asteroid belt in the distant past, but impacts & the gravitational tug of Jupiter pulled many of them away & kept the remaining debris spread out, thereby preventing it from ever coming together to form a single, larger body.

Oort Cloud

Many more planetesimals were sent out even further than the Kuiper belt, creating a spherical distribution of debris called the Oort cloud after Jan Oort (1900-1992).

The asteroid belt exists between the orbits of which planets?

Mars & Jupiter

The smallest terrestrial planet is

Mercury

The albedo of Mercury is 0.1. This means that

Mercury reflects 1/10 of the sunlight falling on it.

Describe how exoplanets are discovered by mircolensing.

Microlensing is the gravitational focusing of light from a distant star or galaxy by a closer object to give a brighter image of the star or galaxy. Just as a light passing through a lens changes direction and is focused, light can also be focused by gravity. As a star with a planet passes between Earth and a more distant star, the light from the distant star is focused toward us making the distant star appear brighter. The focusing of the distant star's light occurs twice, once by the closer star and once by the closer star and its planet making the distant star change brightness.

Kuiper Belt Object (KBO)

Most of them, comprising 1 family, are in relatively circular orbits orbiting in planets close to the ecliptic. The other family of KBOs was probably formed in the inner solar system & was sent outward by the gravitational force from various planets. At least 1500 have been observed.

What fraction of the mass of the Earth is made up of the elements hydrogen & helium?

Much less than 1%

In our solar system, which of the following planets is a member of the terrestrial group?

Neptune

Have astronomers discovered evidence for life on any planets orbiting other stars?

No

Moon (Natural Satellite)

Object orbiting a larger body that is, in turn, orbiting the Sun or another star. Even non scientists are familiar with 1 of the KBOs that the giant planets flung out-Pluto. Pluto became locked in a 2:3 resonant orbit with Neptune, meaning that it orbits the Sun twice in the same time that Neptune orbits 3 times.

Dwarf Planet

Orbits the Sun or another star & has enough mass so that its own gravitational attraction causes it to be spherical, rather than being a large, irregular rock in space; but it doesn't have enough gravitational attraction to clear its neighborhood of other orbiting debris. These objects are smaller than planets, hence with less mass & less gravitational attraction. This definition is the reason that Pluto was demoted from planet status. So far, of the myriad candidate bodies, astronomers have agreed on 5 dwarf planets: Pluto, Ceres, Haumea, Makemake, & Eris, with several other bodies being considered for that classification.

Solar System

Our star, the Sun, and all bodies that orbit it- planets dwarf planets, moons, & various kinds of debris (asteroids, comets, & meteoroids)-make up the solar system. Observations of other star & exoplanet systems & sophisticated computer models, are giving astronomers insights into the process by which the solar system came into existence.

Kuiper Belt

Outward flying planetesimals spread way beyond the original disk (which had a radius of 30 au). Millions of them were flung into a bagel-shaped volume beyond the orbit of Neptune that is now called the Kuiper Belt, named after Gerard Kuiper (1905-1973), who 1st proposed the existence of such a region in 1951.

Describe how exoplanets are discovered by observing stellar motion across the celestial sphere.

Periodically, planets can make their stars move toward & then away from us causing the star to orbit its center of mass. This motion appears as a wobbling of the star across the celestial sphere.

Asteroid

Piece of space debris composed primarily of rock & metal that is larger than about 10 m (no exact minimum size has been agreed upon).

Planetesimal

Pieces of debris that come together during the process of accretion. The formation of planetesimals that had migrated inside the snow line was a turning point in the young solar system's evolution. They collided with countless dust particles as well as with each other. Sometimes colliding planetesimals destroyed each other, & returned to dust. Eventually, however, these collisions led to the growth of several hundred substantial bodies that persisted & eventually reached the size & mass of our Moon. At this size, the gravitational attraction of the planetesimals was strong enough for them to either sweep up the surrounding rubble or fling it far away, thereby clearing out nearby space & limiting their own growth. These bodies led to the formation of the terrestrial planets.

Solar Nebula

Some chemical evidence in space debris that we have analyzed on Earth suggests that the dense core in which our solar system formed may have become Jeans unstable when its gas & dust were hit by the shock wave from a supernova. We call that dense core the solar nebula (proto-solar or pre-solar nebula). The solar nebula initially had a diameter of at least 1000 au (1000 times the average distance from Earth to the Sun) & a total mass about 2 to 3 times the solar system's present mass. At first, that nebula was a very cold collection of gas and dust— well below the freezing point of water. Most of the solar nebula was hydrogen and helium, ice and icecoated dust grains composed of heavier elements (metals) were scattered across this vast volume.

Exoplanet

Planets were 1st observed orbiting other stars in 1992. Astronomers continue to develop innovative methods of detecting planets outside the solar system, called exoplanets or extrasolar planets. More than 3700 exoplanets have been discovered orbiting other stars. Exoplanets ranging in mass from less than the mass Mercury, the lowest-mass planet in our solar system, to many times the mass of Jupiter, the most massive planet orbiting the Sun, have been detected. Most of the exoplanets that have been discovered have masses roughly equal to the mass of Jupiter. Exoplanets of size ranging from 0.3 to 34 Earth diameters have been observed. Some exoplanets are absorbed directly, while most are detected indirectly as a result of their effects on the stars they orbit. Exoplanets orbiting virtually all types of stars have been observed. There are more exoplanets than stars in our Milky Way galaxy. Exoplanets supporting life must be Earth like in composition & in their stars' habitable zones. No life has been discovered on any exoplanet. Some exoplanets that are not orbiting stars have been observed.

Describe 2 methods astronomers use to search for life on exoplanets.

Radial Velocity: Until recently when it was surpassed by transit, this method was responsible for the majority of exoplanets being discovered. Direct Imagery: Astronomers have been able to find exoplanets in the simplest way possible by seeing them, to be able to able to distinguish a planet from a star; it needs to be relatively far away from it.

What is a star's habitable zone?

Region around the star in which a terrestrial planet of the right size could have a surface temperature suitable for liquid. (water & life)

Small Solar System Body (SSSB)

Rings of billions of dust particles & mountain-sized pieces of debris in orbit around Jupiter, Saturn, Uranus, & Neptune are yet another category of solar system object. Everything in the solar system that is not a planet, dwarf planet, moon, or part of a ring system, is called a small solar system body (SSSB). The tricky part of the classification system for objects in the solar system is that several classes overlap. Ex: Largest asteroids are dwarf planets, whereas the vast majority of asteroids, as well as all meteoroids & all comets, are small solar system bodies.

Asteroids, meteoroids, & comets are also classified as what (one) kind of object today?

Small Solar System Bodies (SSSB)

Which planets are terrestrial & which are giants?

Terrestrial: Mercury, Venus, Mars, & Earth Giants: Jupiter, Saturn, Uranus, & Neptune

Which of the following statements is TRUE?

The Earth is the most massive of the terrestrial planets.

Jeans instability is responsible for what event in the evolution of the solar system?

The Jeans Instability is responsible for the collapse of the solar nebula to form the Sun, as well as the rest of the solar system.

According to the Nice theory, where did the Kuiper Belt objects & Oort Cloud objects come from?

The KBO & OCO are icy planetesimals that were flung outward by encounters with the giant planets.

Name & briefly describe 1 small solar system body.

The Oort Cloud is a spherical shell of millions of icy bodies which surrounds the solar system at vast distances & is thought to be the birth place of long-period comets.

What name is given to the concentration of mass that formed at the center of the solar nebula & eventually became the Sun?

The Protosun

Protostar

The vast majority of the mass in each dense core's inner region collapses together due to its own gravitational attraction to form 1, 2, or 3 protostars, which are destined to become stars that orbit together. The gas & dust in the disk that's swirling too fast to join the protostars will either remain in orbit & become parts of planets, moons, or smaller orbiting debris, or it will be heated by energy from the protostar(s) & thereby expelled back into interstellar space.

What is the Nice model's explanation for why there is no planet in the vicinity of the asteroid belt?

There were more bodies in the asteroid belt in the past, but impacts & the gravitational tug of Jupiter pulled some bodies away & kept the remaining debris spread out, thereby preventing it from ever coming together to create a planet.

Nice Model

Transformation of the solar nebula into the solar system of today is best explained by the Nice model & its successor, Nice 2 Model. These scientific theories are based on computer simulations of the evolution of the solar nebula. The Nice 2 Model is more accurate because it includes more gravitational interactions between the debris that were used in the original Nice model. These models propose that in the outer solar system Jupiter formed 1st, followed by Saturn, & then by Neptune & Uranus, which were then flung out to their present orbits by gravitational forces from Jupiter & Saturn.

Describe how exoplanets are discovered by measuring stellar brightnesses.

When a planet is orbiting its star then it will hide some of the starlight causing the star to dim a little bit or more depending on the size of the planet. So when observing the star periodically and if the dimness is seen often then it reveals that there's presence of a planet.

At what point in time do we say that the protosun became the Sun?

When thermonuclear fusion reactions began at its center

Jeans Instability

Wherever clumps of gas is compressed & becomes dense enough so that its own gravity causes it to begin collapsing in on itself, that region has a Jeans instability, named after the British physicist Sir James Jeans (1877-1946), who calculated the necessary conditions of gas density & temperature for such collapse in 1902.

Has liquid water been discovered on any planets orbiting other stars?

While direct evidence of liquid water has yet to be discovered, a growing number of terrestrial exoplanets are observed in their stars' habitable zones, implying the presence of water on them.

Snow Line (Frost Line or Ice Line)

Within a hundred thousand years or so of the disk forming, no gases or ices orbited closer than roughly 3 au, between what are now the orbits of Mars & Jupiter. Temperatures were low enough beyond this distance that gases, primarily hydrogen & helium, & ices remained in orbit around the protosun along with the dust particles out there. The boundary beyond which these gases & ices of water, carbon dioxide, methane, & ammonia persisted.

Have any Earthlike planets been discovered orbiting in the habitable zones around other stars?

Yes, at least 16 Earthlike exoplanets have been observed in their stars' habitable zones.

Have any planets been observed orbiting other stars?

Yes, at least 3700 exoplanets have been observed.

Accretion is the process by which

dust grains & ice crystals coalesce to form planetesimals.

Most meteoroids are

fragments of asteroids.

The average density of a planet is

its total mass divided by its total volume.

The composition of a typical asteroid is

rock & metal.

The asteroid belt is made up of

rocky bodies with diameters from less than a kilometer to hundreds of kilometers.

The most likely mechanism for the Solar System is that

the Sun & planets slowly condensed to their present form from a gas and dust cloud.

The manner in which the large, outer planets formed was

the accretion of planetesimals to form a core, followed by the gravitational capture of hydrogen & helium gas.

The manner in which the terrestrial planets formed was

the accretion of solid planetesimals containing mostly rocky material.

There is very little hydrogen or helium in the inner part of the solar system today. We believe the reason for this is that

the intense radiation from the early Sun drove the light elements out of the inner solar system.

Most of the weight of our bodies comes from heavy elements such as carbon & oxygen. This fact tells us that

the solar system didn't form directly from the material created by the Big Bang.