ASTR 1P01 Lecture 10: Objects in the Solar System Notes

Mercury

Mercury is the closest planet to the Sun.
The Sun appears 3 times larger from Mercury's surface than from Earth and sunlight is 7 times brighter.
Its semi-major axis is ~58 million km or 0.39 AU.
Mercury's orbital eccentricity is 0.206, the largest among the 8 planets, causing great variation in its distance from the Sun.
At aphelion (farthest from the Sun), it's ~70 million km.
At perihelion (closest to the Sun), it's ~46 million km, approximately 2/3 of the aphelion distance.
Mercury is an interior planet, meaning its orbit lies between the Sun and Earth.
From Earth, Mercury appears within 28° of the Sun.
It can be seen near the western horizon after sunset or the eastern horizon before sunrise.
Transits of Mercury occur rarely (13-14 times per century), only in May or November.
The last transit was November 11, 2019, and the next will be November 13, 2032.
A transit is when Mercury passes in front of the Sun, similar to a solar eclipse but Mercury blocks only a tiny part of the Sun.
Mercury's orbital period is ~88 Earth days, the shortest among the planets.
It is tidally locked with the Sun in a 3:2 spin-orbit resonance, spinning 3 times on its axis for every 2 orbits around the Sun.
Mercury is the only tidally locked planet in the solar system.
A solar or synodic day is how long the Sun takes to return to the same position in the sky (24 hours on Earth).
A sidereal day is how long a fixed star takes to return to the same position (23:56 hours on Earth).
On Earth, the solar and sidereal day are almost equivalent.
Mercury's sidereal day is ~58.65 Earth days, but its solar day is ~176 Earth days (exactly 2 Mercury years).
$58.65 \times 3 \approx 176$
3 Mercury sidereal days = 2 Mercury years
Mercury has the smallest axial tilt, ~0.03°, resulting in no seasons.
It has no atmosphere to retain heat, so surface temperatures vary greatly.
Temperatures range from ~100 K (−173 °C) at night to ~700 K (427 °C) during the day at the equator.
Polar regions remain constantly below ~180 K (−93 °C).
Mercury has no moons, likely due to its proximity to the Sun.
Its surface is heavily cratered, similar to the Earth’s Moon, indicating geological inactivity for billions of years.
Mercury's mass is $3.3 \times 10^{23}$ kg, 18 times smaller than Earth’s mass, and the smallest among the planets.
Its radius is ~2,400 km, ~38% of Earth's radius.
Its mean density is ~5,400 kg/m³, the second highest among the planets.
Earth is the most dense planet, at ~5,500 kg/m³ .
Mercury must be composed of heavier materials, such as metals, due to its high density.
It has a metallic iron-nickel core taking up 60% of its mass.
At least part of the core must be liquid, generating a weak magnetic field.
The rest of the planet is primarily made up of silicates (rocks composed of silicon and oxygen).
The internal structure consists of an inner core, a middle mantle, and an outer crust.

Venus

Venus is the second planet from the Sun.
Its semi-major axis is 108 million km or 0.72 AU, roughly twice that of Mercury.
Like Mercury, Venus is an interior planet.
Therefore, it always appears close to the Sun as seen in Earth’s sky.
Venus is the brightest planet in the sky, visible to the naked eye during the day, and can even cast shadows at night.
Like Mercury, Venus does not have any moons.
Venus can get closer to Earth than any other planet: ~40 million km at its closest approach.
It's the easiest planet to reach from Earth (if timed correctly); Venera 1 (1961) was the first spacecraft sent by humans.
Mercury is closer to Earth than Venus most of the time.
Since Venus is an interior planet, it can also pass directly in front of the Sun as seen from Earth. This is called a transit of Venus.
Transits of Venus are extremely rare, occurring in pairs 8 years apart, separated by 105 or 121 years.
The last pair of transits was June 8, 2004, and June 5, 2012.
The next pair will be December 10, 2117, and December 8, 2125.
Historically, Venus transits were used to accurately estimate the size of the solar system, as early as 1639.
Venus is covered by dense clouds of sulfuric acid $(H2SO4)$ , which reflect ~70% of the sunlight.
This makes it very hard to see its surface, even from cameras in orbit around it.
All planets in the solar system revolve around the Sun counterclockwise, as viewed from above the Sun’s north pole.
Most planets also spin around their axes in the same direction.
However, Venus and Uranus have retrograde rotation: they spin around their axes clockwise, opposite to their direction of revolution around the Sun.
On Venus and Uranus, the Sun rises in the west and sets in the east!
Venus's axial tilt is ~177°, very close to 180°, indicating that the rotational axis is tilted “upside-down” compared to the orbital axis.
Uranus’s axial tilt is ~97.8°, close to 90°, indicating that the rotational axis is tilted almost perpendicular to the orbital axis.
Earth’s axis tilt of 23.4°.
One solar year on Venus is ~225 Earth days.
One solar day on Venus is ~117 Earth days.
One sidereal day on Venus (full rotation with respect to fixed stars) is ~243 Earth days.
It is the longest sidereal day among all the planets.
The sidereal day is longer than the solar day, unlike Earth or Mercury, because Venus has retrograde rotation.
Venus has a radius of ~6,050 km, very similar to the radius of Earth (~6,370 km).
Its mean density is ~5,200 kg/m³, slightly lower than Earth’s (~5,500 kg/m³).
Its mass is ~ $4.9 \times 10^{24}$ kg, ~82% of Earth’s mass (~ $6.0 \times 10^{24}$ kg).
Venus is sometimes called Earth’s “sister” or “twin” planet due to their similarity in size and mass.
Venus has a very different climate and atmosphere.
The atmosphere consists mainly of carbon dioxide $(CO_2)$ , and it is the densest and hottest among the 4 terrestrial planets.
The atmospheric pressure at the surface of Venus is ~92 times that of Earth.
The average surface temperature of Venus is 737 K (464 °C).
This is hotter than even the maximum daytime temperature on Mercury, ~700 K (427 °C), and obviously much hotter than Earth.
Venus is a bit closer to the Sun than Earth, so it gets some more intense sunlight.
However, this is not nearly enough to explain why its surface is so much hotter than Earth.
Also, Mercury is even closer to the Sun than Venus, but it is colder!
The reason Venus is so hot is the greenhouse effect.
The greenhouse effect also exists on Earth.
After the Sun warms the surface of the Earth, the surface releases heat back in the form of infrared radiation.
However, greenhouse gases, such as carbon dioxide $(CO2)$ and water vapor $(H2O)$ , prevent heat from escaping back into space.
This causes the overall temperature of the planet to increase.
The same thing happens on Venus. However, Venus has a million times more $CO_2$ than Earth!
This makes the greenhouse effect much stronger, and therefore the temperature gets much hotter.
Venus may have had a climate similar to Earth, with moderate temperatures, water oceans, and $CO_2$ stored in the ocean and rocks.
However, even a small amount of extra heat from the Sun can lead to increased water evaporation and release of gas from rocks.
This increases the greenhouse gases $CO2$ and $H2O$ in the atmosphere.
So this process amplifies itself: more heat → more greenhouse gases → more heat → more greenhouse gases → and so on.
Eventually the oceans boil completely.
This is called the runaway greenhouse effect.
We don’t know much about the internal structure of Venus, but it should have a differentiated structure with a core, mantle, and crust, like Earth and Mercury.

Mars

Mars is the fourth planet from the Sun and the farthest in the inner solar system.
Its semi-major axis is ~228 million km or ~1.5 AU.
It is the second-smallest planet, with a radius of ~3,900 km (larger only than Mercury, which has a ~2,400 km radius).
It has a very thin atmosphere, less than 1% the density of Earth’s atmosphere.
Like Mercury, Venus, and Earth, Mars is a terrestrial planet, composed primarily of silicates and metals.
It has a core of iron and nickel, like Earth.
Mars is sometimes called the Red Planet due to its reddish color, caused by iron oxide in its surface.
A Martian solar day is 24 hours and ~39 minutes, remarkably close to a solar day on Earth.
A Martian year is ~687 Earth days long (~1.9 Earth years).
The axial tilt of Mars is ~25.2°, very close to that of Earth (~23.4°).
This means Mars has seasons, like Earth does, but they are almost twice as long, since the Martian year is longer.
Mars has two moons: Phobos and Deimos.
The surface of Mars has many volcanoes. However, it is unknown if it is still volcanically active today.
Olympus Mons, a volcano on Mars, is the largest volcano and highest known mountain in the solar system.
It is ~22 km high. Compare this with Mount Everest, Earth’s highest mountain, which is only ~8.8 km high!
Valles Marineris, a system of canyons on Mars, is one of the largest canyons in the solar system.
It is ~4,000 km long, ~200 km wide, and ~7 km deep.
Liquid water cannot exist on the surface of Mars due to its low atmospheric pressure.
However, there is ice water on the surface, including both polar ice caps.
There is likely more ice deep underground.
In the past, Mars may have had liquid water on its surface.
The Korolev impact crater on Mars is estimated to contain ~2,200 km³ of water ice.
If Mars had liquid water, then it could have been suitable for life (as we know it).
However, we don’t know if Mars ever had any form of life.
Mars is one of the most likely candidates for life in the solar system, due to its similarities with Earth.
Looking for evidence of life on Mars is a primary objective of past and future Mars missions.
Even if we find life, it will most likely be microorganisms, but that would still be extremely exciting!

Jupiter

Jupiter is the fifth planet from the Sun.
Its semi-major axis is ~778 million km or ~5.2 AU.
It is the largest planet in the solar system, with a mean radius of ~70,000 km, which is ~11 times Earth’s radius.
Jupiter is also the most massive planet, at ~ $1.9 \times 10^{27}$ kg, which is ~318 times Earth’s mass and ~1/1050 the Sun’s mass.
Its mass is more than 2.5 times that of all other planets combined!
Jupiter is the third brightest celestial object in the Earth's night sky, after the Moon and Venus.
Jupiter is primarily composed of hydrogen.
It also contains helium: ~25% of its mass and ~10% of its volume.
As a gas giant, Jupiter does not have a solid surface.
However, it may have a rocky core of heavier elements at its center.
Jupiter’s atmosphere is made of bands, with storms along their boundaries.
The most famous storm is the Great Red Spot, a huge storm which has been raging since at least 1831.
Jupiter has 80 known moons, and possibly many more. We keep discovering new moons all the time.
The 4 largest moons are Io, Europa, Ganymede, and Callisto. They were discovered by Galileo in 1610.
Ganymede is the largest and most massive moon in the solar system: 26% larger than Mercury, but only 45% as massive.
In general, the outer planets have many more moons than the inner planets.
Inner: Mercury and Venus have no moons, Earth has 1, Mars has 2.
Outer: Jupiter has 80+, Saturn has 83+, Uranus has 27+, Neptune has 13+.
Like all the outer planets, Jupiter also has rings. (However, they are not as impressive as Saturn’s rings!)

Saturn

Saturn is the sixth planet from the Sun.
Its semi-major axis is ~1.4 billion km or ~9.6 AU.
Its internal structure is similar to that of Jupiter. From innermost to outermost:
- A core of iron-nickel and rock.
- A deep layer of metallic hydrogen.
- An intermediate layer of liquid hydrogen and liquid helium.
- A gaseous outer layer.
Saturn appears pale yellow due to ammonia crystals in its upper atmosphere.
It has 83 known moons.
Saturn’s most famous feature is its ring system, composed mainly of ice particles, with some rocky debris and dust.
There are hundreds of moonlets (small moons) orbiting inside the rings.
The 7 main rings are labeled in letters from A to G in the order of discovery.
Saturn’s largest moon, Titan, is the second-largest in the solar system (after Ganymede).
Titan is also the only moon in the solar system with a substantial atmosphere. It is composed largely of nitrogen.
It is the only known object other than Earth with evidence of stable bodies of surface liquid.
Titan is primarily composed of ice and rocks, likely differentiated into a rocky core surrounded by various layers of ice.
It also has a crust of ice and a subsurface layer of ammonia-rich liquid water.
Another interesting feature of Saturn is this hexagonal cloud pattern around the north pole.

Uranus

Uranus is the seventh planet from the Sun.
Its semi-major axis is ~2.9 billion km or ~19 AU.
There are two ways to pronounce its name:
- YOOR-a-nes (seems to be preferred among astronomers)
- yoo-RAY-nes
Uranus can be seen with the naked eye. However, it is very dim and moves slowly across the sky (its orbital period is 84 Earth years).
Therefore, it was long thought to be one of the fixed stars.
Recall: planets are “wandering stars”, because they move in the sky.
In 1781, William Herschel observed Uranus with a telescope and realized it wasn’t a fixed star, because it was moving. But he initially thought it was a comet.
Astronomers computed its orbit and found that it is nearly circular. Since most comets have very eccentric paths, they concluded Uranus is most likely a planet.
Both Uranus and Neptune are ice giants. This is different from Jupiter and Saturn, which are gas giants.
Gas giants are composed mainly of hydrogen and helium.
Ice giants are composed mainly of heavier elements such as oxygen, carbon, nitrogen, and sulfur.
None of the giants have solid surfaces.
In astronomy, the word “ice” doesn’t mean the substance is solid or cold, like water ice.
“Gases” have extremely low melting points. For example:
- Hydrogen (H): ~14 K (−259 °C)
- Helium (He): ~1 K (−272 °C)
“Ices” have melting points above ~90 K (–173 °C). For example:
- Water ( $H_2O$ ): ~273 K (0 °C)
- Ammonia ( $NH_3$ ): ~195 K (−78 °C)
- Methane ( $CH_4$ ): ~91 K (−182 °C)
“Ices” in ice giants are generally in liquid or gas form, and very hot!
The interior of Uranus is mainly composed of ices and rocks.
It has a layered cloud structure, with a lower layer of water clouds and an upper layer of methane clouds.
Uranus appears blue due to the methane it contains.
Uranus is unique in the solar system, because its spin axis is tilted sideways compared to its axis of revolution around the Sun.
Its axial tilt is ~97.8°, close to a right angle of 90°.
Thus, its north pole points toward the Sun, rather than upward.
Earth’s axial tilt is 23.4° and Venus' axial tilt is 177°.
This tilt was likely caused by a collision with another large celestial body, which turned the planet “on its side”.
Like the other giants, Uranus has a ring system (with 13 rings) and many moons (27 currently known).
These rings and moons rotate around the equator of Uranus, which means they also rotate perpendicular to the rest of the solar system.
Since Uranus is tilted sideways, it experiences seasons very differently from other planets.
Recall: a solstice is when the Sun is highest or lowest in the sky.
Near the solstices on Uranus (“summer” and “winter”):
- One pole faces the Sun continuously and the other faces away.
- A narrow strip around the equator experiences a rapid day–night cycle, with the Sun low over the horizon.
You can imagine the planet “rolling on its side” along its orbit. (Most planets move along their orbits like spinning tops.)

Neptune

Neptune is the eighth planet from the Sun and the farthest known planet in the solar system.
Its semi-major axis is ~4.5 billion km or ~30 AU.
Since light intensity decreases as the square of the distance, sunlight on Neptune (30 AU) is $30^2 = 900$ times weaker than sunlight on Earth (1 AU).
It also has the longest orbital period: ~165 years.
Neptune is the only planet that cannot be seen with the naked eye.
In fact, Neptune was initially discovered mathematically, rather than by direct observation.
The orbit of Uranus was not behaving as Newtonian physics predicted.
This led astronomers to think there must be another planet exerting its gravitational force on Uranus and changing its orbit.
They were able to calculate where this planet, Neptune, must be located, and observed it there with a telescope in 1846.
Neptune is also an ice giant, like Uranus, and its interior is primarily composed of ices and rock.
Also like Uranus, it is blue due to the methane in its outer regions.
However, unlike Uranus, Neptune has visible weather patterns.
The most famous example is the Great Dark Spot, similar to the Great Red Spot on Jupiter.
Neptune also has the strongest winds of any planet in the solar system, with wind speeds as high as 2,100 km/h.
Neptune has 14 known moons.
Its largest moon is Triton, discovered only 17 days after the discovery of Neptune itself.
Triton is the only large moon in the solar system with a retrograde orbit, opposite to Neptune’s rotation.
Because of that, it is thought to have been a dwarf planet captured from the Kuiper belt.
It took over a century before another moon, Nereid (NEER-ee-id), was discovered.
Like the other giant planets, Neptune also has rings.

Pluto

Pluto is a dwarf planet.
Pluto was discovered in 1930, and was considered the ninth planet from the Sun for a long time.
However, with time, many more similar objects were discovered in the same region, now known as the Kuiper (KIE-per) belt.
The Kuiper belt extends from the orbit of Neptune, at ~30 AU, all the way up to ~50 AU.
In 2006, the International Astronomical Union (IAU) defined a planet as any object that:
- Is in direct orbit around the Sun (not around another object).
- Is massive enough to be rounded by its own gravity.
- “Clears the neighborhood” around its orbit, sweeping out smaller bodies over time until it does not share its orbit with any other bodies of comparable size (except its own moons).
A dwarf planet is an object that satisfies criteria 1 and 2 but not 3.
Since Pluto didn’t clear its neighborhood (it shares it with other Kuiper belt objects), it was reclassified as a dwarf planet.
This redefinition of Pluto in 2006 sparked a great public debate.
People were upset that Pluto was “downgraded” to a dwarf planet.
New Mexico and Illinois even passed resolutions that declared Pluto to be considered a planet in those states (because Pluto’s discoverer, Clyde Tombaugh, lived there).
In reality, Pluto being reclassified as a dwarf planet doesn’t mean it’s any less “important” than it was before. It just means we understand the universe better now, so we can give more precise and useful definitions to things.
Pluto is made primarily of ice and rock, and is much smaller than the inner planets.
Other Kuiper belt objects have similar composition and size.
Pluto has 1/6 the mass and 1/3 the volume of Earth’s Moon. It is smaller than many of the larger moons in the solar system.
Pluto has 5 known moons: Charon (the largest), Styx, Nix, Kerberos, and Hydra.
Trans-Neptunian objects (TNOs) are any planet-like objects, including dwarf planets, that are beyond the orbit of Neptune.
Pluto was the first TNO to be discovered, and there are more than 2,600 other TNOs currently known.
Some TNOs have moons of their own, and at least one, the dwarf planet Haumea, is known to have rings.
Trans-Neptunian objects also exist beyond the Kuiper belt, even all the way out to the Oort (OR-t) cloud, at 2,000 to 200,000 AU (3.2 light-years) from the Sun.
These are sometimes called extreme trans-Neptunian objects (ETNOs).

Small Solar System Bodies

Small solar system bodies (SSSBs) were defined in 2006 as objects in the solar system that are not planets, dwarf planets, or moons.
SSSBs include:
- Asteroids.
- Comets.
- Any trans-Neptunian objects that are not dwarf planets.
- Trojans, which share the orbit of a planet or moon; most share the orbit of Jupiter.
- Centaurs, which have characteristics of both asteroids and comets, and exist between Jupiter and Neptune.

Comets

Comets are small solar system bodies composed of ice, dust, and small rocks.
The nucleus of a comet is the solid part, which can be between a few hundred meters to tens of kilometers across.
When a comet passes close to the Sun, it warms up and begins to release gas.
This produces a visible atmosphere (a coma) and often also a tail.
The coma may be up to 15 times Earth's diameter, and the tail may be as long as 1 AU.
The brightest comets may be seen from Earth with the naked eye.
This happens roughly once per year.
Comets have been observed and recorded since ancient times.
They can make an arc of up to 30° (60 Moons) across the sky.
Usually, comets have highly eccentric elliptical orbits. Their orbital periods range from several years to several millions of years.
Short-period comets originate in the Kuiper belt. Long-period comets are thought to originate in the Oort cloud.
There are currently more than 4,500 known comets, but their true numbers are estimated at ~1 trillion.
Comets have two tails: a dust tail and a gas tail. Each points in a slightly different direction.
The gas tail always points directly away from the Sun, because it is pushed away by the solar wind.
The dust tail is made of larger particles, so it is not strongly affected by solar wind.