Day 4: Planets

Key Moments in History:

The inner planets, Mercury and Venus, and the outer planets, Mars, Jupiter and Saturn, were identified by ancient Babylonian astronomers in the 2nd millennium BC. Aristarchus of Samos, and later in Nicolaus Copernicus came to view the Earth as a planet revolving with the other planets around the Sun, in the following order of distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. The Sun, now situated near the center of revolution, was no longer considered a planet.

Key Concepts

What is the definition of a planet?

Definition: planet: A planet is a spherical ball of rock and/or gas that orbits a star. The Earth is a planet. Our solar system has eight planets. These planets are, in order of increasing average distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The only object visited by humans other than the planet Earth is the Moon, which is not a planet. It is a satellite.

The International Astronomy Union has an official definition for a planet. It states, "A 'planet' is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit."

In the Milky Way, about 1 in 5 stars that are like our sun in size, color and age have planets that are roughly Earth's size and are in the habitable zone where life-crucial water can be liquid, according to intricate calculations based on four years of observations from NASA's now-crippled Kepler telescope. So far Kepler has found 1,235 planet candidates, while 54 of them were orbiting their host star in the so-called "habitable zone". Scientists from the University of California (UC) at Berkeley and the University of Hawaii at Manoa have statistically determined that 20 percent of Sun-like stars in our galaxy have Earth-sized planets that could host life.

Questions of the Day:

1. What is the definition of Habitable or Goldilocks Zone?

2. Are all stars’ Habitable Zones at the same distance? Why or why not?

3. What would happen if a planet weren’t in the Habitable Zone?

4. Would a microbe’s Habitable Zone be closer or further from a star than a human’s Habitable Zone? Explain your answer.

What is the Habitable Zone?
Life as we know it needs liquid water to survive. The Habitable Zone is the distance from a star where liquid water can exist on a planet’s surface. If a planet is too far from its star, water freezes. If a planet is too close to its star, water evaporates. A planet must stay in the Habitable-Zone throughout its orbit in order for water to remain a liquid.

Where is the Habitable Zone?
The distance at which a planet can have water is determined by how much energy is given off by the star. This distance is measured in astronomical units or AU. An astronomical unit is the average distance from Earth to the Sun, which is equal to 149,598,770 km or 93,000,000 miles. For cooler red dwarfs, the Habitable Zone is so close to the star that solar flares and radiation from the star would destroy life. For very hot blue stars, the Habitable Zone is further away. These stars tend to burn at such high temperatures that they have very short lives, lasting only a few million years. (It took 700 million years for life to become established on Earth). Our Sun’s Habitable Zone for larger life forms including humans is between 0.9 AU to 1.5 AU.

Is the Habitable Zone always in the same place?
The Habitable Zone can move as a star changes. As a star grows older, it grows hotter causing the zone to mover further away from the star. At one time, Earth was on the outer edge of the Sun’s Habitable Zone, but now the zone has moved further away, so Venus is no longer in the Habitable Zone. Since stars change, it is important to have a star has a temperature that stays about the same for a long time. It’s also important that the orbit of an Earth-size planet be in the area of the zone that remains in the zone as the zone changes.

If I’m in the Zone, is Survival Guaranteed?
Even if a planet is in the Habitable Zone throughout its entire orbit, human survival is not guaranteed. The planet may not even have water on it to begin with, or the water it has may not be liquid. Mars, for example, has such a low surface pressure that its water cannot be a liquid on the surface. Water goes from a solid to a gas without ever being liquid. Also, the planet may not have the right kind of atmosphere and may not be the right size to hold on to the atmosphere that humans need. Without the right kind of atmosphere to trap heat and maintain a stable temperature, surface water would not be found on a planet. Finally, there may be other dangers, such as large planets, solar flares or radiation, from which humans need protection.

Is the habitable zone the same for all life?
The Habitable-Zone for microbes is much larger than the Habitable-Zone for humans, because microbes can survive under conditions that humans cannot. A microbe is an animal or plant so small it can be seen only with a microscope. A bacterium is an example of a microbe. There are microbes that can survive in the frozen ice of Antarctica and in the extremely hot, thermal vents on the ocean floor. That’s why scientists are looking for life on Mars and one of Jupiter’s moons, Europa.

7 Reasons why life might exist on other planets

1. Theory of evolution

It states that life adapts to environmental conditions. Though Darwin hardly thought about life on other planets when creating his theory of evolution, he argued that in a place habitable for life it will certainly take root. So if to think that our environment is not only our planet, but other planets and star systems, as well as interstellar space, an unusual interpretation of the theory of evolution can be made: maybe life can adapt even to the open space. Maybe, one day we will meet up with really unimaginable living beings.

2. The mystery of the origin of life on Earth

Although there is a recognized theory about the origin of life on Earth, there is a mystery of how carbon molecules came to form fragile membranes, which eventually became cells. The more we learn about a hostile environment that existed on Earth when life was emerging and developing, the more questions arise about the origin of life. According to the panspermia theory, single-celled life could have been brought to Earth by meteorites from somewhere else.

3. Variety and persistence of life on Earth

It is a fact that life on earth evolved in extremely difficult conditions, having survived ice ages, meteorite impacts, powerful volcanic eruptions, droughts, etc. Moreover, life on our planet has become very diverse in a relatively short period of time. Why would not the same thing happen on another planet in our solar system, or even in another star system?

4. Extremophiles on Earth

Extremophiles are organisms that can survive in the extreme conditions of cold, heat, exposure to toxic chemicals, and even in the vacuum. These creatures can live in sub-glacial lakes in the Arctic or in the hot volcanic vents. Tardigrades, amazing tiny organisms, can survive even in the vacuum. Thus we see that life can exist in such a hostile environment on Earth, therefore it can also exist on planets radically different from Earth.

5. Rapidly increasing number of Earth-like planets

In the last decade, a great number of planets outside the solar system were discovered, many of which are gas giants like Jupiter. However, technological advantages have allowed scientists to find smaller, solid planets like Earth. Moreover, some of them are in the so-called “habitable zone”, i.e. at such a distance from their star that they approach the terrestrial temperatures. Given the great number of planets outside our solar system, there is a probability that there are forms of life on one of them.

6. Existence of starting materials on other planets

It is believed that life on Earth began from chemical reactions that led to the formation of cell membrane and DNA. However, these chemical reactions could have begun from complex organic compounds, such as proteins, nucleic acids, etc. There is evidence that such substances already exist on other planets. For example, they were found in Titan’s atmosphere and in the environment of the Orion Nebula. Of course, this does not mean that there is life, but, according to many scientists, these substances contributed to the development of life on Earth. If they are common throughout the universe, it is likely that life emerged in other places too.

7. Oceans and lakes are very common in our solar system

Life on Earth emerged in the ocean, so it could happen in the same way on other planets. There is proof that once there was water on Mars and on Titan, one of the Saturn’s moons. It is also believed that once there was a big ocean on Jupiter’s moon Europa, covered with a thick ice layer. In each of these planets there could be life, and maybe there still is.

Daily Dispatch

Video of the Day

School Activity:

Activity: The Phases of the Moon

This activity requires a darkened room and is ideal for small groups. The second part of this activity fulfills same goal but for a larger group

Materials: Strong light; extension cord; two inch Styrofoam ball; pencils.

1. Darken room and use extension cord to enable light to be placed in middle of room. Each student places moon ball on the end of a pencil and stands in a circle around light.

2. Students imagine that their heads are earth. The ball represents the moon and the light, the sun. They hold their moons at arm's length right in front of the sun.

3. Students move the ball a little to the left of the sun looking at the moon until they can see a crescent shape in light. Get them to figure out if this crescent is facing the sun or away.

4. Students keep moving their moons around their heads (earth). They stop when half the moon is in light. As the moon grows fuller is it moving towards or away from the sun?

5. They move the moon in a circle until they can see it fully lighted. To accomplish this, they must hold the moon above their heads. When they observe the ball fully in light ask if it is between them (earth) and the sun or on the opposite side of them and the sun?

6. Students continue to move the moons until they are half full again. As it move toward the sun is it getting fuller or thinner.

7. Have them move their moons so that they become crescent slivers. Then explain that when the moon passes the sun it usually is just above or below it and we cannot see it. Why not? this is the phase we call the new moon. It is called new because the ancients thought it was newly born each time.

8. Repeat this activity several times making sure that the light source is appropriate so that the phases can be clearly seen.


How long does it take from a new moon to a full moon? To the new moon? What is a lunar cycle? Is it possible to have two full moons ever in one month? If so, how could this be possible?

Links: - Search for Extraterestrial Life Institute - Astrobiology - Life Beyond Earth - Stephen Hawking - Alien Life (Life Beyond Earth - Nova Video) - The "WOW" Signal