Did Venus Give Earth the Moon? Wild New Theory on Lunar History

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In astronomy, a double planet also binary planet is a binary system where both objects are of planetary mass. The term is not recognized by the International Astronomical Union IAU and is therefore not an official classification. At its General Assembly, the International Astronomical Union considered a proposal that Pluto and Charon be reclassified as a double planet, [1] but the proposal was abandoned in favor of the current definition of planet.

Some binary asteroids with components of roughly equal mass are sometimes informally referred to as double minor planets. There is debate as to what criteria should be used to distinguish "double planet" from a "planet—moon system".

The following are considerations. A definition proposed in the Astronomical Journal calls for both bodies to individually satisfy an orbit-clearing criterion in order to be called a double planet. One important consideration in defining "double planet" is the ratio of the masses of the two bodies. A mass ratio of 1 would indicate bodies of equal mass, and bodies with mass ratios closer to 1 are more attractive to label as "doubles.

Some dwarf planetstoo, have satellites substantially less massive than the dwarf planets themselves. The most notable exception is the Pluto—Charon binary theory of the moon. The Charon-to-Pluto mass ratio of 0. The International Astronomical Union IAU currently calls Charon a satellite of Pluto, but has explicitly expressed a willingness to reconsider the bodies double dwarf planets at a future time.

The Moon-to-Earth mass ratio of 0. Consequently, some scientists view the Earth-Moon system as a double planet as well, though this is a minority view. Currently, the most commonly proposed definition for a double-planet system is one in which the barycenteraround which binary theory of the moon bodies orbit, lies outside both bodies. Under this definition, Pluto and Charon are double dwarf planets, since they orbit a point clearly outside of Pluto, binary theory of the moon visible in animations created from images of the New Horizons space probe in June Under this definition, the Earth—Moon system is not currently a double planet; although the Moon is massive enough to cause the Earth to make a noticeable revolution around this center of mass, this point nevertheless lies well within Earth.

However, the Moon migrates outward from Earth at a rate of approximately 3. It is interesting to note that the center of mass of binary theory of the moon Jupiter—Sun system lies outside the surface of the Sun, though arguing that Jupiter is a double star is not analogous to arguing Charon is a double planet; the binary theory of the moon is that one cannot argue that Jupiter is even a star —even a brown dwarf —due to its low mass and associated inability to support any type of fusion.

Binary theory of the moon Asimov suggested a distinction between planet—moon and double-planet structures based in part on what he called a " tug-of-war " value, which does not consider their relative sizes.

This can be shown to equal. This formula actually reflects the relation of the gravitational effects on the smaller body from the larger body and from the Sun. The tug-of-war figure for Saturn's moon Titan iswhich means that Saturn's hold on Titan is times as strong as the Sun's hold on Titan.

Titan's tug-of-war value may be compared with that of Saturn's moon Phoebewhich has a tug-of-war value of just 3. So Saturn's hold on Phoebe is only 3. Asimov calculated tug-of-war values for several satellites of the planets. He showed that even the largest gas giant, Jupiter, had only a slightly better hold than the Sun on its outer captured satellites, some with tug-of-war values not much higher than one. In nearly every one of Asimov's calculations the tug-of-war value was found to be greater binary theory of the moon one, so in those cases the Sun loses the tug-of-war with the planets.

Binary theory of the moon one exception was Earth's Moon, where the Sun wins the tug-of-war with a value of 0. Asimov included this with his other arguments that Earth and the Moon should be considered a binary planet. We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth.

From within the Earth—Moon system, the simplest way of picturing the situation is to have the Moon revolve about the Earth; but if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon's orbit is everywhere concave toward the Sun.

It is always "falling toward" the Sun. Note that this definition of double binary theory of the moon depends on the pair's distance from the Sun. If the Earth—Moon system happened to orbit farther away from the Sun than it does now, then Earth would win the tug of war.

For example, at the orbit of Mars, the Moon's tug-of-war value would be 1. Also, several tiny moons discovered since Asimov's proposal would qualify as double planets by this argument. Neptune's small outer moons Neso and Psamathefor example, have tug-of-war values of 0.

Yet their masses are tiny compared to Neptune's, with an estimated ratio of 1. A final consideration is the way in which the two bodies came to form a system.

Both the Earth-Moon and Pluto-Charon systems are thought to have been formed as a result of giant impacts: However, a giant impact is not a sufficient condition for two bodies being "double planets" because such impacts can also produce tiny satellites, such as the four, small, outer satellites of Pluto.

A now-abandoned hypothesis for the origin of the Moon was actually called the "double-planet hypothesis"; the idea was that the Earth and the Moon formed in the same region of the solar system 's proto-planetary disk, forming a system under gravitational interaction. This idea, too, is a problematic condition for defining two bodies as "double planets" because planets can "capture" moons through gravitational interaction.

For example, the moons of Mars Phobos and Deimos are thought to be asteroids captured long ago by Mars. Such a weak definition would also deem Neptune-Triton a double planet, since Triton was a Kuiper belt body the same size and of similar composition to Pluto, later captured by Neptune. From Wikipedia, the free encyclopedia. Not to be confused with a star orbited by two planets, called a double-planet system. The Pluto — Charon system is closer to binary than the Earth—Moon system distance not to scale.

The Earth — Moon system is sometimes informally referred to as a double planet masses are roughly proportional to volumes, not surface area. Which term one uses relies solely upon the perspective of the observer. From the point-of-view of the Sun, the Moon's orbit is concave; from outside the Moon's orbit, say, from planet Mars, it is convex.

Astronomical Society of the Pacific. Formula derived on p. National University of Singapore: Ripley Crater Vader Crater. Styx Nix Binary theory of the moon Hydra.

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Return to Lesson 11's Main Page. The implication of the Apollo cargo of lunar samples so far does not clarify how the Moon was formed. Several theoretical models have been proposed to explain the origin of the Moon. The older models fall into three broad categories: To these have been added the giant impact model, which is the strongest contender to date.

In , the astronomer Sir George Howard Darwin, son of the biologist Charles Darwin, proposed that the Moon was once part of Earth and broke or fissioned from it due to forces caused by a fast rotation and solar tides.

The fission model proposed the large basin of the Pacific Ocean as the place from which the Moon was ejected. The difference in density between Earth and the Moon might at first glance seem to rule out the fission theory, but the Earth's crust does have a density close to that of the Moon. If the Moon formed from material from the Earth's crust, we should expect the density to be just as we find it. There is a problem with the fission theory, however. Astronomers have difficulty explaining how an object as massive as the Moon might have torn away from the Earth.

No satisfactory mechanism for this event has been proposed. In addition, the Moon does not orbit in the plane of the Earth's equator as it would if it were ejected from a spinning Earth. Early in the twentieth century, another theory was proposed. It holds that the Moon was originally a separate astronomical body that happened to come near Earth, became bound to the Earth's gravitational field, and subsequequently settled into orbit as the Moon.

This is the capture model. There are problems with the capture model. If one astronomical object comes close to another, each of their paths will be changed by the gravitational force between them, but one will not capture the other unless there is contact between the two or unless a third object is involved, so that the interaction of the three objects results in one of them being slowed down to an orbital speed.

Such a near-collision between three objects is remote in nature. The binary accretion sister or coformation model , which was suggested in the early s, is the oldest.

It holds that as Earth formed from a spinning disk of material, not all of that material coalesced to form Earth. A small part of it was left orbiting Earth and formed into the Moon. This model is totally consistent with the models that explain the formation of the solar system. Nevertheless, a simple comparison of densities seems to rule out this model, for if the Moon formed along with Earth, the two bodies should have about the same density. The Earth's density is 5.

In addition, astronomers have difficulty explaining the orientation of the Moon's orbit in this model. The leftover material would orbit in the plane of the Earth's equator. The Moon does not orbit in this plane as it should if it formed from leftover material.

Refer to your reading assignment for more on these models. The Giant Impact Model. In the s, A. Cameron and William Ward proposed a new model. They suggested that early in the Earth's history, it was struck at a glancing angle by a large object, that the impact resulted in a fusion of the two objects, and that material was thrown off of the two to form the present Moon.

Computer simulation of such a collision shows that if the impacting object has a mass nearly as great as Mars, heat resulting from the collision would vaporize material and eject enough of it into orbit to account for the mass of the Moon, once the material coalesced. The large impact model , as it is called, is able to explain both the similarities and the differences in the compositions of Earth and the Moon. Since the mids, a consensus has been building among astronomers that the large impact model fits the data better than the other three models.

Recent theoretical work on the formation of the planets indicates that without large impacts, Earth would rotate every hours instead of every 24 hours. A glancing impact by a large object explains its present rotation rate. Like all new theories, the large impact model will be tested against both existing data and new data as the years pass.

Refer to your reading assignment for more on the impact theory. Key Terms refer to your text for some these terms. Review Questions refer to your text to answer some of these questions 1. Explain how the Moon could have formed by the fission model. Describe the capture model. What is the binary accretion model?

What are the problems associated with each of the three older models? Astronomers are now favoring the large impact model for the formation of the Moon. Advanced Question refer to your text to help you answer this question. Do you need help with this topic? CCAC offers on-campus and online tutoring services.

Online tutoring is available through "Smarthinking. Choose a members of the Physics and Physical Science Faculty from the "Participating Faculty" drop down list who have agreed to conduct some of their office hours online. A separate window will open with information about the selected faculty member.

Please ask permission to join one of the sessions if you are not a student of that faculty member. As an option, you may post your questions to a message board. Each tutor and faculty member has a message board. Answers to your questions will be posted within 24 hours.

You can also review questions and answers that have been sent to the message board during the past seven days. Do you need more help with this topic? Older Models The implication of the Apollo cargo of lunar samples so far does not clarify how the Moon was formed. For this topic, study the true and false, fill in the blanks self-test, and review questions at the end of the Chapter s of your reading assignment. In addition, learn the key words and answer all questions that follow: Key Terms refer to your text for some these terms fission model binary accretion model coformation theory capture model large impact model.