That is the important difference to understand -- and it will allow us to understand brown dwarfs as well. As a star forms from a cloud of contracting gas , the temperature in its center becomes so large that hydrogen begins to fuse into helium -- releasing an enormous amount of energy which causes the star to begin shining under its own power.
A planet forms from small particles of dust left over from the formation of a star. These particles collide and stick together. There is never enough temperature to cause particles to fuse and release energy.
In other words, a planet is not hot enough or heavy enough to produce its own light. Brown dwarfs are objects which have a size between that of a giant planet like Jupiter and that of a small star. In fact, most astronomers would classify any object with between 15 times the mass of Jupiter and 75 times the mass of Jupiter to be a brown dwarf.
There are three criteria that we consider when trying to distinguish among stars, giant planets, and brown dwarfs: how they formed, how massive they are, and what they look like. All of the planets we study today — including the ones in our solar system — orbit a star. These patterns indicate that planets form out of the circumstellar disks encircling young stars.
While we have observed such structures, scientists in the 18th century first conceived the idea. On the other hand, we usually find brown dwarfs as isolated objects or in pairs, suggesting that they form like stars.
It also assumes we fully understand planet and star formation, but recent discoveries with the Spitzer, Kepler, and Herschel space telescopes indicate that we have much to learn about these processes. Mass is another way to distinguish planets from brown dwarfs. Deuterium is an isotope of hydrogen that contains a proton and a neutron in its nucleus a hydrogen nucleus contains just a proton.
Deuterium is the easiest atom to fuse, so objects that cannot fuse deuterium cannot fuse anything , which sets a threshold for distinguishing the two classes of objects. Mass is also useful because we can directly measure it through, for example, the orbital motion of bodies in a binary system or microlensing observations which utilize a trick of gravity.
Unfortunately, the mass definition occasionally conflicts with the formation definition. Plus, not all planets and brown dwarfs have masses that are easy to measure.
Distinguishing a planet from a brown dwarf based on its appearance is probably the most challenging approach. As you point out, we now know of brown dwarfs that have low atmospheric temperatures like giant planets and have the same atmospheric compositions mostly hydrogen and helium , so the spectra we measure from brown dwarfs and planets are similar.
We have increasing evidence that clouds in the atmospheres of brown dwarfs and giant planets might be different, but such research remains work in progress. In practice, we use a combination of these characterizations to determine whether an object is a planet or a brown dwarf, and the International Astronomical Union has proposed a definition that combines the formation and mass criteria described above.
Last year, I conducted a survey of astronomers, students, and the public, asking them to classify 10 objects as planets, brown dwarfs, or something else. The group did not classify any of the objects unanimously as a planet or brown dwarf, and in some cases the vote was evenly split.
Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy. By signing up you may also receive reader surveys and occasional special offers. Phillip E. Jun 14, Jupiter is a planet, brown dwarfs are too big to be planets.
Explanation: A brown dwarf is an object which is too big to be a planet but too small to be a star. Related questions Which planets is least like earth in size and composition? In what ways are Terrestrial planets different from Jovian planets? Why are planets and satellite round while asteroids irregular? Why do Uranus and Neptune appear to be blue?
0コメント