Sub-Brown Dwarfs
Sub-brown dwarfs (SBDs) are intriguing celestial objects that lie between the smallest stars and larger planets in size. Formed from the same gas and dust processes that create stars, SBDs are too small to initiate the thermonuclear fusion that powers stars and even brown dwarfs, which can at least fuse deuterium. These "failed stars" are characterized by their initial temperatures of approximately 1,500 to 2,000 kelvins, gradually cooling over time and radiating heat and dim light. SBDs are typically faint and are primarily identified using infrared imaging techniques, making them challenging to observe; it's believed that the Milky Way may harbor as many SBDs as stars, though this remains unproven.
The study of SBDs is significant because scientists speculate they could potentially support life, especially if they possess atmospheres that can retain heat and possibly harbor liquid. While these objects share similarities with brown dwarfs and extrasolar planets, they are distinct in their classification, primarily defined by mass rather than size. The ongoing research aims to refine the definitions of these celestial bodies and deepen our understanding of their roles in the universe.
Sub-Brown Dwarfs
FIELDS OF STUDY: Astronomy; Observational Astronomy
ABSTRACT: Sub-brown dwarfs (SBDs) are celestial bodies that have attributes similar to stars and planets. SBDs are similar to brown dwarfs (BDs) and extrasolar planets, but they have some important differences. SBDs form in the same way stars form, but SBDs are smaller and do not produce thermonuclear fusion. Although scientists believe that BDs and SBDs are common in the galaxy, these objects are difficult to identify and confirm. Scientists study SBDs in part because they believe that SBDs could possibly be hospitable for extraterrestrial life.
Brown Dwarfs and Sub-Brown Dwarfs
Sub-brown dwarfs (SBDs) are celestial bodies that are generally smaller than stars and larger than planets. They formed in the same way stars form, but they are too small to create the thermonuclear reaction that stars have. Brown dwarfs (BDs) are similar low-mass celestial objects that cannot burn hydrogen but can at least burn deuterium, a heavy form of hydrogen. SBDs do not have enough mass for this fusion, either. Sometimes SBDs are surrounded by circumstellar disks. Circumstellar disks are made up of gas and dust, and they surround stars and other objects. Scientists believe that circumstellar disks produce planets.
SBDs and BDs are sometimes called "failed stars." This is because their mass is too small, and they failed to produce the thermonuclear fusion that powers other stars. However, these bodies do give off some thermal energy. When SBDs are first created, they have about the same temperature as a very cool star, about 1,500 to 2,000 kelvins (1227 to 1727 degrees Celsius, or 2,240 to 3,140 degrees Fahrenheit). SBDs radiate their thermal energy in the form of heat and dim light. As SBDs age, they become cooler and cooler.
SBDs and BDs are mostly observed through infrared imaging. This is because these failed stars are too faint to see with traditional telescopes. BDs that are up to about one hundred light-years away can be detected through infrared imaging. This is not a very far cosmic distance. Scientists have not been able to observe many of these objects. The imaging problem becomes even more complex when researchers try to observe SBDs. These objects are even smaller than BDs. Scientists cannot see many of the BDs and SBDs that they believe exist in the universe. Some scientists believe that the Milky Way galaxy has about as many BDs and SBDs as it has stars. However, this cannot yet be proven. As part of the US National Aeronautics and Space Administration (NASA) funded Backyard Worlds: Planet 9 project launched in 2017, astronomers identified ninety-five new BDs. They used data submitted by amateur astronomers as well as from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) space telescope.
SBDs and Other Celestial Bodies
SBDs are related to, but not the same as, both BDs and extrasolar planets. BDs are celestial objects that formed in a similar way as stars. Stars, BDs, and SBDs all formed when a ball of gas and dust collapsed under its own weight. Scientists believe that planets formed from circumstellar disks of gas and dust that surround stars, BDs, and SBDs.
Stars have to be quite large so that they can create a thermonuclear reaction that causes them to produce light and heat. Stars usually have to be over seventy-five times as large as Jupiter to create fusion. Objects between thirteen and seventy-five times the size of Jupiter are BDs.
Some objects form in the same way as BDs, but they are smaller than thirteen times the size of Jupiter. Some of these objects are extrasolar planets. Extrasolar planets orbit stars. Objects that are thirteen times the size of Jupiter and smaller but do not orbit stars are SBDs.
Mass, not radius, distinguishes the different types of objects. This phenomenon is an important part of why stars achieve thermonuclear fusion and less massive objects do not. An object compresses as it becomes more massive, and the gases inside the object become hotter and hotter. Once the gases become very hot, hydrogen molecules can fuse together to make helium, causing thermonuclear fusion.
The differences among stars, BDs, SBDs, extrasolar planets, and planets are still being discussed among scientists. It is clear that these objects are different, but their exact definitions are being refined. As scientists learn more about these objects, they will be able to further refine the definitions.
Studying SBDs
One reason to study SBDs that scientists believe these objects might be capable of supporting life. The temperature of an SBD is hottest when the object is first created, but scientists believe that SBDs can have fairly stable temperatures for long periods of time. Some SBDs may have atmospheres that hold in their heat. Because SBDs can capture their own heat, they might have liquid on them, which could allow for life to form. Earth has liquid water that allows life to exist, but scientists believe life could develop in places where liquids other than water are present. However, life that evolved on an SBD would most likely be much different from life that evolved on Earth.
PRINCIPAL TERMS
- brown dwarf: a celestial object that forms like a star but does not have enough mass to produce thermonuclear fusion like a star.
- thermal energy: energy that is in the form of heat.
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