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Dwarf Destroyers
Astronomers find planets where they... shouldn't be
One of the most interesting and amazing objects in the Universe are white dwarfs. Astronomers believe that white dwarfs are dead stars. Despite the fact that their mass is close to the mass of the Sun, they are considered one of the dimmest stars. They shine 10 thousand times weaker than the Sun. In this sense, white dwarfs are an exception to the astrophysical rules, because they do not correspond to a clear relationship between the mass of a cosmic object and its brightness.Each white dwarf was once very similar to our Sun and shone as brightly, and perhaps brighter than our star. But over time, the thermonuclear fuel gradually burned out, and the star began to die.
Objects of this kind are rare in the Universe. Astronomers have found more than a dozen white dwarfs in our Galaxy, around which asteroids, comets and maybe even planets revolve. But most of all, these systems are more like space graveyards. When the star died out, it swallowed up all the worlds that existed around it. Looking at this, you can imagine what the solar system will look like in 5 billion years.
Astronomers have always assumed that planetary systems can form not only around stars similar to the Sun. Although hopes of finding a system similar to the solar one were never lost. However, 15 years ago, when a real boom in astronomical discoveries began, it became clear that planetary systems could be very different from the solar one.
The first discovery was the sun-like star 51 Pegasi, which, as it turned out, has its own planet, weighing more than Jupiter, but rotating in an orbit smaller than that of Mercury. As instruments became more powerful and sensitive, astronomers discovered even more surprising systems. For example, the sun-like star HD 40307 turned out to have three planets, whose masses are four to ten times the mass of the Earth, and orbits are half the size of the orbit of Mercury. The star 55 Cancri A has as many as five planets with masses 10 to 1,000 times that of Earth and orbits whose radii range from one-tenth the radius of Mercury to about the same as Jupiter.
However, white dwarfs prove that stars don't have to be like the Sun at all. Planets and the matter from which they are created can revolve around stars, which may even be smaller than them. Moreover, the number of such stars seems to be no less than the number of systems with ordinary stars. Astronomers even suggest that solar systems like ours may not be the most common in the Universe.
Thus, it turned out that many white dwarfs even have dust disks, which serve as the basis for the creation of planets. Only scientists could not understand whether these were disks from which planets never formed, or whether they were the ashes of other celestial bodies destroyed by the dwarf. Strange disks were discovered by scientists using infrared spectrometers. In 1987, astronomers at an observatory on the top of Hawaii's Mauna Kea recorded a strong infrared flash from the white dwarf G29-38. It was possible to establish that the temperature of the object is only 1200 Kelvin, i.e. much lower than that of a normal star, where it is 12 thousand kelvins. At first, scientists thought that another, cooler star was orbiting the dwarf, but in 1990 it turned out that the infrared flares were coming from a disk that was heated by the dying star.
G29-38 has another interesting property. Its bark contains heavy elements. The presence of calcium and iron is surprising because the gravitational field near the white dwarf's surface is so strong that these elements must sink inward. In 2003, a simple explanation was proposed for both oddities: a white dwarf had recently destroyed an asteroid (which contains iron and calcium) that fell into its gravitational field. A cascade of collisions crushed the debris into dust, from which the disk was formed.
Using Earth-based telescopes and the Spitzer Space Telescope, astronomers found 15 white dwarfs with the same infrared oddities and elemental anomalies.
The disks around white dwarfs are much smaller than the disks that form planets orbiting newly born stars. Judging by the intensity of infrared radiation, the mass of these disks is the same as that of an asteroid with a diameter of only 30 km. This suggests that these disks were most likely formed as a result of the disintegration of an asteroid. The presence of the disks also proves that some of the planetary material survived the demise of the star.
Theoretical calculations show that Earth-like planets and asteroids can survive the death of their star if they orbit from it at a distance greater than 1 AU. (about 149,597,870.610 km). For example, when the Sun goes out, Mars may survive, but the Earth has a 50/50 chance.
To study which parts of a planetary system might survive, Spitzer observed the white dwarf WD 2226-210, so young that its outer layers remain visible.
WD 2226-210 became the missing link between Sun-like stars and old white dwarfs such as G29-38. The disk of cosmic dust around the star is located at a distance of 100 AU. This is much further away than the disks around other white dwarfs. The dust that makes it up was formed as a result of the collision of asteroids and comets. Similar disks of fragments exist around the Sun and other stars.
The discovery of WD 2226-210 confirms that distant asteroids and comets can outlive their star. But if asteroids and comets can survive, then why can’t planets do the same? As the WD 2262-210 cools, it will illuminate the dust less and less. Over time, the distant belt of asteroids and comets will become invisible. Only occasionally will an object in this belt come dangerously close to a white dwarf, only to be destroyed.