The discovery of the new planet raises doubts about the role of dust and ice in the formation of planets
Shay Zucker
Recently, we were informed of the discovery of a new planet, which was nicknamed "Methuselah" in the media, because of its apparent extreme age. This planet is part of a rather strange system of three objects. At the center of the system are two small and very compact objects, which orbit each other once every six months. The planet circled around both of them in a relatively distant orbit and completes its entire orbit every hundred years or so. The whole system is inside a dense cluster of tens of thousands of stars called a "globular cluster".
The new discovery is the culmination of a scientific chain of events that began in 1988 when the PSR B1620-26 pulsar was discovered. A pulsar is a neutron star - a highly compressed object whose mass is greater than the mass of our Sun and whose radius is only a few kilometers. A neutron star is the remnant of a star (Sun) that ended its life in a violent explosion - a "supernova". Many neutron stars spin around their axis at a very fast rate of hundreds of revolutions per second. The star then acts as a sort of beacon emitting a rotating radio beam. Whenever the beam "hits" us, we receive a pulse of radiation, hence the name "Pulser". The extreme regularity of the pulses makes pulsars one of the most accurate "clocks" in nature.
When deviations from this exact periodicity are nevertheless observed, this may indicate disturbances in the pulsar's motion due to the presence of additional objects surrounding it. Such was the case with the pulsar: PSR B1620-26. The analysis of its anomalies from the cycle indicated the presence of another object that collided with it in an elliptical orbit. It turns out that the other bone is a "white dwarf". White dwarfs are another type of remnants left over from the end of the life cycle of ordinary stars.
A white dwarf is less compressed than a neutron star and can be the size of Earth. The continuation of the observations showed more anomalies of the cycle that were not explained by the presence of the white dwarf and it was necessary to assume the presence of another small object to explain them. In the 90s, there was a debate about the essence of the extra bone, which has now been given the affectionate name "Methuselah".
The team of researchers, led by Stein Sigurdsson from the University of Pennsylvania, analyzed observations from the archive of the Hubble Space Telescope, in which the holiday white dwarf around the pulsar was seen for the first time. From the observations the team estimated the temperature of the white dwarf and its color. The existing theory about white dwarfs makes it possible to estimate their mass using their temperature and color, and this is how the researchers got an estimate of the mass of the white dwarf. This figure was the missing figure in the equations until now, which made it possible to deduce the mass of the third object and finally determine that it is a planet, whose mass is 2.5 times greater than the mass of the planet Jupiter.
This is not the first time that planets have been discovered in orbit around pulsars. The first ones were discovered by the researchers Welchen and Fraile in 1992 and aroused the interest of astronomers. This is because the supernova explosion that creates the pulsar is supposed to be so violent that no planet would survive such an orbit. Therefore, it is assumed that the planets accompanied by pulsars reached their current state after the death of the star. In the case of PSR B1620-26 there are several other special circumstances that sharpen the question of how the system seen today was created. First, it is the first planet found orbiting a distant pair of relatively close objects (the pulsar and the white dwarf). In all the previous cases where a planet was discovered in a double star system, it orbited only one of the partners. Secondly, as mentioned, the system is inside a globular star cluster, which the researchers named M4, which contains several tens of thousands of high-density stars, much more than the density of stars in the vicinity of our solar system. It is very likely that the gravitational force of the neighboring stars will strongly influence the development of the system and the orbits of the various objects that make it up. In fact, the closer the system is to the center of the cluster, the more likely it is that the various gravitational forces will "tear" it apart and separate its three components.
A clue to the turbulent history of the system is the "young" age of the white dwarf. In addition to the mass, the measurements of the space telescope made it possible to estimate that its age is about half a billion years, while the entire cluster is an "old" cluster, about 12.5 billion years old. Therefore, the researchers assume that the planet was initially a companion of one star, the one that later became a white dwarf. The question still remains as to how the pulsar joined the two. After running many computer simulations, the researchers arrived at the following scenario: In the beginning, the planet alone revolved around its star. The whole system moves towards the center of the globular cluster. Because of the high density in the central region, it was only probable that the system would "meet" another double star system, one of whose components is a pulsar. The two systems "collided" with each other, and at the end of the process, the star and the pulsar found themselves in orbit around each other, with both orbiting the planet in a distant orbit. The original partner of the pulsar was "ejected" from the system in an unknown direction. The recoil from the collision caused the system to move away from the center of the cluster again and reach its current location. At some point, about half a billion years ago, the star ended its life and became a white dwarf.
The presence of the system in the globular cluster M4 poses another problem: according to the accepted theory, planets form inside a "disk" of gas, dust and ice that surrounds a star in its initial stages of development. The process is supposed to begin with the "adhesion" of the dust and ice grains, which leads to the formation of objects that grow more and more tightly - "proto-planets". At some point the protoplanets can reach a mass sufficient for them to absorb gas from the disk, which will form their atmosphere. A necessary component of the process is the presence of a sufficient amount of dust and ice, which are composed of relatively heavy elements. The problem is that in the M4 cluster the amount of heavy elements is very small and not enough to create protoplanets in this process.
Thus, an alternative theory regarding the formation of planets suddenly gained popularity. This theory, developed by Alan Boss of the Carnegie Institution in Washington, does not require the presence of dust and ice to form planets. Instead, she maintains that in various places in the disk, a spontaneous "collapse" of the gas suddenly begins, which attracts more material to it due to the force of gravity. It is accepted by researchers that a process very similar to this underlies the formation of stars. Naturally, Alan Boss uses the new discovery as some confirmation of his theory.
