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The first detection of gravitational waves took place via an instrument that must force to overcome the constant background of vibrations and jolts that occur on the Earth. His success has helped push the pursuit of a project that would exceed all this noise. LISA – the space antenna for laser interferometer – would detect gravitational waves using the same technique as LIGO, but would place its material in space without any vibration on the ground. Preliminary tests of hardware prototypes have shown that the idea should work.
LISA should not be put in place until the 2030s, but that did not stop astronomers and physicists from thinking about things that it could possibly detect. Two of these astronomers, Nicola Tamanini and Camilla Danielski, now suggest that LISA could be used to identify a very strange class of planets: the heavy planets gravitating around binary pairs of white dwarf stars. But because of its exquisite sensitivity, LISA could potentially spot them in orbit around our own galaxy.
How would it work?
Gravitational waves are produced when two mass objects interact, but they are too small to be detected, unless the objects in question are both massive and close to each other. The LIGO detector is sensitive enough to detect objects such as neutron stars and black holes, all extremely incredibly dense and whose masses are of the same order as those of the Sun. But thanks to its increased sensitivity and the frequencies of the gravitational waves to which it will be sensitive, LISA will be able to capture dense objects but not so massive.
A prime candidate here is a white dwarf star, which represents the remains of a Sun-like star after burning most of its hydrogen and helium, producing a nucleus made up primarily of carbon and carbon. 39; oxygen. Without the energy provided by fusion, gravity will crush these objects into a dense ball of atoms, but they will not have enough mass to crush the atoms themselves. If there are no other sources of mass, they simply stay as they are and gradually shine as they lose the heat with which they had begun.
By itself, a white dwarf will not produce gravitational waves. But many stars similar to those of the Sun exist in binary systems, and some of them are so-called "near binaries". These stars are close enough that, if they grow late in life, both members of the binary share a single envelope. The friction of this orbit can bring closer their nuclei. When this step is complete, the two resulting white dwarfs can be in orbit narrow enough to produce gravitational waves.
LIGO is unable to detect these waves. LISA, on the other hand, would do it.
Stars
But detecting the binary system does not mean detecting a planet in orbit. Although a massive planet in close orbit is not directly detectable via the gravitational waves that it creates, it would alter the orbits of the two white dwarfs. And these changes would be detectable because they would change the frequency of the gravitational waves produced. This method looks a bit like the way we currently detect planets based on the Doppler shifts that they create in the light of a star when they move it back and forth during their orbit.
The authors themselves make this comparison specifically. But this is not correct, because the technique will only work for planets much more massive than the Earth and only if they revolve around binary white dwarfs. But in exchange, there are a lot of advantages. Tamanini and Danielski write that LISA "has the advantages of observing all over the galaxy, is not affected by star activity and requires no observation reference". In fact, they calculate that LISA might even be sensitive to binaries located near the neighboring Andromeda galaxy, which means that it will certainly be able to detect anything in the dwarf galaxies that gravitate around the Milky Way.
What can we learn?
Pairs of dead stars may seem extremely rare, and the gigantic planets that orbit them are also rare. But scientists estimate that 95% of the stars in the Milky Way will end up as white dwarfs, greatly increasing the chances of finding systems to watch. Calculations indicate that LISA should be able to detect approximately 25,000 of these binary systems.
And finding something would be informative. At present, we get a clear picture of the formation of the planet around isolated stars, but binary stars are common. We have some indications that planets can form when stars orbit at a distance, but we have a lot to learn about how they form around nearby binaries. It is possible that a single disc forming a planet will form around the two stars, but we have little evidence to judge. In addition, as Sun-like stars develop late in life, they release huge amounts of dust and gas, which can trigger the formation of a late planet.
In addition to telling us about planet formation near binary stars, LISA's broad reach can make it possible to draw conclusions about planet formation outside the Milky Way. If we find similar frequencies of large planets orbiting white dwarves in the Milky Way and in the dwarf galaxies that surround it, this would support the idea that the mechanisms of planet formation are universal.
What LISA can not tell us is very much about the planet itself. Since we do not know what angle the planet is gravitating around the line of sight with the Earth, we can not say whether it is a relatively light planet in orbit close to the plane or whether it is is a huge planet orbiting around a wide angle. This is because both should create similar changes in the binary system. To truly determine what is happening, gravitational wave data should be combined with visual observations using traditional telescopes, which requires stars to be relatively close.
LISA has been in space for more than 10 years, and Tamanini and Danielski note that the Transiting Exoplanet Survey satellite, currently in service, should tell us about the planets surrounding binary star systems well in advance. Nevertheless, the scientific arguments in favor of LISA are not based on the detection of exoplanets. But when it comes into orbit, this kind of preliminary work can ensure that we have the software in place to extract that information from its data, as well as all the data that originally justified the continuation of the mission.
Nature Astronomy, 2019. DOI: 10.1038 / s41550-019-0807-y (About DOIs).
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