(ORDO NEWS) — Polish astronomers from the OGLE project have discovered the smallest exoplanet in our Galaxy that is not associated with a single star. Such celestial bodies are called free floating, orphans, outcasts. They are not massive and difficult to spot. Such objects are detected by indirect signs, and how they are formed is still unknown.
According to the catalog created by the Paris Observatory, 4374 exoplanets have been recorded in our Galaxy, and another 2550 candidates are awaiting confirmation. The vast majority are outside the solar system, they revolve around parent stars or binary systems. And about a dozen objects are quite unusual. Scientists have not yet established their gravitational connection with any star – it seems that they are flying alone in outer space on their own in hyperbolic orbits.
The formation and evolution hypotheses of the planets predicted the existence of such wandering bodies. This was confirmed in the late 1980s by Polish and American scientists in the Optical Gravitational Lensing Experiment (OGLE). This method is used to observe dark compact bodies – black holes and rocky planets. They do not emit, therefore, they are optically invisible, it is also impossible to see them by the transit method, since they do not revolve around the stars and cannot obscure them. But they have a lot.
Any massive object outrages space-time around it. If it finds itself on the axis between an observer on Earth or its orbit, and a bright star, then gravitational waves form a kind of lens around it. And the brightness of the star increases for a short time, but long enough to find this peak in the telescope data. This is called the gravitational microlensing method. With its help, free-floating planets are discovered.
More than stars in the galaxy
In 2012, British scientists suggested that there should be a lot of free-floating planets in the Milky Way – several times more than stars. Perhaps they formed a little later than the Big Bang. Every 25 million years they cross the solar system, accumulating dust microparticles and germs of life, that is, they can carry biological material across the Galaxy.
As the data was collected, however, doubts arose. An analysis of the observations of the OGLE experiment for 2010-2015 showed: most likely, in our Galaxy there are no more than 0.25 of such rogue planets with a mass of about Jupiter per star. And the stars, according to various estimates, are 200-400 billion. That is, there are still a lot of freely floating planets.
At the end of October, scientists from the University of Warsaw, as part of the OGLE collaboration, announced the discovery of a candidate for the smallest free-floating planet in the Milky Way – its mass is equal to that of Earth or even Martian. It is possible that OGLE-2016-BLG-1928 has a star, but it was not found within a radius of eight astronomical units. The authors of the work note that the microlensing method is very sensitive, it is able to detect the effect of a lens with a duration of only 45 minutes. This means that the new object is very compact, such as an Earth-like planet.
The three rogue planets were discovered using the South Korean Microlensing Telescope Network located in Chile, South Africa and Australia. The next candidate, KMT-2017-BLG2820, was announced by scientists in October.
Astronomers have high hopes for the Nancy Grace Roman Space Telescope, which NASA is building specifically to search for free-floating planets. Observations from orbit, where the atmosphere does not interfere, are more accurate. The device will be able to detect objects weighing less than Mars. Although “Roman”, as the telescope is called for short, will survey a small sector of the sky, it will add hundreds of orphan planets to the catalog.
Scientists are leaning towards the idea that free-floating planets have been “kicked” from their star systems. According to the theory of planet formation, when the gas collapses and a star lights up, the material gravitating towards it stratifies, and planetary embryos appear in it. Gas giants form on the outskirts of protoplanetary disks and, continuing to collect gas, migrate to the center. Because of the instability that occurs, less massive planets, especially rocky ones, the size of Earth and Mars, are thrown out of the system and the gravitational field of the parent star.
By the way, they turn to this idea to describe the discrepancies in the structure of the solar system, the possible existence of the ninth planet and the future fate of Mercury – there is a possibility that it will be “thrown out” by the instability-creating forces of Jupiter.
Perturbation of the planetary system and the ejection of part of the material can also occur for external reasons, for example, due to the passage of a single star or galactic tides. Binary star systems, according to scientists, produce more orphan planets than single ones.
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