According to NASA, there are over 5,000 exoplanets which have been discovered and confirmed in the Milky Way galaxy alone. This means that we are almost familiar with the arrangement where planets orbit a parent star or stars. However, scientists estimate that our home galaxy is also populated with a huge number of free-floating or rogue planets.
Astronomers believe that our galaxy may in fact contain as many as a quadrillion rogue planets. These isolated worlds may have been ejected from their home planetary systems by gravitational interactions with other planets or passing stellar bodies.
TESS's First Rogue Planet
According to NASA, its Transiting Exoplanet Survey Satellite (TESS) may have discovered its first "orphaned" planet which roams the universe all alone without a star. The terrestrial planet, likely bigger than Earth, was discovered within the analysis of 1.3 million light curves gathered by space observatories.
Team co-leader Michelle Kunimoto reported that they discovered the first signal in TESS data which is consistent with what they would expect from microlensing. If it is verified that the signal actually indicates a free-floating exoplanet, it would likely be a planet 6,500 light years away and has a mass a few times that of Earth.
A Little Help From Einstein
This discovery demonstrates the capability of TESS to use a phenomenon first suggested by Albert Einstein more than a century ago. Since rogue planets do not orbit any stars, the usual techniques for detecting exoplanets do not work. Fortunately, Einstein's theory of general relativity predicts a phenomenon which can be used to observe the free-floating exoplanets.
Einstein suggests that in our universe, anything with mass curves the fabric of space and time, with gravity arising from this curvature. When light passes one of the curved spots, its path gets bent. This means that light from a background source can take different paths around the intervening object, arriving at an observer's vision at various times.
This is called gravitational lensing. This phenomenon results when the position of the background source shifts from the perspective of the observer or appears in many places in the same image.
Since rogue planets have very little mass, the lensing effect would be weak, thus it is called microlensing. Yet, this event can cause a brightening of a background source which is visible to astronomers and indicate the presence of a rogue exoplanet.
TESS is designed to search for planets which are closely bound to their host stars by looking for transits. This means that this space telescope may not be the right instrument for hunting rogue planets.
However, gravitational lensing can also cause a background star to brighten as a lensing object transits between that star and Earth. Since TESS is sensitive to small changes in the light of a star, it can also recognize the brightening episodes which are a feature of microlensing caused by rogue planets.
As with many other discoveries, the detected exoplanet candidate still needs to be verified. According to Kunimoto, it is difficult to discern the nature of any specific signal since microlensing events do not repeat. Because of this, astronomers are cautious about the origin of the event, and the planet was identified as a rogue candidate since it is consistent with the signal they would expect from such a world.
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