Circumstantial evidence points at the existence of Planet Nine, the hypothetical planet that some astronomers think lies in the outer region of the Solar System beyond Neptune.
Scientists think that the existence of this world could explain the strangle looping of the trans-Neptunian objects or TNOs at the edge of the star system. Unfortunately, no telescope has so far been able to spot this elusive world.
Invisible To Current Observatories
California Institute of Technology astronomer Michael Brown said there is a reason to believe that if the ninth planet does exist, it could be essentially invisible to existing observatories.
In 2016, Brown and colleague Konstantin Batygin predicted that Planet Nine weighs five to 20 times more than the Earth's mass and that it follows an elliptical orbit up to 1,000 times the distance between the Earth and the sun.
Planets twice as far away as Earth appear 16 times dimmer since the intensity of sunlight weakens by a factor of four going out and by another four times coming back.
This means that at an orbital distance of 600 astronomical units (AU), a body could be 160,000 dimmer than Neptune, which orbits 30 AU away from the sun. At 1,000 AU, the elusive Planet Nine would appear more than a million times weaker.
Astronomers said there is a brick wall at a distance of 1,000 AU, and this could be part of the reason why spotting the planet has proved difficult.
Brown and colleague, Carnegie Institution for Science astronomer Scott Sheppard, now spearhead the search for the planet using the Subaru telescope in Hawaii. Researchers use Subaru's wide field of view to survey potential search area.
The efforts, however, do not guarantee the discovery of the planet as it could remain hidden amid the light pollution of the Milky Way. It may also be invisible in the glare of a bright star.
Worse, the body could be in its orbit where it is beyond the 1,000-AU wall and it can take thousands of years before the planet would swing back around.
Next Generation CMB Experiment
Scientists now consider other options to detect the planet. Physicists, for instance, think that Planet Nine, which is smaller and colder than the gas giants, would shine in the millimeter part of the spectrum between the microwaves and infrared light.
University of Illinois cosmologist Gilbert Holder is optimistic that the millimeter telescopes in Antarctica and Chile would be able to detect the Planet Nine today if it happens to stray across their search field.
Holder thinks that once operational, the Next Generation CMB Experiment could pick up bodies as small as earth at an orbital distance of 1,000 AU.
"There would be nowhere for Planet Nine to hide once this thing was turned on," he said.