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In a research paper, published in arxiv.org on Jan. 31, 2018, authors Xinyu Dai and Edwardo Guerras claim to have detected trillions of planets in a galaxy far, far away.
My first reaction was “that can’t possibly be true”. We can detect only subtle hints of planets that are 3000 light-years (18,000 trillion miles) from Earth. How could anyone detect planets a million times farther away.
But, they may be right.
Before explaining this observation, some might ask (as my fearless editor did):
(1) Are any of those planets habitable?
(2) Why look for planets in other galaxies?
(3) Why even look for planets in our galaxy?
(1) As discussed below, all the “detected” planets in this remote galaxy are unbound, meaning they are solitary bodies in interstellar space (not orbiting stars), where temperatures might be as “warm” as 20K (–423ºF) — not very inviting. Planets orbiting stars may also exist, but they could not be detected with Dai and Guerras’ technique.
(2) Scientists seek to understand everything in the universe — that’s our job description. Some things are more important than others, but until a thing is discovered, how can we be sure it isn’t important?
(3) SciFi-enthusiasts may dream of colonizing Earth 2.0 and saving humanity from the eventual extinction of all life on Earth 1.0. That’s a good story, and I too like good stories. Unfortunately, that is extremely unrealistic. I’ve written extensively about the many rare and vital properties of Earth, and also about the immense challenges of even short voyages to other star systems — references are provided on the website given below.
Now, back to the search for exoplanets (planets outside our solar system).
Most important, please know that exoplanet “discoveries” fall far short of a beautiful picture worth framing. Of the 3500 exoplanets “discovered” so far in our galaxy, only 10 have been directly photographed in the usual sense, and those images show only tiny specks of light. The other 3490 exoplanets were detected indirectly, primarily from their tiny effects on their host stars.
Gravitational lensing has revealed 19 of the 3500 exoplanets detected in our galaxy. As Einstein predicted, massive bodies bend light. As sketched below, the light we see from a distant quasar can be partially focused by a galaxy (gray blob) that happens to lie in our line of sight.
If the gray blob is replaced by a single star or planet, the effect is much smaller and is called microlensing. The following graph shows a 3-fold brightening of light from a distant source as a nearby star passes through our line of sight. The tiny peak on the right may be microlensing by a planet orbiting that star.
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Dai and Guerras of the University of Oklahoma push microlensing to a cosmic scale. Their light source is a quasar 6 billion light-years away. X-rays from the accretion disk of the quasar’s supermassive black hole (SMBH) are focused as they pass near massive bodies in a lensing galaxy 3.8 billion light-years from Earth.
Based on intricate mathematical analyses, Dai and Guerras say the observed changes in these x-rays prove unbound planets account for at least 0.01% of the lensing galaxy’s mass.
If so, this galaxy contains trillions of unbound planets — at least 2000 times more than the number of its stars.
The key physics entails the optics of caustic rays, light paths with much higher intensity than neighboring paths. Caustics arise when light passes through inhomogeneous matter. This image, by Brocken Inaglory, shows underwater caustics due to the ocean’s chaotic surface.
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Other scientists will certainly try to confirm or refute this discovery, and it may be several years before we will know if this extraordinary claim is true.
For readers interested in the details of Dai and Guerras’ technical analysis, I provide a description here on my website.
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Best Regards,
Robert
April 17, 2018
Note: Previous newsletters can be found on my website
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