Space: Why Can’t We Find Planet Nine? (Yet)

Quanta Magazine said:

Many astronomers remain convinced that a once-in-a-generation discovery is in the offing — one that would rewrite textbooks down to the elementary school level. “Every time we take a picture,” said Surhud More, an astronomer at the University of Tokyo, “there is this possibility that Planet Nine exists in the shot.”

Planet Nine, the hypothetical body thought to be lurking far beyond Neptune, continues to accumulate circumstantial evidence for its existence. But no telescope has yet been able to spot it. Michael Brown, a champion of the Planet Nine hypothesis and an astronomer at the California Institute of Technology, feels “eternally optimistic” that someone will soon find it, but there’s reason to believe that Planet Nine, if it exists, might be essentially invisible to existing observatories. At a recent Caltech workshop, two dozen physicists gathered to share the latest news and to discuss more creative ways to hunt.

The first evidence for Planet Nine surfaced in 2014, when the discovery of a planetoid revealed that a handful of mini ice-worlds beyond the Kuiper belt followed suspiciously similar paths around the sun. “If things are in the same orbit, then something’s pushing them,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science and the co-discoverer of the 2014 planetoid. Brown and his colleague Konstantin Batygin made a specific prediction two years later: The “perturber,” as they call it, should weigh between 5 and 20 Earth masses and follow an elliptical orbit hundreds or even 1,000 times more distant from the sun than Earth.

Out there, space gets dark alarmingly fast. Planets twice as far away look 16 times dimmer — the intensity of the sunlight weakens by a factor of four going out, and then four times again coming back. At an orbital distance of 600 astronomical units (1 AU is the distance between Earth and the sun), Planet Nine would be 160,000 times dimmer than Neptune is at 30 AU. At 1,000 AU, it would appear more than 1 million times weaker. “There’s really a brick wall, basically, at 1,000 AU,” said Kevin Luhman, an astronomer at Pennsylvania State University.

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