Caltech researchers answer skeptics' questions about Planet 9

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Caltech researchers dropped a bomb on the scientific community Wednesday by unveiling new research that suggests there is a ninth planet ranging through our solar system that has so far escaped our notice.

The team came up with the idea as they tried to explain why six objects in an area beyond Neptune, known as the Kuiper Belt, were behaving so strangely. These icy chunks of space rock showed an odd, elliptical orbit that seems to point them in the same direction.

Caltech's Konstantin Batygin said at first they laughed at the notion of a mysterious planet lurking billions of miles from Earth causing this effect.

"We literally looked for every possible explanation you could find where you don't need to invoke this additional massive body in the distant solar system," he explained.

But after exhausting all other theories, they decided to run computer models imagining a gas giant, ten times the mass of Earth, looping through that area of space. 

It was that model that proved best at explaining the strange behavior of the Kuiper Belt objects. In essence, the massive gravitational pull of a gas giant in that part of solar system seemed to be the most probable reason for why the six objects were orbiting the way they did. 

The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. The new research from Konstantin Batygin and Mike Brown show that a planet with 10 times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration.
The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. The new research from Konstantin Batygin and Mike Brown show that a planet with 10 times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration. Caltech/R. Hurt (IPAC); [Diagram created using WorldWide Telescope.]

If it's true, it means a planet roughly the size of Neptune has eluded our detection despite all our increasingly sophisticated observational tools.

It's no surprise then that this big claim has raised some big questions. Here are some of the things skeptics are asking.

Does it have to be a planet causing these strange orbits?

Plenty of scientists have tried to explain strange findings in the past by imagining a mysterious planet that locks its neighbors in a gravitational vice. 

Batygin said his first instinct was to look for a less flashy explanation. He examined whether the odd orbits of the Kuiper Belt objects could have happened randomly. Calculations show just a  0.007 percent chance of that happening, making it very unlikely.

He also looked at whether the solar system as it is currently set up could have somehow led to this configuration.

"We tried... we really failed," he said.

They even used computers to see if the Kuiper Belt itself could create enough gravitational force to be the culprit, but under those models, the belt would need to have 100 times the mass it does today.

In the end, the Planet 9 theory seemed the best option.

Still, Emily Lakdawalla, senior editor with the Planetary Society, noted that just because this theory works the best for now doesn't mean it must be right.

"Any theoretical paper describing one possible explanation for weird features of the solar system often leaves out what could be other possible explanations that we haven't even thought of yet," she said.

If this planet is so big, why are only six objects influenced by it?

Anything the size of this hypothetical Planet 9 should leave lots of gravitational fingerprints.

However, the initial research here is based on the orbits of just six objects -- each roughly the size of Los Angeles.

That's not a lot of evidence, said Laura Danly, an astronomer and curator with the Griffith Observatory.

"I think we'd want to see more observations of objects that share those strange orbital behaviors to help put more solid foundation on this hypothesis," she said.

The team at Caltech agree, and they said they'd love more data to work with. The problem is Kuiper Belt objects are very distant and hard to spot, so we don't have many known objects to examine.

Still, Caltech's Mike Brown said his team has been able to use the Planet 9 hypothesis to explain the strange behavior of some other distant objects in the same region of space.

"It turns out there are other effects that this predicts that we have started to find too," Brown noted.

Now that this Planet 9 theory is out there, plenty of researchers will be looking for traces of it in other data to either support the idea or bring it down.

"That's the way science works, people should go off and try and prove that we are wrong. I assume that people are starting that today," Brown said.

How did this planet end up so far from the sun?

It's believed our planets formed from a disk of gas and dust swirling around our newly formed sun more than 4.5 billion years ago. Clumps of material stuck together then smashed into other clumps forming the seeds of the planets.

As those seeds orbited the sun, they eventually gathered enough gravity to pull in surrounding objects and eventually cleared all the debris from their path.

The problem with something like Planet 9 is that it lies so far from the sun, that it's likely the disk of dust and gas there was too thin to create a body that size.

Batygin and Brown address this by suggesting perhaps Planet 9 formed closer to our sun but was eventually knocked away by say Jupiter or Saturn, sending it into its current orbit.

This isn't as wild as it may sound since researchers believe our early solar system was a busy place where planets might have bounced off each other like balls on a pool table.

However, if that's the case, it raises another question.

Why wasn't this gas giant flung out of our solar system altogether?

It would take a huge amount of energy to bounce something the size of Planet 9 out of its original orbit and send it elsewhere. If that were to happen, the planet would likely keep going, perhaps leaving the solar system all together.

Once again, Batygin says the nature of the early solar system might help explain why this didn't happen.

He argues that when our celestial home was forming, it was part of a cluster of thousands of neighboring stars, each affecting the space around it in major ways.

"Once you get far away from the sun, the gravitational pull of neighboring stars can modify your orbit very, very significantly," he said.

Batygin thinks perhaps some of these stars interacted with Planet 9 as it flew from our inner solar system, turning it around and putting it in a very wide but consistent orbit. 

Given its size, why didn't we spot this before?

You think you'd notice something ten times the mass of Earth hanging around in the solar system. So why is this the first time we are hearing about a possible ninth planet?

In fact, in 2009 NASA launched the Widefield Infrared Survey Explorer (WISE) satellite and later tasked it with completing a survey of the sky to find heat from large planets.

Planet 9 didn't show up.

However, that survey was designed to find objects the size of Saturn or larger. Planet 9 would have been too small to be detected.

Planet 9 is also believed to have a very long orbit, taking anywhere from 10,000 to 20,000 years to make one trip around the sun. That means it might have spent recent history quite far from Earth, making it tough to spot.

Still, there are a few telescopes that are up for the job, like the W. M. Keck Observatory and the Subaru Telescope, both in Hawaii.

It's possible the planet also showed up in past photographs of space, but scientists missed it because they weren't looking for it. Brown and Batygin are encouraging researchers to comb through images taken of areas believed to be in Planet 9's orbit to see if the celestial body made a cameo.

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