• Orbit of Planet Nine, Caltech
  • Trumpler 14, NASA, ESA, and J. Maíz Apellániz (Institute of Astrophysics of Andalusia, Spain)

Planet Nine in Outer Space!

We may yet again have a ninth planet in the Solar System.

No, Pluto has not regained its planetary status. Instead, astronomers published evidence in the Astronomical Journal this week of a ninth planet in the Solar System—one that is at least ten times more massive than Earth, roughly the diameter of Neptune, and orbits the Sun in a highly tilted and elliptical orbit that never brings it closer than 200 times the distance of Earth to the Sun.

It is important to emphasize that the announcement this week was not a discovery announcement—there are no direct observations or images of the proposed “Planet Nine,” as researchers have dubbed it—but rather a presentation of the strongest evidence yet of another large planet skirting the extreme edge of our solar system.

So if we have not seen this proposed planet, what evidence do we have that it’s actually there?

The investigation is led by two professors from Caltech, Konstantin Batygin and Mike Brown. (Brown, an Academy fellow who goes by the moniker “plutokiller” on Twitter, is a leading dwarf planet researcher and the discoverer of the dwarf planet Eris. He is known as the “demoter” of Pluto, and he presented here last October about his research into Planet Nine as part of our Benjamin Dean Astronomy Lecture Series). In 2014, inspired by a paper by Chad Trujillo and Scott Sheppard that noted similarities in the orbits of 13 Kuiper Belt objects, the pair began to analyze the orbits of other far-flung objects in the outer reaches of the Solar System: in particular, six larger icy bodies, including the dwarf planet Sedna, that have distinct, elliptical, tilted, but stable orbits. What is surprising, according to Batygin, is that their orbits all point into the same overall direction, in stark contrast with the rest of the Kuiper Belt. The odds of such an alignment occurring on coincidence are, well, astronomical—about 0.007 percent! (Seth Shostak of the SETI Institute compares it to “spilling a handful of pencils onto a table and finding that they pretty much all point in the same direction.”) So it seems likely that something is influencing them.

The two astronomers began to build and run computer simulations of what might cause such an arrangement. They started by testing if the Kuiper Belt itself had enough mass to influence the orbits. Then they moved on to the concept of a closer planet within the bulk of the Kuiper Belt, and eventually, they ran a simulation of a larger planet, farther away from the Sun, with an orbit essentially opposite of the six objects of interest. The latter simulation showed that repeated close encounters with this potential planet would nudge the smaller objects into paths that all had similar characteristics. Batygin calls it “a beautiful match to the real data.” Taking it a step further, the simulation also predicted that a few Kuiper belt objects would have been knocked into orbits perpendicular to the plane of the Solar System—and five such objects have been previously observed!

(You can take a look at the data yourself! The Caltech press release has links to videos that show the relationship between the errant objects’ orbits as well as the proposed path of Planet Nine. You can find a great visualization of the orbits on YouTube as well. And the data will be made available via WorldWide Telescope, too, so you can analyze the orbits firsthand—check back here on Monday, January 25, for a link.)

Predicting the existence of a planet based on mathematical models prior to observational discovery is not unprecedented: Neptune’s existence was also predicted before it was found due to the irregularities it causes in Uranus’s orbit. But it certainly puts Batygin and Brown in good scientific company.

If confirmed, the new world would be the third “ninth planet” discovered after Pallas (now classified as an asteroid) and Pluto (now a dwarf planet). What makes this planet different than the former ninth planets? Why would this distant world deserve the title “planet” when Pluto does not? Brown has a straightforward way of looking at the cutoff for planethood: If the object is massive enough that it has cleared its path around the Sun and “bullied” its region of the solar system, then it’s a planet. “There are some truly dominant bodies in the Solar System, and they are pushing around everything else,” Brown notes. “This is what we mean when we say planet.” Under this definition, Pluto clearly doesn’t have the size to influence its region of the Solar System in the manner of a true planet. Planet Nine, with at least 10 times the mass of Earth (or 5,000 times the mass of Pluto) clearly does. Maybe Planet Nine, a more robust world with no debate on its planethood and theorized by American astronomers, could replace Pluto in the hearts of many.

The next step, of course, is direct observation of the planet. However, that is not as easy as it sounds. While an approximate orbit of the world has been narrowed down, its location along its highly elliptical orbit has not. If it currently resides in a closer part of the orbit—still some 200 astronomical units (AU) away, or 200 times farther from the Sun than Earth—sky surveys may pick it up. If it lies farther from the Sun, which could be anywhere from 500 and 1,200 AU out, then Earth’s largest telescopes such as those on Mauna Kea in Hawaii will be needed to spot it. Brown hopes for visual confirmation of the world in the next five years. –Elise Ricard

That Parade of Planets Is NOT an “Alignment”

As we reported last week, early-risers will be able to see the five naked-eye planets at the same time, weather-permitting, next week. Yes, it’s still happening—four worlds (Venus, Saturn, Mars, and Jupiter) are already visible, and Mercury will soon start briefly peeking out of the morning twilight. Joining them—from January 27 through February 6—the Moon will move along the line from planet to planet. After that, Venus and Mercury will eventually disappear into the Sun’s glow, but Saturn, Mars, and Jupiter will remain visible for some time, gradually moving into the evening sky.

However, a number of reports in the media are describing the planetary configuration as an “alignment,” as if the planets are physically arranging themselves into a domino-like row in the Solar System. Some follow up and clarify the situation, but readers who latch on to the buzzword “alignment” may be misled into thinking there’s more going on than what’s actually happening.

The only “line” forming is the apparent one that the planets form along the plane of the Solar System (called the “ecliptic”), as viewed from Earth, and they do that all the time. It’s like looking at a handful of marbles scattered randomly on a tabletop: from overhead, the true, haphazard distribution can be seen, but when viewed along the tabletop, the marbles appear to be distributed in a straight line. This is the case with the planets we see in the sky, because the planets’ orbits—including Earth’s—all lie on the same plane, and when we look into the sky, we’re looking along that plane and see the planets distributed along the “ecliptic.” The naked-eye planets are spread out across nearly 120 degrees as seen from Earth, and when the relative distances of the planets are taken into consideration, any semblance of a line breaks down. This can be seen at websites that display the current positions of the planets as seen from above.

Not an “alignment” of any kind, but observers can certainly appreciate the sight of five planets in the sky at the same time, with the Moon as an added bonus. Hope for clear weather and enjoy the show! –Bing Quock

Just a Pretty Picture?

To round out our celebration of Space Friday, we’ll draw your attention to a really lovely image from the Hubble Space Telescope, taken as part of the Hubble Heritage program.

The snapshot of the star cluster Trumpler 14 reveals not just the bright and brilliant stars but also the colorful wisps of gas in which they are embedded. A mere half million years old, these youthful stars give off so much radiation that the surrounding gas and dust glow not just with reflected light but also with emission from ionized atoms. The stars themselves are not long for this universe: they will likely flare out spectacularly in the relatively near future (in astronomical terms, that is), within a few million years or so. But never fear! The cluster lies 8,000 light years away, so we have nothing to fear from the blast when these massive stars go supernova (unlike hypothetical aliens near ASAS-SN-15lh, the “superluminous supernova” we described last week). We can just enjoy the view, courtesy of Hubble, from afar.

There’s a zoomable version of the image available, too, so you can scan in it all its lush detail. –Ryan Wyatt

Image(s): Orbit of Planet Nine, Caltech; Trumpler 14, NASA, ESA, and J. Maíz Apellániz (Institute of Astrophysics of Andalusia, Spain)

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