Tikalon Blog by Dev Gualtieri

Solar Tilt and Planet Nine

November 7, 2016

As all scientists know, nature is fond of throwing us curveballs. Just when we think we have things figured out, the data points head in an unexpected direction. A baseball curveball drops downwards from a straight trajectory, an effect caused by the pitcher putting a forward spin on the ball as it leaves his hand.

There's an example of a confounding and stealthy spin in an early physics experiment. While this experiment is sometimes called "Einstein's only experiment," more likely than not, Albert Einstein didn't actually do the experiment. The experiment was probably performed by his colleague, Wander Johannes de Haas (1878-960), who improved on an experiment by Owen Richardson (1879-1959).[1-2] De Haas was a Dutch physicist known especially among condensed matter physicists for the De Haas–van Alphen effect.

Owen Richardson (1879-1959).

In 1901, Richardson discovered the exponential dependence of current on filament temperature in a vacuum diode. This is called Richardson's law in his honor.

(Wikimedia Commons image, modified for artistic effect.)

Although Richardson was the first to do the experiment, what was found is called the Einstein–de Haas effect, and not the Richardson effect, since Einstein and de Haas gave the results a theoretical framework. However, we need not feel too sorry for Richardson, since he went on to win the 1928 Nobel Prize in Physics for his work on thermionic emission.

The Einstein–de Haas effect demonstrated the existence of atoms using the simple apparatus shown in the figure. An iron cylinder is suspended inside a solenoid coil. When the switch is engaged, the current through the solenoid produces a magnetic field parallel to the axis of the cylinder. Despite the symmetry of the system, the iron cylinder unexpectedly twists.

Experiment demonstrating the Einstein-de Haas effect

An experimental demonstration of the Einstein-de Haas effect.

An iron cylinder hanging from a thin wire twists when a magnetic field, generated by a solenoid, is applied along its axis.

The small twist is measured by light deflected from a mirror.

(Wikimedia Commons image by Jasper Olbrich, modified.)

The twist is a consequence of an axial angular momentum given to the iron cylinder. Since angular momentum must be conserved in a classical system, and the cylinder was initially at rest, there must be a contrary angular momentum developed inside the iron cylinder. This is the angular momentum of the invisible electrons of the iron atoms. What's even more interesting is the Barnett effect, an effect complementary to the Einstein-de Haas effect, in which a rotating iron cylinder becomes magnetized.

There's another mysterious spin that occurs on a larger scale than this laboratory experiment. Our Sun rotates, a fact known since the telescope enabled sunspot observations. English astronomer and polymath, Thomas Harriot (1560-1621), who was the first to draw the Moon as observed through a telescope on July 26, 1609, was also the first to telescopically observe sunspots in December, 1610. However, it was Johannes Fabricius (1587-1616) who noticed their movement and an implied solar rotation, now known to be about 25 days at its equator.

The mystery is not the Sun's spin; rather, it's the angle of its spin. The orbits of our eight Solar System planets are very nearly on a plane known as the invariable plane, which is very nearly the same as the plane of Earth's orbit (the ecliptic). However, the rotation axis of the Sun, termed its obliquity, is tilted 7.25° from this plane. Since all the Solar System bodies coalesced from a protoplanetary disk, the Sun's obliquity should be near zero.

This unusual solar obliquity has been an astrophysics topic for at least 65 years.[3] In 2014, Chadwick A. Trujillo, an astronomer at the Gemini Observatory (Hilo, Hawaii), and Scott S. Sheppard of the Carnegie Institution for Science (Washington, DC), surmised the existence of a Trans-Neptunian object now known as Planet Nine.[4-5] It's now been hypothesized that Planet Nine is the cause of the solar obliquity.[6-8] I wrote about Planet Nine in a recent article (Planet Nine, June 27, 2016).

Dark and mysterious.

Artist's impression of Planet Nine.

(Caltech image by R. Hurt.)

Here are a few facts about Planet Nine.[9]

• Planet Nine is a hypothetical planet at the present. If its location were known, the Hubble Space Telescope would image it as just a few pixels.

• The mass of Planet Nine is estimated to be about ten times the mass of Earth. It should resemble a smaller version of Neptune.

• Planet Nine has an orbit about twenty times farther from the Sun than that of Neptune.

• The orbital period of Planet Nine is about 10,000-20,000 years.

• Planet Nine has a greater gravitational affect on the Solar System than any other known planet.

The orbit of Planet Nine and other Trans-Neptunian objects.

(Wikimedia Commons image by Karl432.)

Planet Nine's apparent affect on the Sun's obliquity arises from its large mass, and its imputed thirty degree inclination to the invariable plane. Evidence for Planet Nine's influence on the Sun comes from the observation that it would also affect the orbits of quite a few objects, kicking their orbits far out of the invariant plane, and such objects have been observed.[4-6] One of these is Sedna, a minor planet three times farther from the Sun than Neptune. Sedna has an inclination of nearly 12°. Six Kuiper Belt objects have orbits that appear to be in small ratios with a massive planet like Planet Nine.[8]

Elizabeth Bailey, Konstantin Batygin, and Michael E. Brown, planetary astronomers at Caltech, announced their solution to the solar obliquity problem on October 18, 2016, at the American Astronomical Society Division for Planetary Sciences 2016 annual meeting in Pasadena, California.[8] A paper on their research has been accepted for publication in a future issue of the The Astrophysical Journal.[7]

The physical reason for Planet Nine's large affect on the Sun and outer planets is a consequence of angular momentum, as is the reason for the Einstein-de Haas effect that I presented above. In the case of a planet, the angular momentum is the product of its mass and its distance from the Sun. Planet Nine has a large mass, and it's positioned quite far from the Sun.[7] Says Bailey, a graduate student who was lead author of the study,

"Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment."[7]

With this solution to the solar obliquity problem, there's a confluence of evidence that points to the existence of Planet Nine.[7-8] Michael Brown is quoted in The Guardian as saying, "...if there’s not a massive planet out there it has to be that there was one there yesterday and disappeared."[8] The planet's actual discovery should occur within the next few years.[7-8]

Solar Tilt and Planet Nine

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