More Evidence Presented in Defense of Planet 9

In January of 2016, astronomers Mike Brown and Konstantin Batygin published the first evidence that there might be another planet in our Solar System. Known as “Planet 9” (“Planet X” to those who reject the controversial 2006 Resolution by the IAU), this hypothetical body was believed to orbit at an extreme distance from our Sun, as evidenced by the fact that certain Trans-Neptunian Objects (TNOs) all seem to be pointing in the same direction.

Since that time, more and more evidence has been produced that show how the presence of Planet 9 affected the evolution of the Solar System, leading it to become as it is today. For example, a recent study by a team of researchers from the University of Michigan has shown how Planet 9 may have kept certain TNOs from being destroyed or ejected from the Solar System over the course of billions of years.

The study, which was recently published in the Astronomical Journal under the title “Evaluating the Dynamical Stability of Outer Solar System Objects in the Presence of Planet Nine“, was led by Juliette Becker, a graduate student with the University of Michigan’s Department of Astronomy. It was supported by Professors David Gerdes and Fred Adams, as well as graduate and undergraduate students from UofM’s Depart of Physics.

Diagram showing how the six most distant known objects in the Solar System with orbits beyond Neptune (TNOs) all mysteriously line up in a single direction. Credit: Caltech/R. Hurt (IPAC)

For the sake of their study, Becker and her colleagues conducted a large set of computer simulations that examined the stability of Trans-Neptunian Objects (TNOs) who’s orbits are believed to have been influenced by Planet 9. In each simulation, the researchers tested a different version of Planet 9 to see if its gravitational influence would result in the Solar System as we know it today.

From this, they uncovered two key findings. First, the simulations showed that Planet 9 may have led to the current Solar System by preventing these TNOs from being destroyed or ejected from the Solar System. Second, the simulations indicated that TNOs can jump between stable orbits, a process they refer to as “resonance hopping”. This would prevent these same TNOs from being thrown out of the Kuiper Belt.

As Becker explained in a University of Michigan press statement:

“From that set of simulations, we found out that there are preferred versions of Planet Nine that make the TNO stay stable for longer, so it basically increases the probability that our solar system exists the way it does. Through these computer simulations, we were able to determine which realization of Planet Nine creates our solar system—the whole caveat here being, if Planet Nine is real.”

Next, Becker and her team examined the TNOs to see if they experienced resonance with Planet 9. This phenomena, which occurs as a result of objects exerting a gravitational influence on each other, causes them to line up in a pattern. What they found was that, on occasion, Neptune will push a TNOs out of its orbital resonance, but does not disturb it enough to send it towards the Sun.

Artist's impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign
Artist’s impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign

A plausible explanation for this behavior was the gravitational influence of another object, which serves to catch any TNOs and confine them to a different resonance. In addition, the team also considered a newly-discovered TNO that was recently detected by The Dark Energy Survey collaboration – a group of 400 scientist from 26 institutions in seven countries, which includes several members from the University of Michigan.

This object has a high orbital inclination compared to the plane of the Solar System, where it is tilted at 54° relative to the Sun’s ecliptic. After analyzing this new object, Becker and team concluded that the object also experiences resonance hopping, which is consistent with the existence of Planet 9. This, along with other recent studies, are creating a picture where it is harder to imagine the Solar System without Planet 9 than with it.

As Becker explained, all that remains now is to observe Planet 9 directly.”The ultimate goal would be to directly see Planet Nine—to take a telescope, point it at the sky, and see reflected light from the sun bouncing off of Planet Nine,” she said. “Since we haven’t yet been able to find it, despite many people looking, we’re stuck with these kinds of indirect methods.”

Further Reading: University of Michigan, The Astronomical Journal

Did you Know There are X-rays Coming from Pluto? That’s Strange, What’s Causing it?

Once held to be the outermost planet of the Solar System, Pluto‘s designation was changed by the International Astronomical Union in 2006, owing to the discovery of many new Kuiper Belt Objects that were comparable in size. In spite of this, Pluto remains a source of fascination and a focal point of much scientific interest. And even after the historic flyby conducted by the New Horizons probe in July of 2015, many mysteries remain.

What’s more, ongoing analysis of the NH data has revealed new mysteries. For instance, a recent study by a team of astronomers indicated that a survey by the Chandra X-ray Observatory revealed the presence of some rather strong x-rays emissions coming from Pluto. This was unexpected, and is causing scientists to rethink what they thought they knew about Pluto’s atmosphere and its interaction with solar wind.

In the past, many Solar bodies have been observed emitting x-rays, which were the result of interaction between solar wind and neutral gases (like argon and nitrogen). Such emissions have been detected from planets like Venus and Mars (due to the presence of argon and/or nitrogen in their atmospheres), but also with smaller bodies like comets – which acquire halos due to outgassing.

Artist’s impression of New Horizons’ close encounter with the Pluto–Charon system. Credit: NASA/JHU APL/SwRI/Steve Gribben

Ever since the NH probe conducted its flyby of Pluto in 2015, astronomers have been aware that Pluto has an atmosphere which changes size and density with the seasons. Basically, as the planet reaches perihelion during its 248 year orbital period – a distance of 4,436,820,000 km, 2,756,912,133 mi from the Sun – the atmosphere thickens due to the sublimation of frozen nitrogen and methane on the surface.

The last time Pluto was at perihelion was on September 5th, 1989, which means that it was still experiencing summer when NH made its flyby. While studying Pluto, the probe detected an atmosphere that was primarily composed of nitrogen gas (N²) along with methane (CH4) and carbon dioxide (CO²). Astronomers therefore decided to look for signs of x-ray emissions coming from Pluto’s atmosphere using the Chandra X-ray Observatory.

Prior to the NH mission’s flyby, most models of Pluto’s atmosphere expected it to be quite extended. However, the probe found that the atmosphere was less extended and that its rate of loss was hundreds of times lower than what these models predicted. Therefore, as the team indicated in their study, they expected to find x-ray emissions that were consistent with what the NH flyby observed:

“Given that most pre-encounter models of Pluto’s atmosphere had predicted it to be much more extended, with an estimated loss rate to space of ~1027 to 1028 mol/sec of N² and CH4… we attempted to detect X-ray emission created by [solar wind] neutral gas charge exchange interactions in the low density neutral gas surrounding Pluto,” they wrote.

Images sent by NASA’s New Horizons spacecraft show possible clouds floating over the frozen landscape including the streaky patch at right. Credit: NASA/JHUAPL/SwR

However, after consulting data from the Advanced CCD Imaging Spectrometer (ACIS) aboard Chandra, they found that x-ray emissions coming from Pluto were greater than what this would allow for.  In some cases, strong x-ray emissions have been noted coming from other smaller objects in the Solar System, which is due to the scattering of solar x-rays by small dust grains composed of carbon, nitrogen and oxygen.

But the energy distribution they noted with Pluto’s x-rays were not consistent with this explanation. Another possibility that the team offered is that they could be due to some process (or processes) that focus the solar wind near Pluto, which would enhance the effect of its modest atmosphere. As they indicate in their conclusions:

“The observed emission from Pluto is not aurorally driven. If due to scattering, it would have to be sourced by a unique population of nanoscale haze grains composed of C, N, and O atoms in Pluto’s atmosphere resonantly fluorescing under the Sun’s insolation. If driven by charge exchange between [solar wind] minor ions and neutral gas species (mainly CH4) escaping from Pluto, then density enhancement and adjustment of the [solar wind] minor ion relative abundance in the interaction region near Pluto is required versus naïve models.”

For the time being, the true cause of these x-ray emissions is likely to remain a mystery. They also highlight the need for more research when it comes to this distant and most massive of Kuiper Belt Objects. Luckily, the data provided by the NH mission is likely to be poured over for decades, revealing new and interesting things about Pluto, the outer Solar System, and how the most distant worlds from our Sun behave.

The study – which was accepted for publication in the journal Icarus – was conducted by astronomers from the Johns Hopkins University Applied Physics Laboratory (JHUAPL), the Harvard-Smithsonian Center for Astrophysics, the Southwest Research Institute (SwI), the Vikram Sarabhai Space Center (VSCC), and NASA’s Jet Propulsion Laboratory and Ames Research Center.

Further Reading: CfA, arXiv