Getting to know our new neighbour

Tantalising evidence for another planet in our Solar System was presented early this year. Details on ‘Planet Nine’, however, were scarce. Here, astrophysicist Esther Linder tells us how she and her team are attempting to find out what it is made of, what it might look like and even why we have never seen it. 

In January, we heard about the exciting possibility of another planet in the Solar System¹. Caltech astronomer Mike Brown and his colleague Konstantin Batygin predicted an object of 10 Earth masses at a distance of 700 Astronomical Units (1 AU = distance Sun-Earth) after studying the peculiar orbits of distant Kuiper Belt Objects. Having an additional companion in the Solar System would be a challenging case for planetary formation theory, since from a formation point of view; it is not easy to conceive that such a big object forms so far away from the Sun. It is therefore unclear how this planet could have ended up at its predicted present-day position.

[caption id="attachment_54480" align="alignnone" width="550"] Theories surrounding Planet 9 surfaced early this year.[/caption]

Finding another ice giant in the Solar System would also be helpful regarding the understanding of the internal structure of Uranus and Neptune. Because, even if these two ice giants look similar at first glance, Uranus has to date no measurable internal heat flux, but Neptune has, and this difference is so far unexplained. Having an additional ice giant to study would therefore be very interesting from a planetary evolution theory point of view. So, if there was another major body in the Solar System, we wondered what it would look like – how big and warm it would be and especially, how bright it would be in various bands of the electromagnetic spectrum. Could we discover reasons for why it has never been seen? To answer these questions we performed various simulations. In our group we have a model called Completo21, initiated six years ago by Christoph Mordasini, and then further developed by him and his graduate students. Completo21 is a model of planetary evolution and begins its calculation at the moment in time when the parent protoplanetary disc has vanished. The planet structure is simplified by assuming that the core consists of iron. This is wrapped in a silicate mantle followed by a possible water ice layer and finally a H/He envelope. Such an internal structure is inferred from assuming that Planet 9 is a smaller version of Neptune.

Completo21 takes into account the cooling and contraction of the core and the gaseous envelope. In the nominal (reference) mode, we assume an adiabatic interior where heat is transported outwards by sinking and rising gas blobs. As input various planetary parameters like the core and envelope mass, the semi-major axis, the ice fraction in the core and the metallicity are needed. The output is, among other things, the radius, temperature and luminosity at all times during the evolution. We were also interested in calculating magnitudes; the brightness of the object in various observational bands in the visual and infrared domain. This is important for the planet’s detectability.

Finding another ice giant in the Solar System would also be helpful regarding the understanding of the internal structure of Uranus and Neptune.

Observational bands, such as V (visual), R (red) or I (infrared), are wavelength intervals in the electromagnetic spectrum where astronomical observations are made. Magnitudes are defined via the flux of an object in a specific filter band relative to a standard. This standard is the spectrum of the star Vega. To calculate the flux of an object, a filter transmission curve is needed. This curve tells us how much flux goes through the filter at a specific wavelength. Combining these things and assuming a blackbody spectrum for the planet, we could calculate magnitudes in various filter bands. Moreover, we also wanted to study the impact of various model settings – like the planet’s mass or semi-major axis – on the magnitude. Therefore we performed a range of simulations, varying one parameter at a time relative to the nominal starting configuration. We also performed some simulations where the planet is allowed to circle the Sun in an orbit given by solving Kepler’s equation. In this scenario, the planet is not fixed at one specific distance to the Sun but its closest point to the Sun is at 280 AU, and its furthest at 1120 AU.

[caption id="attachment_54478" align="alignnone" width="620"] Reasons for Planet 9 not being discovered sooner include telescopes not looking at the right part of the sky from Earth and also the magnification power of telescopes used by astronomers in the past.[/caption]

We found that an object of 10 Earth masses at a distance of 700 AU is brightest in the mid- and far infrared. Its effective temperature is 47 Kelvin, which is much more than 10 Kelvin, the temperature it would have had if it would only be heated by irradiation from the Sun at a distance of 700 AU. Therefore, it is still cooling and contracting, radiating away the energy that is left over from its formation. The blackbody spectrum of the planet shows two major contributions. Namely, the part that comes from the reflected light from the Sun, as well as its intrinsic radiation that contributes in the mid- and far infrared. Visual filters therefore pick up the reflected light, whereas filters working in the infrared can detect the intrinsic light. With a heavier mass, the planet gets significantly brighter. Also with a change of the semi-major axis, the brightness changes, making it more detectable in the visual bands when it’s closer to the sun, since then the reflected flux is bigger.

We also wondered why such an object, if it exists in the Solar System, has remained undetected until now. Therefore we studied past astronomical surveys. We found that there were basically two problems. Either the telescope that were used were not able to detect such an object because it is too faint, or they did not look at the right position in the sky, i.e. did not cover the planet’s orbit. Today, it is not trivial to find or definitely rule out the existence of another major body in the Solar System. Because even if the orbit of the possible planet is getting more and more constrained due to additional analyses being performed of the orbits of various bodies in the Solar System, the exact position of the predicted object is not clear at all. Since Planet Nine takes approximately 20000 years to orbit around the sun, there is a wide area on the sky that needs to be monitored in order to observe the planet’s orbital motion. Moreover, the planet might be in front of the Milky Way, making it harder to detect because of confusion with the many background stars.

Its effective temperature is 47 Kelvin, which is much more than 10 Kelvin, the temperature it would have had if it would only be heated by irradiation from the Sun at a distance of 700 AU.

Teams all around the world are currently working hard to detect or definitely rule out candidate Planet Nine. At latest, future telescopes such as the Large Synoptic Survey Telescope will have the needed depth in magnitude to be able to find or rule out candidate Planet Nine. These are exciting times.

Author: Esther Linder is a PhD student of Professor C. Mordasini in the Department Space Research & Planetary Sciences at Bern University, Switzerland.

Featured: Professor Mordasini leads the research group “PlanetsInTime” at the University of Bern. He is interested in many aspects of planet formation and evolution theory, but he is also involved in the observational search for extrasolar planets with large telescopes in Chile.

References: 1. http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22/meta