The search for Planet 9, 10, and beyond

Planetary scientists and cosmologists at Penn work together to find planets that might be hiding in the far reaches of the solar system.

a large black circle in the center of the milky way galaxy with a very small Sun in the background
Artist’s impression of Planet Nine, depicted as a dark sphere with the Milky Way in the background. Neptune’s orbit is shown as a small ellipse around the Sun. The sky view and appearance are based on the conjectures of Planet Nine’s co-proposer, Mike Brown (Image: Tom Ruen).

Students learning about the solar system might use the mnemonic “My Very Educated Mother Just Served Us Nachos” to remember the order of the planets—much to the chagrin of those who were served “Nine Pizzas” before Pluto lost planet status in 2006. And, while astronomers found evidence in 2015 that there might be planets beyond Neptune, efforts to confirm the existence of Planet Nine and, more recently Planet Ten, have yet to find anything conclusive.  

Researchers at Penn hosted a first-of-its-kind workshop by bringing together experts in planetary science and cosmology to develop a strategy for finding new planets and studying objects in the outer reaches of the solar system. Cullen Blake and Bhuvnesh Jain hosted the two-day workshop, which featured presentations from field-leading experts, including Penn researchers Gary Bernstein; Pedro Bernardinelli, a graduate student working with Bernstein; and researcher Eric Baxter

The challenge with finding a planet-sized needle in a cosmic haystack is that there’s a lot of sky to search with only a limited number of surveys, equipment, and time. Planet Nine “is faint, and the problem is that you have to scan the whole sky and you have to see it move,” says Blake. “There’s a lot of sky to observe, and we don’t really have a single facility to look for it.” 

By studying and finding new trans-Neptunian objects, pieces of rock and ice that are found in a region beyond Neptune known as the Kuiper belt, researchers can gain insights into how the solar system formed, a fundamental question that astronomers are still trying to answer. 

a color-coded map of the sky showing differences in temperature in the universe
CMB data collected by the Wilkinson Microwave Anisotropy Probe (WMAP). CMB radiation is a remnant of the Big Bang, with surveys revealing 13.77 billion years of minute temperature fluctuations, shown as differences in color (Image credit: NASA). 

Combining different types of surveys could help astronomers find planets beyond Neptune. Cosmic microwave background (CMB) surveys, for example, can be used to determine the age and composition of distant materials, while optical surveys can spot moving objects as they pass in front of stars as well as measuring the movement itself. The problem is that the data collected from CMB and optical surveys aren’t easy to analyze together. 

A white paper, which was led by researchers at Penn, provides guidance on how to combine survey data, and was a starting point for discussions between more than 20 researchers with expertise in CMB and optical surveys and planetary science. “Penn is a very special place for this topic,” says Jain. “We have very strong representation of the two types of datasets of CMB and optical surveys, experts in planetary astronomy, and real interactions between researchers in these diverse fields.”

At the workshop, researchers delved into the data which show abnormal orbits of several trans-Neptunian objects and discussed how the existence of additional planets was the best hypothesis for this phenomenon. 

Many were excited about the upcoming Large Synoptic Survey Telescope (LSST) and its ability to detect planets 9 and 10. LSST is an optical survey that will be able to detect a single Earth-mass planet as far away as 1000 astronomical units, or 1,000 times the distance between Earth and the sun. Workshop participants also talked about how to use other surveys including WFIRST, Euclid, TAOS, the South Pole Telescope, the Atacama Cosmology Telescope, SPHEREx, and the Transiting Exoplanet Survey Satellite (TESS).

Gil Holder, a co-author of the white paper, and Baxter both discussed how to use CMB surveys to find planets. They noted that the thermal “glow” of distant planets peaks near the microwave band that is targeted by CMB surveys. Many agreed that CMB surveys would be useful for spotting potential planets which could then be validated using optical data. 

Researchers discussed the technical and the computational hurdles of trying to combine CMB and optical data. Bernardinelli, an optical astronomer who collaborates with CMB researchers like Mark Devlin and John Orlowski-Scherer, says these discussions were fruitful. “It helped me to understand the challenges, both on the optical side and on the CMB side,” he says. 

a telescope dome bathed in red light with the milky way galaxy behind it
The Blanco Telescope dome, where the Dark Energy Camera collected the data used at Penn to look for Planet Nine, with the Milky Way in the background (Photo credit: Reidar Hahn, Fermilab)

The workshop concluded with a presentation by Bernardinelli on a Penn-led project using data from the Dark Energy Survey (DES) to search for objects in the Kuiper belt, where Bernardinelli and his collaborators have so far found more than 250 new trans-Neptunian objects, roughly 10 percent of all known distant solar system objects. Now that data collection is complete, a final analysis of the complete dataset will mean more chances to find new objects, including new planets.

While there’s no concrete evidence of Planet 9 or 10 yet, finding new objects in the Kuiper belt is still exciting. “It’s actually more scientifically useful,” says Masao Sako. “We can study things like the distribution of the orbital elements because the way that they are distributed tells us things about the formation and history of the solar system.” And after the New Horizons flyby of Ultima Thule earlier this year, the ability to track small, distant objects is becoming increasingly important for missions across the sky and on the ground. 

Cullen Blake is an assistant professor in the Department of Physics & Astronomy in the School of Arts and Sciences at the University of Pennsylvania.

Bhuvnesh Jain is the Walter H. and Leonore C. Annenberg Professor in the Natural Sciences and the co-director of the Center for Particle Cosmology in the Department of Physics & Astronomy in the School of Arts and Sciences at the University of Pennsylvania.

Masao Sako is an associate professor and undergraduate chair in the Department of Physics & Astronomy in the School of Arts and Sciences at the University of Pennsylvania.