Planets’ Wacky Orbits Solved

By combining nearly 1,500 observations with sophisticated computer models, astronomers have shed light on a nearby planetary system, proving that the planets' bizarre orbits will actually remain stable for the next 100 million years.

This star map will reveal 55 Cancri. International Astronomical Union / Sky & Telescope Magazine

The 55 Cancri planetary system orbits the star labeled ρ1, which is slightly left of the top star in the constellation Cancer.
International Astronomical Union / Sky & Telescope Magazine

If it’s clear tonight, step outside just after sunset. In the south, trailing behind Orion is Cancer, where on the darkest of nights you can just barely see 55 Cancri as the slightest speck. Shining from 40 light-years away, this faint star has five known planets in its retinue — ranging from a scorching-hot super-Earth followed by two hot-Jupiters within Mercury’s orbit to a cooler gas giant near Venus’s orbit and a Jupiter-like planet near Jupiter’s orbit.

Despite the fact that the system is located at a prime spot for study, until now astronomers have failed to understand the orbits of its two massive planets, which orbit closer to 55 Cnc than Mercury does to our Sun. Why hasn’t one of the planets been flung into the star, or even into the other?

A new study led by Benjamin Nelson (Pennsylvania State University and University of Florida) has provided the first complete model of the system’s orbital dynamics over time. The team mapped 1,418 radial-velocity observations from four observatories across the U.S. with computer simulations, revealing that the system will remain surprisingly stable for at least another 100 million years.

A simple solution is to use computer simulations to map how the exoplanets gravitationally interact with one another over time. However, these simulations often require a huge number of calculations and are therefore time-consuming on even the fastest supercomputers.

Astronomers will often sidestep this issue by simplifying their simulations. But Nelson and colleagues pioneered a new method. The team used graphics cards to accelerate the simulations, making them 4 to 5 times faster than they would have been otherwise.

 This illustration shows the orbital distances and relative sizes of the four innermost planets known to orbit the star 55 Cancri A (bottom) in comparison with planets in own inner Solar System (top).  Both Jupiter and the Jupiter-mass planet 55 Cancri "d" are outside this picture, orbiting their host star with a distance of nearly 5 astronomical units (AU), where one AU is equal to the average distance between the Earth and the Sun. Center for Exoplanets and Habitable Worlds, Penn State University

This illustration shows the orbital distances and relative sizes of the four innermost planets known to orbit the star 55 Cancri A (bottom) in comparison with planets in own inner Solar System (top). Both Jupiter and the Jupiter-mass planet 55 Cancri d are outside this picture.
Center for Exoplanets and Habitable Worlds (Pennsylvania State University)

This new result allowed the team to understand, for the first time, the complex orbital dynamics of the system as a whole and confirm that it could in fact remain stable. It also provided a few unique surprises, by shedding light on the individual exoplanets.

Previously astronomers thought the two massive planets 55 Cancri b and c were in a 3:1 resonance, meaning that the inner planet completes three orbits in the time it takes the outer planet to just complete one. These resonances often lead to unstable interactions, where the two planets alter or constrain each other's orbits. But the new results show the two planets are close, but not precisely in an orbital resonance with each other, allowing them to remain stable for another 100 million years.

55 Cancri e — the innermost planet — has a rapid orbit (clocking in at 18 hours) that drives temperatures to skyrocket. Astronomers have recently speculated that this carbon-rich planet might be heated high enough that it’s actually a diamond-rich planet. New constraints provided by this research may lead to new insight into its interior composition.

In contrast, 55 Cancri d — the outermost planet — has a slow and distant orbit. The planet is nearly 4 times the mass of Jupiter and orbits at 5.45 Earth-Sun distances, similar to Jupiter’s 5.2. These new constraints push the planet further toward being the closest known Jupiter analog in the exoplanet population.

The study is part of a much larger effort to develop new techniques to find Earth-like worlds. Many have focused their efforts toward developing state-of-the-art instrumentation for the world’s largest telescopes. But Nelson and colleagues are pairing those efforts with developing state-of-the-art computational and statistical tools, which will help detailed provide further analyses such as this one.


B. Nelson et al. “The 55 Cancri Planetary System: Fully Self-Consistent N-body Constraints and a Dynamical Analysis,” 2014

2 thoughts on “Planets’ Wacky Orbits Solved

  1. Roderick

    55 Cnc the host star is said to be 10 billion years old or so. The 100 million years of stable orbit for the exoplanet system falls far short of the host star age published. Is there a problem or conflict here not being reported?

  2. Joe S

    Computer simulations of planetary systems are not reliable beyond a certain time horizon. Also, the planetary orbits can become chaotic, in the sense of exponential sensitivity to uncertainties in the initial conditions. We only know the current state of our solar system (the initial condition for a computer simulation), or that of 55 Cnc, to a finite number of digits, so the predictions into the future are limited. Long-term stability of the solar system is still a difficult and unanswered theoretical question, as far as i know (i’ve been out of school a long time now).

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