This illustration contrasts our familiar Earth with the exceptionally strange planet known as 55 Cancri e, which revolves around its sun-like star in the shortest time period of all known exoplanets: about 18 hours. (Credit: NASA/JPL-Caltech/R. Hurt (SSC) via Flickr)

Giant planets orbit close but don’t crash into star

Astronomers say they have a clearer picture why two giant planets are able to orbit close to their star without getting flung into the star or colliding into each other.

The star is named 55 Cancri. The two giant planets are located closer to 55 Cancri than Mercury is to our Sun.

Scientists combined thousands of observations with new statistical and computational techniques to measure the planets’ properties more accurately. The results suggest that the planets particular masses and orbits are preventing the system from self-destructing anytime soon.

“The 55 Cancri planetary system is unique in the richness of both the diversity of its known planets and the number and variety of astronomical observations,” says Eric Ford, an astronomy and astrophysics professor at Penn State and a coauthor of the paper published in the Monthly Notices of the Royal Astronomical Society.

“The complexity of this system makes it unusually challenging to interpret these observations,” says Ford.

Eight times the mass of Earth

This artist's conception compares a hypothetical solar system centered around a tiny "sun" (top) to a known solar system centered around a star, called 55 Cancri, which is about the same size as our sun. (Credit: NASA/JPL-Caltech/T. Pyle (SSC) via Flickr)
This artist’s conception compares a hypothetical solar system centered around a tiny “sun” (top) to a known solar system centered around a star, called 55 Cancri, which is about the same size as our sun. (Credit: NASA/JPL-Caltech/T. Pyle (SSC) via Flickr)

Benjamin Nelson, a graduate student at Penn State, and Ford collaborated with computer scientists to develop a tool for simulating planetary systems using graphics cards to accelerate the computations.

By combining multiple types of observations, they determined that one of the planets in the system (55 Cnc e) has eight times the mass of Earth, twice the distance of Earth’s radius, and the same density as that of Earth. This planet is far too hot to have liquid water because its surface temperature is estimated to be 3,800 degrees Fahrenheit, so it is not likely to host life.

It was only in 2011, 8 years after the discovery of this inner-most planet (55 Cnc e) that astronomers recognized it orbited its host star in less than 18 hours, rather than nearly 3 days, as originally thought. Soon after, astronomers detected the shadow of the planet passing over the Earth, allowing astronomers to measure the size of the planet relative to the size of the star.

“These two giant planets of 55 Cancri interact so strongly that we can detect changes in their orbits. These detections are exciting because they enable us to learn things about the orbits that are normally not observable,” says Nelson.

“However, the rapid interactions between the planets also present a challenge since modeling the system requires time-consuming simulations for each model to determine the trajectories of the planets and therefore their likelihood of survival for billions of years without a catastrophic collision.”

“One must precisely account for the motion of the giant planets in order to accurately measure the properties of the super-Earth-mass planet,” Ford says. “Most previous analyses had ignored the planet-planet interactions. A few earlier studies had modeled these effects, but had performed only simplistic statistical analyses due to the huge number of calculations required for a proper analysis.”

Visible to the naked eye

The 55 Cancri planetary system is just 39 light years away in the constellation Cancer. The system shines brightly when viewed from Earth because it is so close, so astronomers have been able to directly measure the radius of its star—an observation that is practical only for some of our closest stellar neighbors.

Knowing the star’s radius made it possible for astronomers to make precise measurements of its mass—nearly the same mass as our Sun—as well as the size and density of its super-Earth-size planet.

“Because 55 Cancri is so bright that it can be seen with the naked eye, astronomers have been able to measure the velocity of this star from four different observatories over a thousand times, giving the planets in this system much more attention than most exoplanets receive,” says Jason Wright, an assistant professor at Penn State who led a program to scrutinize this and several other planetary systems.

Astronomers first discovered that 55 Cancri is orbited by a giant planet in 1997. Long-term observations by Wright and colleagues later made possible the detection of five planets orbiting the star, ranging from a cold giant planet with an orbit very similar to that of Jupiter to a scorching-hot “super-Earth”—a type of planet with a mass higher than Earth’s but substantially below that of Neptune, which has a mass 17 times greater than Earth.

The study’s coauthors include scientists at the University of Florida, Yale University, the Max-Planck Institute for Astronomy in Germany, the University of Hawaii, and the Harvard-Smithsonian Center for Astrophysics.

Funding from NASA supported the work.

Source: Penn State

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