Family of planets found

Artist's conception of the Kepler-11 system

This artist's conception of the newly discovered planetary system shows six planets orbiting the Sun-like star Kepler-11.

  • The star Kepler-11 has six planets
  • Some of them are not much bigger than Earth
  • Discovery was made using the Kepler space telescope

A REMARKABLE PLANETARY SYSTEM discovered by NASA’s Kepler space telescope has six planets circling a Sun-like star, including five small planets in tightly packed orbits.

Astronomers at the University of California, Santa Cruz, and their co-authors analysed the orbital characteristics of the system to determine the sizes and masses of the planets, and figure out their likely compositions.

All of the information gleaned is based on Kepler’s measurements of the changing brightness of the host star (called Kepler-11) as the planets passed in front of it, producing mini-eclipses called ‘transits’.

The five inner planets in the Kepler-11 system range from 2.3 to 13.5 times the mass of the Earth. Their orbital periods are all less than 50 Earth days, which means they are very close to their host star…so close that they would fit inside the orbit of Mercury in our Solar System.

The sixth planet is larger and farther out, with an orbital period of 118 days and an undetermined mass.

“Not only is this an amazing planetary system, it also validates a powerful new method to measure the masses of planets,” said Daniel Fabrycky, a Hubble postdoctoral fellow at UC Santa Cruz, who led the orbital analysis.

Fabrycky and Jack Lissauer, a scientist at NASA’S Ames Research Centre, are the lead authors of a paper on Kepler-11 published in the February 3 issue of Nature.

“Of the six planets, the most massive are potentially like Neptune and Uranus, but the three lowest mass planets are unlike anything we have in our Solar System,” said Jonathan Fortney, assistant professor of astronomy and astrophysics at UCSC, who led the work on understanding the structure and composition of the planets, along with UCSC graduate students Eric Lopez and Neil Miller.

Comparison of Kepler planet sizes with Earth and Jupiter

Planet comparisons. The six newly discovered Kepler-11 planets are represented by the circles on the bottom row. Earth, Jupiter, and estimated sizes of other Kepler planets are shown above. (RE stands for 'radius of Earth'.)

How the work was done

The Kepler space telescope detects planets that “transit” or pass in front of their host star, causing periodic dips in the brightness of the star as measured by the telescope’s sensitive photometer.

The amount of the brightness reduction tells scientists how big the planet is in terms of its radius. The time between transits tells them its orbital period.

To determine the planets’ masses, Fabrycky analysed slight variations in the orbital periods caused by gravitational interactions among the planets.

The timing of the transits is not perfectly regular, which is an indication that the planets are gravitationally interacting, says Fabrycky. The scientists’ computer models show that the system can remain stable on time scales of millions of years.

Previously, detections of transiting planets have been followed up with observations from powerful ground-based telescopes to confirm the planet and determine its mass using Doppler spectroscopy, which measures the “wobble” in the motion of the star caused by the gravitational tug of the planet.

Artist's drawing of the Kepler space telescope

Artist's drawing of the Kepler space telescope

With Kepler-11, however, the planets are too small and the star (2,000 light-years away) is too faint for this method to work.

And this is likely to be the case with many of the planets detected by the Kepler mission, the main goal of which is to find small, Earth-size planets in the habitable zones of their stars. See our related story.

A remarkable system

More than 100 transiting planets have been observed by Kepler and other telescopes, but the vast majority of them are Jupiter-like gas giants, and almost all of them are in (so far as is known) single-planet systems.

The Kepler-11 system is remarkable in terms of the number of planets, their small sizes, and their closely packed orbits. Before this, astronomers had determined both size and mass for only three exoplanets smaller than Neptune.

Now, this single planetary system has added five more.

The sixth planet in Kepler-11 is separated enough from the others that the orbital perturbation method can’t be used to determine its mass, Fabrycky said.

As is the case in our Solar System, all of the Kepler-11 planets orbit in more or less the same plane. This finding reinforces the idea that planets form in flattened discs of gas and dust (‘stellar discs’) spinning around a star, and the disc pattern is conserved after the planets have formed, Fabrycky said.

“The ‘coplanar’ orbits in our Solar System inspired this theory in the first place, and now we have another good example. But that and the Sun-like star are the only parts of Kepler-11 that are like the Solar System,” he said.

What are they like?

The densities of the planets provide clues to their compositions. All six planets have densities lower than Earth’s.

“It looks like the inner two could be mostly water, with possibly a thin skin of hydrogen-helium gas on top, like mini-Neptunes,” Fortney said. “The ones farther out have densities less than water, which seems to indicate significant hydrogen-helium atmospheres.”

Diagram comparing the Kepler-11 system to our Solar System

Diagram comparing the Kepler-11 system to our Solar System, showing how Kepler-11's five small, inner planets would fit within the orbit of Mercury in our Solar System.

That’s surprising, because a small, hot planet should have a hard time holding onto a lightweight atmosphere.

These planets are pretty hot because of their close orbits, and the hotter it is the more gravity you need to keep the atmosphere,” Fortney said.

“My students and I are still working on this, but our thoughts are that all these planets probably started with more massive hydrogen-helium atmospheres, and we see the remnants of those atmospheres on the ones farther out,” Fortney added. “The ones closer in have probably lost most of it.”

Comparing Neptunes and Jupiters

One reason a six-planet system is so exciting is that it allows scientists to make these kinds of comparisons among planets within the same system.

“That’s really powerful, because we can work out what’s happened to this system as a whole,” Fortney said. “Comparative planetary science is how we’ve come to understand our Solar System, so this is much better than just finding more solitary hot Jupiters around other stars.”

For example, the presence of small planets with hydrogen-helium atmospheres suggests that this system formed relatively quickly, he said. Studies indicate that stellar discs lose their hydrogen and helium gas within about 5 million years.

“So it tells us how quickly planets can form,” Fortney said.

The inner planets are so close together that it seems unlikely they formed where they are now, he added.

“At least some must have formed farther out and migrated inward. If a planet is embedded in a disc of gas, the drag on it leads to the planet spiralling inward over time. So formation and migration had to happen early on.”

Adapted from information issued by UCSC.

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