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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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Planets galore!

Artist's impression of an exoplanet system

Artist's impression of an exoplanet system. NASA's Kepler space telescope has now found 1,235 planet candidates within a small region of our Milky Way galaxy.

NASA’S KEPLER SPACE TELESCOPE has discovered its first Earth-size planet candidates and its first candidates in the habitable zone, a region where liquid water could exist on a planet’s surface.

Five of the potential planets are nearly Earth-size and orbit in the habitable zone of smaller, cooler stars than our Sun.

Candidates require follow-up observations to verify they are actual planets.

Kepler also found six confirmed planets orbiting a Sun-like star, Kepler-11. This is the largest group of transiting planets orbiting a single star yet discovered outside our Solar System. See our related story.

“In one generation we have gone from extraterrestrial planets being a mainstay of science fiction, to the present, where Kepler has helped turn science fiction into today’s reality,” said NASA Administrator Charles Bolden.

The discoveries are part of several hundred new planet candidates identified in new Kepler mission science data, released on Tuesday, February 1.

Diagram explaining a Kepler transit observation

Kepler detects the dip in starlight as a planet passes in front of a star, called a 'transit'.

A multitude of planets

Kepler looks for signs of planets by measuring tiny decreases in the brightness of stars caused by planets crossing in front of them. This is known as a transit.

The planets are too far away and too small for images of them to be made, but Kepler’s transit method can detect their presence.

The findings increase the number of planet candidates identified by Kepler to-date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter.

Of the 54 new planet candidates found in the habitable zone, five are nearly Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size—up to twice the size of Earth—to larger than Jupiter.

The findings are based on the results of observations conducted May 12 to September 17, 2009, of more than 156,000 stars in Kepler’s field of view, which covers approximately 1/400 of the sky.

“The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting Sun-like stars in our galaxy,” said William Borucki of NASA’s Ames Research Centre, the mission’s science principal investigator.

“We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone, some of which could have moons with liquid water.”

Star with a large family

Among the stars with planetary candidates, 170 show evidence of multiple planetary candidates.

Kepler-11, located approximately 2,000 light years from Earth, is the most tightly packed planetary system yet discovered. All six of its confirmed planets have orbits smaller than Venus, and five of the six have orbits smaller than Mercury’s.

The only other star with more than one confirmed transiting planet is Kepler-9, which has three.

Diagram comparing Kepler-11 system with the Solar System

A comparison of the orbits of the six planets in the Kepler-11 system with the orbits of Mercury and Venus in our Solar System.

“Kepler-11 is a remarkable system whose architecture and dynamics provide clues about its formation,” said Jack Lissauer, a planetary scientist and Kepler science team member at Ames.

“These six planets are mixtures of rock and gases, possibly including water. The rocky material accounts for most of the planets’ mass, while the gas takes up most of their volume,” he added.

“By measuring the sizes and masses of the five inner planets, we determined they are among the lowest-mass confirmed planets beyond our Solar System.”

All of the planets orbiting Kepler-11 are larger than Earth, with the largest ones being comparable in size to Uranus and Neptune. The innermost planet, Kepler-11b, is ten times closer to its star than Earth is to the Sun.

Moving outward, the other planets are Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11f, and the outermost planet, Kepler-11g, which is half as far from its star as Earth is from the Sun.

The planets Kepler-11d, Kepler-11e and Kepler-11f have a significant amount of light gas, which indicates that they formed within a few million years of the system’s formation.

Kepler field-of-view star chart

This star chart illustrates the large patch of sky that NASA's Kepler mission is staring at during its three-and-a-half-year mission. The planet hunter's full field of view covers 100 square degrees.

Follow up needed

Since transits of planets in the habitable zone of Sun-like stars occur about once a year and require three transits for verification, it is expected to take three years to locate and verify Earth-size planets orbiting Sun-like stars.

The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planetary candidates and other objects of interest the spacecraft finds.

The star field that Kepler observes in the constellations Cygnus and Lyra can only be seen from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.

“The historic milestones Kepler makes with each new discovery will determine the course of every exoplanet mission to follow,” said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington.

Adapted from information issued by NASA. Image credits NASA, Tim Pyle, Software Bisque, G. Bacon/STScI.

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