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Unseen planet uncovered

Artist's conception of exoplanet Kepler-19c

The "invisible" world Kepler-19c, seen in the foreground of this artist's conception, was discovered solely through its gravitational influence on the companion world Kepler-19b—seen as a dot crossing the star's face. Kepler-19b is slightly more than twice the diameter of Earth, and is probably a "mini-Neptune." Nothing is known about Kepler-19c, other than that it exists.

USUALLY, RUNNING FIVE MINUTES LATE is a bad thing since you might lose your dinner reservation or miss out on tickets to the latest show. But when a planet runs five minutes late, astronomers get excited because it suggests that another world is nearby.

NASA’s Kepler spacecraft has spotted a planet that alternately runs late and early in its orbit because the gravity of a second, “invisible” world is tugging on it. This is the first definite detection of a previously unknown planet using this method. No other technique could have found the unseen companion.

“This invisible planet makes itself known by its influence on the planet we can see,” said astronomer Sarah Ballard of the Harvard-Smithsonian Centre for Astrophysics (CfA). Ballard is lead author on the study, which has been accepted for publication in The Astrophysical Journal.

“It’s like having someone play a prank on you by ringing your doorbell and running away. You know someone was there, even if you don’t see them when you get outside,” she added.

Found in transit

Both the seen and unseen worlds orbit the Sun-like star Kepler-19, which is 650 light-years from Earth.

Kepler locates planets by looking for a star that dims slightly as a planet transits the star—that is, passing across the star’s face from our point of view.

Transits give one crucial piece of information—the planet’s physical size. The greater the dip in light, the larger the planet relative to its star.

However, the planet and star must line up exactlyfor us to see a transit.

Artist's impression of the Kepler spacecraft

Artist's impression of the Kepler spacecraft

In this case, the known planet, called Kepler-19b, transits its star every 9 days and 7 hours. It orbits the star at a distance of 13.5 million kilometres, where it is heated to a temperature of about 480 degrees Celsius.

Kepler-19b has a diameter of 29,000 kilometres, making it slightly more than twice the size of Earth. It may resemble a “mini-Neptune,” however its mass and composition remain unknown.

Given away by gravity

If Kepler-19b were alone, each transit would follow the next like clockwork. Instead, the transits come up to five minutes early or five minutes late.

Such transit timing variations show that another world’s gravity—dubbed Kepler-19c—is pulling on Kepler-19b, alternately speeding it up or slowing it down.

Historically, the planet Neptune was discovered similarly. Astronomers tracking Uranus noticed that its orbit didn’t match predictions. They realised that a more distant planet might be nudging Uranus and calculated the expected location of the unseen world. Telescopes soon spotted Neptune near its predicted position.

Multiple p

“This method holds great promise for finding planets that can’t be found otherwise,” stated Harvard astronomer and co-author David Charbonneau.

Very little known

So far, astronomers don’t know anything about the invisible world Kepler-19c, other than that it exists. It weighs too little to gravitationally tug the star enough for them to measure its mass.

And Kepler hasn’t detected it transiting the star, suggesting that its orbit is tilted relative to Kepler-19b.

“Kepler-19c has multiple personalities consistent with our data. For instance, it could be a rocky planet on a circular 5-day orbit, or a gas-giant planet on an oblong 100-day orbit,” said co-author Daniel Fabrycky of the University of California, Santa Cruz (UCSC).

The Kepler spacecraft will continue to monitor Kepler-19 throughout its mission. Those additional data will help nail down the orbit of Kepler-19c.

Future ground-based instruments will attempt to measure the mass of Kepler-19c. Only then will we have a clue to the nature of this invisible world.

Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics. Graphics by David A. Aguilar (CfA) and NASA.

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How to weigh a star using a moon

Artist’s concept of an exoplanet and its moon transiting a star

Artist’s concept of an exoplanet and its moon transiting (passing in front of) a sun-like star. Such a system could be used to directly weigh the star.

How do astronomers weigh a star that’s trillions of kilometres away and much too big to fit on a bathroom scale? In most cases they can’t, although they can get a best estimate using computer models of stellar formation.

New work by astrophysicist David Kipping says that in special cases, we can weigh a star directly. If the star has a planet, and that planet has a moon, and both of them cross in front of their star, then we can measure their sizes and orbits to learn about the star.

“I often get asked how astronomers weigh stars. We’ve just added a new technique to our toolbox for that purpose,” said Kipping, a pre-doctoral fellow at the Harvard-Smithsonian Centre for Astrophysics.

Astronomers have found more than 90 planets that cross in front of, or transit, their stars. By measuring the amount of starlight that’s blocked, they can calculate how big the planet is relative to the star.

But they can’t know exactly how big the planet is unless they know the actual size of the star. Computer models give a very good estimate but in science, real measurements are best.

Kipping realised that if a transiting planet has a moon big enough for us to detect (by also blocking starlight), then the planet-moon-star system could be measured in a way that lets us calculate exactly how large and massive all three bodies are.

“Basically, we measure the orbits of the planet around the star and the moon around the planet. Then through Kepler’s Laws of Motion, it’s possible to calculate the mass of the star,” explained Kipping.

The process isn’t easy and requires several steps. By measuring how the star’s light dims when planet and moon transit, astronomers learn three key numbers: 1) the orbital periods of the moon and planet, 2) the size of their orbits relative to the star, and 3) the size of planet and moon relative to the star.

Plugging those numbers into Kepler’s Third Law yields the density of the star and planet. Since density is mass divided by volume, the relative densities and relative sizes gives the relative masses. Finally, scientists measure the star’s wobble due to the planet’s gravitational tug, known as the radial velocity. Combining the measured velocity with the relative masses, they can calculate the mass of the star directly.

“If there was no moon, this whole exercise would be impossible,” stated Kipping. “No moon means we can’t work out the density of the planet, so the whole thing grinds to a halt.”

Kipping hasn’t put his method into practice yet, since no star is known to have both a planet and moon that transit. However, NASA’s Kepler spacecraft should discover several such systems.

“When they’re found, we’ll be ready to weigh them,” said Kipping.

Adapted from information issued by CfA / David A. Aguilar.

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More planets found

Artist's impression of two Saturn-like planets orbiting very close to the star Kepler-9b.

Artist's impression of two Saturn-like planets orbiting very close to the star Kepler-9b.

Two planets, similar in size to Saturn, have been spotted by NASA’s Kepler space telescope moving in front of, or transiting, their star.

The transit technique has been used to detect many so-called exoplanets (ones that orbit stars beyond our Solar System), but this is the first confirmed detection via this method of two planets transiting the same star.

The star is known as Kepler-9, and the planets have been dubbed Kepler-9b and Kepler-9c.

Launched in early 2009, Kepler is focusing on over 150,000 candidate stars in the hunt for small, Earth-sized planets. In particular, the aim is to find Earth-sized planets in stars’ habitable zones…orbital slots that are neither too near nor too far from the stars, so that the planets are neither too hot nor too cold.

Kepler can’t see planets—they’re too small and too far away—but its sensitive instruments can see the small dip in starlight as a planet moves in front of its star. From the amount and duration of the dip, the size and orbit of each planet can be derived.

And if there are small deviations in the regularity of the dips, they could indicate the presence of other planets that aren’t on orbits that produce transits.

In June 2010, Kepler scientists published data on 700 potential planets seen in data collected in the first 43 days of the mission. A handful of the stars seemed to have more than one transiting planet.

“Kepler’s high quality data and round-the-clock coverage of transiting objects enable a whole host of unique measurements to be made of the parent stars and their planetary systems,” said Doug Hudgins, the Kepler program scientist at NASA Headquarters in Washington.

Artist's impression of NASA's Kepler space telescope

NASA's Kepler space telescope is dedicated to hunting for planets orbiting distant stars by spotting the dip in starlight as a planet moves in front, or transits, a star.

Scorching orbits

Follow up observations using the giant telescopes of the W.M. Keck Observatory in Hawaii have shown that both planets are slightly less massive than Saturn, and that Kepler-9b is the larger of the two.

The planets orbit very close to their star, which means their years are very short—just 19 Earth days in the case of Kepler-9b, and 38 days for Kepler-9c. Their close proximity to the star means the planets also would be very, very hot.

Seven months of observations have revealed slight variations in the timing of the two planet’s transits, which is exactly what would be expected as each planet gravitationally tugs on the other.

Their observations of the Kepler-9 system also have given the scientists tantalising hints that there could be another, much smaller planet in orbit around the star. The data suggest it could be about 1.5 times the size of Earth and orbiting scorchingly close to the star, taking just 1.6 Earth days to complete one orbit. This would make it a “hot Earth”.

The scientists still have work to do to confirm the presence of the third planet, though, as what can sometimes seem to be a transiting planet can turn out to be an unrelated phenomenon.

Story by Jonathan Nally, editor Images courtesy NASA.

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