Year in Review – Part 2: Kepler makes astonishing discoveries

by Chris Gebhardt

Following on the planetary accomplishments of NASA’s fleet of unmanned solar system explorers in 2011, the U.S. space agency was also extremely busy in the field of extra-solar studies via observations of other solar systems with the impressive Kepler Space Telescope.

Understanding our uniqueness – Kepler sheds light on extra-solar planets:

It was a difficult start to 2011 for the Kepler team. Beginning the year with an anomaly resolution stemming from the spacecraft putting itself into safe mode on December 22, 2010, the Kepler project team successfully returned the spacecraft to normal operations on January 6, 2011 after determining that the condition was caused by an “unexpected noise in the signal from Kepler’s sun sensors that erroneously indicated Kepler might be pointing too close to the sun.”

It wasn’t long after this that the Kepler team announced the confirmation of the first discovery of our rocky planet outside of our own solar system.

The planet, dubbed Kepler-10b, measures 1.4 times the size of Earth and was, at the time, the smallest planet ever discovered outside our own solar system.

While a primary goal of the Kepler mission is to discover rocky planets that lie within the habitable zone of their parent stars, Kepler-10b was quickly dismissed as a habitable candidate because the planet orbits its parent star every 0.84 days, making the planet more than 20 times closer to its star than Mercury is to the sun.

Nevertheless, the discovery of this planet was a proof of concept for the Kepler team, demonstrating that the telescope’s ultraprecise photometer could in fact measure the tiny decrease in a star’s brightness that occurs when a small, Earth-sized planet crosses in front of it.

In a statement following this announcement, NASA stated that “The discovery of Kepler-10b, a bona fide rocky world, is a significant milestone in the search for planets similar to our own.

“Although this planet is not in the habitable zone, the exciting find showcases the kinds of discoveries made possible by the mission and the promise of many more to come.”

But this discovery would prove to be just the first of many for Kepler in 2011.

One month later, scientists announced the discovery of a six-planet system made up of a mix of rocky and gas giant planets orbiting a single, sun-like star.

Located approximately 2,000 light years from Earth, the Kepler-11 star system was the first such extra-solar planetary system discovered to have more than three confirmed planets.

All the planets discovered in the Kepler-11 system are larger than Earth, with the largest ones comparable to the size of Uranus and Neptune.

The system is extremely compact, with the outermost confirmed planet, Kepler-11g, orbiting its star at a distance twice as close as planet Earth orbits the sun.

The discovery of the system by the Kepler telescope was independently confirmed by ground based observatories as well as the Spitzer Space Telescope.

Moreover, the same day of the announcement of the Kepler – 11 system also saw the announcement of 54 new planet candidates where their orbits could lie within the habitable zone of their respective parent stars.

Since the start of the Kepler mission in March 2009, the number of Earth-sized planet candidates has grown from 0 to 68 while the number of planet candidates in the habitable zone of their parent stars has grown from 0 to 54.

The habitable zone, defined as the region in a planetary system where liquid water could exist on a planetary surface, is of particular interest for the Kepler team in the search for habitable planets like Earth.

But Kepler is not just searching for habitable planets outside our solar system. The telescope is also helping scientists learn more about the stars in our galactic neighborhood.

Following two safe mode events in February and March, NASA announced that the Kepler telescope had aided University of Sydney astrophysicists in their study of red giant stars.

The study, which revealed new information on the evolution of red giant stars, was aided by the Kepler telescope through the use of its high precision brightness measurements of the various stars in its field of view.

Specifically, Kepler provided scientists a view of hundreds of red giants at a level of precision and duration that ground-based telescopes are not capable of.

Less than two weeks later, further information was revealed about Kepler’s insight into the study of the internal structure of stars by observing miniscule pulsations in the stars’ brightness.

And still the discoveries kept coming.

By the end of May 2011, Kepler’s team had found an additional planet in the Kepler-10 system. This confirmed planet, with a radius of 2.2 times that of Earth, completes an orbit of its parent star every 45 days – making it an extremely hot world that lies too close to its parent star to be habitable.

But perhaps more excitingly by this point in its existence, the Kepler space telescope had identified more than 1,200 planetary candidates, 408 of them residing in planetary systems with two or more planets.

Furthermore, in August of this year, astronomers announced the discovery of the darkest-known exoplanet.

Described as a Jupiter-sized gas giant known as TrES-2b, the planet was found to reflect less than 1 percent of the starlight reaching the upper layers of its atmosphere.

While the planet was first discovered using the Trans-Atlantic Exoplanet Survey (TrES) method in 2006, new data from the Kepler telescope allowed scientists to determine the reflectivity of the planet. This led to the discovery that TrES-2b lacks reflective clouds due to its high temperature – a direct result of its 3-million-distance orbit of its parent star.

This close proximity to its parent star yields an average temperature of 1,800-degrees F, which is too hot for high-reflectivity clouds, like ammonia clouds, to form.

In place of those ammonia clouds, scientists have determined that the atmosphere of this planet contains light absorbing chemicals; however, none of these light absorbing chemicals can fully explain the extreme low-reflectivity of TrES-2b.

Click here for:
*Kepler Launch Article*
*Kepler 2010 Review Article*

Moreover, the planet is believed to be tidally locked with its parent star, meaning that one side of the planet always faces the star.

Further observations from the Kepler telescope also showed that the planet has changing phases as it orbits its star, causing the total brightness of the star and its planet to vary slightly during observational periods.

Furthermore, direct observations from Kepler of TrES-2b yielded the detection of the smallest-ever change in brightness from an exoplanet at just six parts per million – making Kepler the first telescope to detect such a minute change in brightness of an exoplanet.

Following this dark world discovery, the Kepler team soon announced the discovery of an invisible world orbiting a sun-like star in the Kepler-19 system, located some 650 light years from Earth in the direction of the constellation Lyra.

The discovery of this invisible world was made possible by direct observations of the planet Kepler-19b which, based on its 8.4 million mile distance from its parent star, should complete an orbit every 9 days and 7 hours.

However, +/- 5 minute variations in the orbital times of Kepler-19b led astronomers to the discovery of the invisible world accompanying Kepler-19b.

As related by Kepler researchers, “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 is pulling on Kepler-19b, alternately speeding it up or slowing it down.”

For context within our own solar system, the planet Neptune was similarly discovered when researchers noticed that Uranus orbit didn’t match predictions. It was soon understood and that the perturbations in Uranus’ orbit were being caused by an unseen planet at a greater distance from the sun than Uranus.

Ground based telescopes soon discovered Neptune near its predicted position based on the observed perturbations in Uranus’ orbit.

While nothing aside from the gravitationally-mandated existence of the invisible world, named Kepler-19c, is known, observations of the Kepler-19 system indicate that Kepler-19c’s orbit is tilted relative to Kepler-19b, meaning that the planet does not transit the Kepler-19 star and therefore cannot be directly observed by the Kepler telescope or from ground-based observatories on Earth.

But, like before, the discoveries and confirmations from Kepler just kept coming.

By mid-September, the Kepler team announced the discovery of a planet orbiting a binary star system.

Residing in a star system 200 light years from Earth, the planet, called Kepler-16b, marked the first confirmation of an unambiguous circumbinary planet –  a planet that orbits two stars in the same system.

Demonstrating the diversity of planets within our own galaxy, Kepler-16b is cold and lies outside of its parent stars’ habitable zone.

The planet was discovered using the transit method of detection, viewing the relative dimming and brightening of a star (or stars in this case) as a planet passes between the star and Kepler’s line of sight.

Observations of the stars’ interaction with Kepler-16b confirmed the planet to be roughly the size of Saturn with a rocky and gaseous composition.

The planet was confirmed to orbit the two stars every 229 days, placing it – if it were in our own solar system – in nearly the precise orbit of Venus, which takes 225 days to orbit the sun.

However, because the two stars in the Kepler-16 system are cooler than our sun, Kepler-16b in fact lies well beyond the habitable zone of the Kepler 16 system.

Furthermore, by early October, Kepler had aided in the discovery of an “unusual multi-planet system” in which a super Earth and two Neptune-size planets all orbit in resonance with each other.

The confirmed three-planet system, if superimposed over a map of our own solar system, would lie complete within the orbit of Mercury. But while all these previous discoveries and confirmations were exciting in their own right, nothing could compare to the final three Kepler announcements of 2011.

On December 2, the discovery of a super-Earth was confirmed around one of the brightest stars in Kepler’s field of view.

Dubbed Kepler-21b, the planet is roughly 1.6 times the size of Earth and 10 times Earth’s mass.

It orbits its parent star every 2.8 days at a distance of only 6 million kilometers – ten times closer to its star than Mercury is to the sun.

The surface temperate on Kepler-21b is estimated to be roughly 2,960 degrees F. Thus, the planet does not lie within the habitable zone of its parent star, the habitable zone still only defined as the zone around a star in which liquid water could exits on the surface of a rocky planetary body.

The star itself, HD 179070 is 352 lights years from Earth.

But this still could not compare to the announcement that came on December 5: the first confirmed planet to lie within the habitable zone of its parent star – which is a sun-like star to boot.

The planet, called Kepler 22b, was, as of December 5, the smallest-yet confirmed planet found to orbit completely within the habitable zone of a star similar to the sun – a G-type star.

The planet lies 600 light years away from Earth and orbits its parent star every 290 days.

At approximately 2.4 times the radius of Earth, Kepler-22b has not yet been confirmed as a rocky planet.

As NASA stated, “This is a major milestone on the road to finding Earth’s twin. Kepler’s results continue to demonstrate the importance of NASA’s science missions, which aim to answer some of the biggest questions about our place in the universe.”

At this time, Kepler-22b is the first of 54 habitable zone planet candidates, as reported in February 2011, to be independently confirmed by follow-up observations after its initial discovery.

As of December 5, the number of planet candidates from Kepler totaled 2,326 – up 89 percent from February 2011. Of that number, roughly 207 are Earth-sized planets, 680 are super Earth-sized, 1,181 are Neptune-sized planets, 203 are Jupiter-sized planets, and 55 are larger than Jupiter.

 Moreover, the number of habitable zone planet candidates decrease from 54 in February to 48, representing a shift in the definition and placement of the habitable zone around stars to account for atmospheric heating which subsequently moves the habitable zone further out from a star.

But that was not the end for Kepler in 2011. The final announcement came two weeks ago with the confirmation of actual Earth-sized planets.

Kepler-20e and Kepler-20f, lying in a star system approximately 1,000 light years from Earth, orbit a sun-like star.

Most exciting is the small size of these planets, with Kepler-20e being slightly smaller than Venus at 0.87 times the radius of Earth and Kepler-20f being 1.03 times the radius of Earth.

Together, these are the two smallest exoplanets yet discovered.

Kepler-20e orbits the host star every 6.1 days while Kepler-20f takes 19.6 days to orbit the star. This places the two worlds too close to their parent star to be habitable by current definitions. If superimposed over our own solar system, the entire five planet system would lie completely within the orbit of Mercury.

In fact, the most-distant confirmed planet in the Kepler-20 system only takes 77.6 days to orbit the star, compared with Mercury’s 88 day orbital period around the sun.

Furthermore, the Kepler-20 system is helping expand our understanding of the composition of other solar systems in our galactic neighborhood.

While our solar system is arranged with the smallest planets closest to the sun and the largest planets farther away, the Kepler-20 system is arranged in an alternating pattern of large, small, large, small, large – all and all, an amazing discovery to cap an amazing year for Kepler.

(All images via NASA).

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