This is really nifty: a visualization of the 1,235 exoplanet candidates observed by Kepler in the recently released data, created by Jer Thorp. In the video, all the candidates are shown as if orbiting a single star – just for the purposes of comparisons. The size of the colored dot is proportional to the size of the planet, and two of the most promising candidates for habitability are highlighted (KOI 326.01 and KOI 314.02).
With the startling new finding of dozens of Earth-sized extrasolar planets, NASA’s Kepler planet hunting space telescope has just revolutionized our understanding of Earths place in the Universe and the search for Extraterrestrial Life. And the historic science discovery is based on data collected in just the first few months of operation of the powerful telescope as it scans only a tiny portion of the sky.
The discovery of 1235 new extrasolar planet candidates was announced today (Feb.2) by NASA and Kepler scientists at a media briefing. 68 of these planet candidates are Earth-sized. Another 288 are Super-Earth-size, 662 are Neptune-size and 165 are Jupiter-size. Most of these candidates orbit stars like our sun.
Even more significant is that 54 of the planet candidates are located within the ‘habitable zone’ of their host stars and 5 of those are Earth-sized. Before today we knew of exactly ZERO Earth-sized planets within the habitable zone. Now there are 5.
Are We Alone ?
“We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone – a region where liquid water could exist on a planet’s surface. Some candidates could even have moons with liquid water,” said William Borucki of NASA’s Ames Research Center, Moffett Field, Calif.. Borucki is the science principal investigator for NASA’s Kepler mission.
“Five of the planetary candidates are both near Earth-size and orbit in the habitable zone of their parent stars.”
Earth-sized water worlds are the most conducive to the formation and evolution of alien life forms. Water is an essential prerequisite for life as we know it.
“Kepler’s blown the lid off everything we know about extrasolar planets,” said Debra Fischer, professor of Astronomy at Yale University, New Haven, Conn
Kepler is the first NASA mission capable of finding Earth-size planets in or near the habitable zones around their parent stars. The mission uses the transit method to detect the tell tale signatures of planets. The goal is to determine how common are planets the size of Earth orbiting inside the habitable zone of stars like our sun.
Kepler measures the miniscule decreases in the brightness of stars caused by planets crossing in front of them and blocking the starlight. Imagine calculating the difference in light transmission caused by a flea sitting on a cars headlight.
Follow up observations over a period of several years will be required to confirm these results, the scientists explained. Astronomers expect that over 80% of the candidate planets will be positively confirmed as real planets by utilizing ground based observatories and the Spitzer Space Telescope.
For an Earth-sized planet orbiting a sun-like star inside the habitable zone, transits occur about once per year. Since three transits are required to verify a planets status, it will therefore take about three years to reach a definitive conclusion.
These remarkable new planet discoveries are based on observations from only the first four months of Kepler’s telescopic operations – May 12, 2009 to Sept. 17, 2009. The space based observatory continuously monitors more than 156,000 stars using 42 CCD detectors with a field of view that covers only 1/400 of the sky.
“Kepler is making good progress towards its goals,” said Borucki
“We have found over twelve hundred candidate planets – that’s more than all the people have found so far in history.”
“Imagine if we could look wider. Kepler looks at one 400th of the sky. If we had 400 of these fields of view, we’d see 400 times that number of candidates. We would see 400,000 candidate planets.”
“The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting stars like our sun in our galaxy,” Borucki amplified. “Our results indicate there must be millions of planets orbiting the stars that surround our sun.”
“If we find that Earth’s are common in the habitable zones of stars, very likely that means life is common around these stars.”
“Kepler has shown that planetary systems like our own are common,” said Debra Fischer.
“The search for planets is motivated by the search for life,” Fischer added.
“We have allowed the public to participate though the website Planethunters.org,” she added. “And now we have over 16,000 dedicated users. The public is excited to be a part of research and history.”
“Thanks to Kepler for this treasure chest of data!” Fisher concluded.
Kepler is just the first step in finding Earth sized and Earth like planets. “We are building the foundation for future generations of explorers,” said Borucki.
“Future missions will be developed to study the composition of planetary atmospheres to determine if they are compatible with the presence of life. The design for these missions depends on Kepler finding whether Earth-size planets in the habitable zone are common or rare.”
The first planets beyond our solar system were discovered in 1995. Up to today there were just over 500 known extrasolar planets.
Kepler now has 15 confirmed extrasolar planet discoveries and over 1200 possible candidates.
In January 2011, Kepler confirmed the discovery of its first rocky planet, named Kepler-10b. The molten world measures just 1.4 times the size of Earth and is the smallest planet ever discovered outside our solar system.
NASA’s Kepler spacecraft was launched on March 6, 2009 from Launch Complex 17-B atop a Delta II rocket at Cape Canaveral Air Force Station in Florida. See spacecraft and launch photos below
Kepler’s science operations are currently funded for three and one half years of operations until November 2012. The mission’s lifetime – and its goal of discovering multitudes of new planets as small as Earth – can be extended if NASA funding is approved by Congress and the President.
William Borucki – Explains Keplers Discovery of Earth Sized Planets
Science principal investigator for NASA’s Kepler mission, NASA’s Ames Research Center
Video Caption: NASA’s Kepler mission 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 near Earth-size and orbit in the habitable zone of smaller, cooler stars than our sun.
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. Located approximately 2,000 light years from Earth, Kepler-11 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.
What is an Earth like planet ? Explantion here
David Charbonneau, an exoplanet researcher at Harvard University, explains what scientists mean when they say “earthlike planet” and “super Earth.” This interview was recorded at NASA’s Goddard Space Flight Center on December 10, 2010, by NASA science writer Daniel Pendick.
More Kepler photos courtesy of nasatech.net here
View of Launch Complex 17 B and cryogenic storage tanks by Ken Kremer
NASA’s Kepler Media Briefing on Feb. 2, 2011
Using data from the Kepler space telescope, scientists have discovered a horde of six planets orbiting a sun-like star, approximately 2,000 light years from Earth. This is the largest group of planets detected so far around another star. The planets in this newly found solar system are relatively small – they range from 2.3 to 13.5 times the mass of the Earth – and are amazing mix of rock and gases. All six planets are crowded within an orbit the size of Venus’ orbit around our Sun; however, the inner five are closer to their star than any planet in our solar system.
“This is a surprisingly flat and compact system of six transiting planets,” said Jack Lissauer, co-investigator on the Kepler mission, speaking at a press conference on February 2, 2011. “The five inner planets are especially close together, something we didn’t think would happen for worlds of this size. This discovery forces us to go back and look at formation models of planets.”
Lissauer added that the close proximity of the six worlds around the star — now called Kepler 11 — also means that the planets are perturbing each others’ orbits. While having a multi-planet system makes it difficult to untangle the signals from each planet, it has the added benefit of providing more information about each of the worlds.
“In a system where the planets are tugging on one another, that means we can weigh the planets,” Lissauer said. “We have found they are low density planets; some are fluffy, sort of like marshmallows. But they are not all gas, so maybe like a marshmallow with a little hard candy at the core.”
Lissauer was incredibly enthusiastic about the discovery.
“We really were just amazed at his gift that nature has given us,” he said. “With six transiting planets, and five so close and getting the sizes and masses of five of these worlds, there is only one word that adequately describes the new finding: Supercalifragilisticexpialidocious.”
Kepler finds planets by using the transit method. The planets’ orbits are edge-on as seen from Earth, so when they pass in front of their star they block a small portion of its light. That dip in brightness is what Kepler detects.
Lissauer explained the animation (seen at the top of this article): “This is the view of Kepler, and it looks like a very special clock, one with six hands moving at six different rates, and we interpret this as six planets orbiting near the same plane. Then, you can see how it might look face on. This is the most compact system of planets every discovered by any technique anywhere.”
The time between transits provides the orbital period. To determine the planets’ masses, the scinetists analyzed slight variations in the orbital periods caused by gravitational interactions among the planets.
Lissauer said the five close inner bodies tug on one another’s orbit, and sometimes the pull can retard the transit time by 10-20 minutes.
“The timing of the transits is not perfectly periodic, and that is the signature of the planets gravitationally interacting,” said Daniel Fabrycky, a Hubble postdoctoral fellow at UC Santa Cruz, who led the orbital dynamics analysis. “By developing a model of the orbital dynamics, we worked out the masses of the planets and verified that the system can be stable on long time scales of millions of years.”
Five of the planets’ orbital periods are all less than 50 days, and the sixth planet is larger and farther out, with an orbital period of 118 days and an undetermined mass.
Finding a large multiplanet system has many people wondering when Kepler will discover an Earth-like world. The scientists on the panel today estimated it will take three years of Kepler data to find another Earth.
“No one is more eager to get to the point of an Earth-like planet than the Kepler team,” said Douglas Hudgins, Kepler program scientist. That will require at least 3 years of Kepler data and painstaking follow-up observations from ground-based before those types of discoveries will emerge from the data.”
Hudgins reminded everyone that the first 15 years of exoplanet searches from ground-based observing produced about 500 planets, and that last year the Kepler team announced 750 exoplanet candidates from just the first three months of Kepler observations. With the release of more Kepler data today, there are now more than 1,200 planet candidates.
“The key thing to remember about every planet candidate,”Hudgins said, “ is that every time we see in data evidence of a signal, there is required analysis and follow-up data and observations to determine it is actually planet and not something masquerading as a planet.”
Translation: this takes time and won’t happen overnight.
But with the release of more data, the Kepler team said they wants to harness the horsepower of the whole planetary community, as well as citizen scientists to scour through the data. The Planet Hunters program from Galaxy Zoo has been a successful project that allows anyone to contribute the science of finding extrasolar planets.
The public has made over 1.3 million classification using just the first 30 days of publicly released Kepler data,” said Debra Fischer, professor of Astronomy at Yale University who heads up the Planet Hunters project. “We are really excited and appreciative that NASA and the Kepler mission has essentially quadrupled the amount of public data with the early release of their latest data.”
Not only is the Kepler spacecraft hunting down extrasolar planets, but it also provides the ability to study stars in unprecedented detail. “We knew that if Kepler had the sensitivity of detecting Earth-size planets, that it would have capability to transform our knowledge of stars themselves,” said Natalie Batalha of San Jose State University in California, a co-investigator on the Kepler Astroseismic Science Consortium. This international partnership of over 400 astronomers uses the Kepler spacecraft to “listen” to tiny oscillations, or “star quakes,” in red giant stars, allowing scientists to do groundbreaking work in deducing the fundamental properties of stars.
In just the first year of Kepler’s operation, the team has been able to study thousands of stars using astroseismology, while previously only a few dozen of stars had been “listened to” using this technique.
“We can say Kepler is listening to thousands of musicians in the sky,” said Daniel Huber, a graduate student at the University of Sydney, during a webcast of a press conference about the new findings.
“From first year of the Kepler mission we moved from having a couple of dozen of stars with a couple of weeks of data,” said Travis Metcalfe, scientist at The National Center for Atmospheric Research, responding to a question posed by Universe today “to having one month to study each of a several thousands of stars. This is an enormous expansion of our capability to study this type of star and what the oscillations tells us.”
Similar to how seismologists study earthquakes to probe the Earth’s interior, astroseismology measures the natural pulse of light waves from stars to gain new insights into stellar structure and evolution.
“Kepler allows us to study the periods of stellar oscillations, and we use them to study the cores of stars — in a way to touch the stars — and get the most accurate measurements of stars we have ever made,” said Hans Kjeldsen, associate professor, KASC, Aarhus University in Denmark.
They can measure size and age with extreme precision and they have now characterized the structure and life cycle of over 1,000 red giants. What they have found so far confirms the current principals of stellar evolution and allows for better predictions of what might happen to our Sun in several billion years.
Kjeldsen said they are getting data of amazing quality. “We can now actually study stars of all phases and evolutionary stages, of different mass, and all different types. This is the amazing thing for me. Instead of looking one star for awhile and then moving on to next star, we now have access to thousands of stars at once. And having said that, there are still thousands and thousands of stars we still need to study.”
Metcalfe said astroseismology listens to the oscillations of the star, and can hear a tone so low that even a whale would have a hard time hearing it. Kepler can see even tiny oscillations as a flickering in the star.
“Sound waves travel into the star and bring information up to the surface, which Kepler can see as a tiny flickering in brightness of the star,” said astronomer Travis Metcalfe of The National Center for Atmospheric Research.
That flickering has a tone like the notes of a musical instrument. “We essentially measure the tone of these musical notes from the star,” he said. “Larger stars flicker in lower tones while smaller stars in higher tones.”
One star that Metcalfe has been focusing on is a red giant, that measured twice the size of the Sun. KIC 11026764 now has the most accurately known properties of any star in the Kepler field. In fact, few stars in the universe are known to similar accuracy, the team said. The oscillations reveal that this star is 5.94 billion years old and is powered by hydrogen fusion in a thin shell around a helium-rich core.
In this consortium, no dollars are actually exchanged between nations. The US provides the Kepler instrument and software pipeline, while the international partners are supplying human resources of ingenuity and scientific expertise.
“We’re not just getting a great legacy of scientific results, but also a valuable symbiosis and partnership here,” said Batalha.
You can watch the press conference on Universe Today at this link.
Get the news of the latest findings regarding stars and their structures during a press conference that will be streamed live from Aarhus University in Denmark today at 11 am EDT (1500 GMT). Using data from NASA’s Kepler spacecraft, an international research team has examined and characterized thousands of stars by using the natural pulse of stellar light waves, thereby gaining new insights into stellar structure and
Two astronomers have written a paper and say that the first Earth-like, habitable exoplanet will be announced in May of 2011. Do they have inside information, a crystal ball, or amazing powers of prediction? No, they base their projection on math and trends from the past 15 years of exoplanet discoveries. And if the discoveries continue at their present rate, the researchers say next year is the year of the long awaited holy grail of finding another Earth-like planet out in the cosmos.
Samuel Arbesman from Harvard Medical School in Boston and Gregory Laughlin at the University of California, Santa Cruz take a scientometric approach to their prediction. Scientometrics is the science of measuring and analyzing science, and is often done using bibliometrics which is a measurement of the impact of scientific publications. Arbesman and Laughlin said this type of work highlights the usefulness of predictive scientometric techniques to understand the pace of scientific discovery in many fields.
They use the properties of previously discovered exoplanets along with external estimates for the discovery of the first potentially habitable extrasolar planet.
In their paper they indicate that since astronomers have been discovering extrasolar planets at an increasing rate since 1995 and the discoveries follow a well understood pattern, it should be easy to predict when planet searchers will hit the jackpot.
The first exoplanets found were the massive Jupiter or larger-sized planets which were the easiest to find, and then as techniques improved over the past 15 years, astronomers have found smaller planets, some just a few times more massive than Earth.
Arbesman and Laughlin took that rate of discovery, and they also needed to factor in all the variables for what we think will make a planet habitable: the surface temperature must allow liquid water to exist, so that life as we know it can appear, and that depends on the size of the star, how far the planet orbits from its star, and what type of surface the exoplanet has.
They conclude there is a 66 per cent probability of finding another Earth by 2013, a 75 per cent probability by 2020, and a 95 per cent probability by 2264, but the median date of discovery is in May 2011. And not just sometime in May, but “early May.”
In June 2010, the Kepler Telescope team revealed they had found 750 exoplanet candidates, and a fair number of those confirmed might be Earth-sized. They expect they can confirm and announce some of these candidates in February 2011. But Arbesman and Laughlin predict it might take longer. “Because of the limited time base line of the mission to date, the Kepler planet candidates to published in February 2011 may be too hot to support significant values for H (which is their habitability metric),” they wrote in their paper.
So, if their prediction comes true, that might mean another team, such as the HARPS, or Keck, or CoRoT, or other exoplanet-finding wizards might make the discovery.
“It must be noted that by publicizing our prediction, there is a concern that it will become accurate,” Arbesman and Laughlin write in their paper, “simply due to the well-studied Hawthorne Effect. However, due to the large number of observations and long periods of time required to confirm an extrasolar planet discovery, it is unlikely that our prediction at this time will appreciably affect the announcement of the discovery of an Earth-like planet. Therefore, it is reasonable to use the habitability metric curve as a rough prediction for when the first potentially habitable planet will be discovered, in this case, as early as May 2011, and likely by the end of 2013.”
It will be interesting to see how accurate their prediction turns out to be!
Additional Source: Technology Review Blog
Mainstream media (MSM) is funny. Well, maybe funny isn’t the right word, especially when they hose things up and create a story when there really isn’t one. Or when they miss the real story. MSM recently succeeded in spades on both accounts in regards to the Kepler mission. Just last month, the Kepler team announced they had found over 750 candidates for extrasolar planets, and 706 of these candidates potentially are planets from as small as Earth to around the size of Jupiter, with the majority having radii less than half that of Jupiter. This is such incredible news, especially when you factor in that the data was from just 43 days of observations! But MSM seemed to miss all this and instead focused on the fact that the Kepler team got approval from NASA to keep over half of their data for an additional six months to verify and confirm their findings, rather than releasing all of it, as per NASA’s standard policy which requires astronomers to release their data from publicly funded instruments in one year. Then over this past weekend, from a TED talk by Kepler co-investigator Dimitar Sasselov, MSM finally realized that Kepler has found a boat-load of potential Earth-sized exoplanets. Well, yes. That’s what they said in June.
But then MSM took things out of context and exaggerated just a tad.
Even though in his talk, Sasselov used the words “potential” and “candidates” and said the planets are “like Earth, that is, having a radius smaller than twice Earth’s radius,” MSM reported news that NASA has found rocky planets with land and water.
And now some people are saying that Sasselov “leaked” the proprietary Kepler data, and some say he is in trouble for doing so. Today, the Kepler team said via Twitter that they are “working hard to thoughtfully respond to the media flurry surrounding the TEDGlobal talk.”
Let me use one of my mother’s favorite admonitions: For Pete’s sakes!
Watch the TED talk. In my opinion, Sasselov does a good job of getting people excited about exoplanets and he doesn’t say we have actually found another Earth. He also does a good job of presenting what the Kepler team has found without revealing any really huge proprietary data, even though he used this graph:
But really, this is pretty much what the Kepler team said in June, that they expected half of the 750 planet candidates would turn out not to be planets, and a fair number of those might be Earth-sized. The graph takes into account the amount of potential planets that Kepler found, plus the planets found previously by other telescopes and missions.
While it is exciting to think about the potential of finding Earth-sized and maybe even Earth-like planets, we’re likely a long way off from actually finding and then actually confirming another Earth. Additionally, right now, we’re only capable of finding planets that orbit relatively close to their parent star, which most likely wouldn’t put them in the “Goldilocks Zone” of being habitable.
You can read our original article from June here, where the Kepler team announced their findings. There’s also an explanation there of why the team requested to keep part of their data for an extra six months.
UPDATE: 10 pm Tuesday: Sasselov has written an blog post at the Kepler website, bascially saying that there is a big difference between Earth-sized and Earth-like. You can read it here.
Possible habitable zones around stars. Credit: Kepler mission
The Kepler mission announced the discovery of 5 new extrasolar planets today at the American Astronomical Society meeting in Washington, DC, each with some very unusual properties. But additionally, the space telescope has spotted some Jupiter-sized objects orbiting stars, and these objects are hotter than the host star. The science team has no idea what these objects could be, but they are part of 100 planetary candidates Kepler has observed that are still being analyzed.
The Kepler mission’s objective is to search for Earth-size planets in the habitable zones of other stars, and the planets announced today are comparable in size to Neptune, Jupiter and the other gas giants of our solar system but are substantially less dense. This first set of five new planets discovered by the Kepler mission was discovered in the first six weeks of the telescope’s operation. “The quick discovery indicates that Kepler is performing well,” said William Borucki, from NASA’s Ames Research Center.
One of these new planets is similar in many ways to Neptune, although its irradiation level is much higher. A second planet is one of the least dense planets ever discovered, and along with the other three, confirms the existence of planets with densities substantially lower than those predicted for gas giant planets. Borucki said Kepler 7b has the density of styrofoam, at .17 grams per cubic centimeter, basically a density of zero.
The smallest planet, Kepler 4b, is 4.31 earth radii, or about Neptune-sized. The other four about the size of Jupiter. All five planets have short orbital periods, and follow-up observations will be made with ground-based telescopes.
Since these planets are close to their host stars, they are very hot, hotter than about 1500 K. 1300 K is the temperature where molten lava flows.
Kepler launched in March 2009 and the mission is expected to last 3½ years. The team now has an additional 8 months of data are now available to analyze. Borucki said in 2010 Kepler will focus on the discovery of smaller planets, with an Earth-sized planet being the “holy grail” of exoplanet discoveries.
Other objects detected by Kepler include unusual variable stars, including binaries, oscillating stars, pulsating variables, and more, including other extrasolar planets, but declined to divulge more, saying his team has to be patient and do the confirmations on all the objects before.
Borucki also said data from Kepler will be released to the public on a regular basis starting in June 2010.
Source: AAS Press conference
A glitch in the Kepler spacecraft’s electronics means the space telescope will not have the ability to spot an Earth-sized planet until 2011, according to principal investigator William Borucki. Noisy amplifiers are creating noise that compromises Kepler’s view, and the team will have to generate and upload a software fix for the spacecraft. “We’re not going to be able to find Earth-size planets in the habitable zone — or it’s going to be very difficult — until that work gets done,” said Borucki, who revealed the problem last week to the NASA Advisory Council.
The team knew about the problem before launch, as the noisy amplifiers were noticed during ground testing before the device was launched. “Everybody knew and worried about this,” says instrument scientist Doug Caldwell. But he said the team thought it was riskier to pry apart the telescope’s electronic guts than to deal with the problem after launch.
Kepler launched on March 6, 2009 and is designed to look for the slight dimming of light that occurs when a planet transits, or crosses in front of a star.
The problem was is caused by amplifiers that boost the signals from the charge-coupled devices that form the heart of the 0.95-metre telescope’s 95-million-pixel photometer, which detects the light emitted from the distant stars. Three of the amplifiers are creating noise, and even though the noise affects only a small portion of the data, Borucki says, but the team has to fix the software — it would be “too cumbersome” to remove the bad data manually — so that it accounts for the noise automatically.
The team is hoping to fix the issue by changing the way data from the telescope is processed, and looks to have everything in place by 2011.
Borucki pointed out that the team was probably going to have to wait at least three years to find an extrasolar Earth orbiting in the habitable zone anyway. Astronomers typically wait for at least three transits before they confirm a planet’s existence; for an Earth-sized planet orbiting at a distance similar to that between the Earth and the Sun, three transits would take three years. But Borucki said that the noise will hinder searches for a rarer scenario: Earth-sized planets that orbit more quickly around dimmer, cooler stars — where the habitable zone is closer in. These planets could transit every few months.
The delay for Kepler could mean ground-based observers could now have the upper hand in the race for the holy grail of planet hunting: finding an Earth-like planet.
Kepler and CoRoT (Convection, Rotation and Planetary Transits) both look for transiting planets while the ground-based telescopes use radial velocity, looking for tiny wobbles in the motion of the parent stars caused by the planets’ gravity. The journal Nature quoted astronomer Greg Laughlin from the University of California at Santa Cruz, saying that the delay for Kepler makes it “more likely that the first Earth-mass planet is going to go to the radial-velocity observers”.
“The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit” That’s Kepler’s third law. In other words, if you square the ‘year’ of each planet, and divide it by the cube of its distance to the Sun, you get the same number, for all planets.
(The other two are “the orbit of each planet is an ellipse with the Sun at a focus”, and “a line between a planet and the Sun sweeps out equal areas in equal times”.)
Copernicus, Kepler, and Newton dealt a one-two-three knockout blow to the idea – thousands of years old – that the Sun (and planets) moved around the Earth. Copernicus put the Sun at the center, Kepler modified Copernicus’ circular motions (and provided a simple, quantitative description of the actual motion), and Newton explained how it all worked (gravity).
Kepler worked out his three laws from detailed records of observations of the positions of the planets (known at the time, Mercury, Venus, Mars, Jupiter, and Saturn) – especially Mars – painstakingly compiled by Tycho Brahe.
Kepler’s third law (in fact, all three) works not only for the planets in our solar system, but also for the moons of all planets, dwarf planets and asteroids, satellites going round the Earth, etc. Well, not quite; if the secondary body – a planet, say – has a mass that’s a significant fraction of the primary one (the Sun, say), then the law needs a small tweak.
By showing how Kepler’s laws could be derived from his universal law of gravitation, Newton united heaven and earth, perhaps the greatest revolution in science (OK, Darwin’s revolution may be greater). Before Newton, the heavens were thought to work according to rules quite different from the ones which governed things on Earth.
NASA’s Imagine the Universe! has a neat demonstration of Kepler’s laws, and this PDF file (from the University of Tennessee Knoxville’s Maths Department) gives a simple derivation of Kepler’s laws, from Newton’s universal law of gravitation.