# How can we tell how many exoplanets a star has?

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Part of this answer (discussing the moving center of mass of our solar system) explains that this movement is one way we know a star has planets:

Bonus: We use this phenomenon to find planets outside the Solar System! If a distant star is observed to 'wobble' or oscillate about it's mean position, we can use that data to infer the presence of one or more exoplanets, and calculate their mass.

It makes sense to me that we could tell that a start has some planet(s) in orbit around it, based on how they affect the star's position. But is this movement really precise and predictable enough to pin down how many exoplanets, and how large they are? Can we produce a "map" of a distant solar system based on this movement?

If needed, we can even restrict the discussion to planets that are "big enough" to detect. For example, when performing this exercise on our Sun, would we be able to accurately predict at least the four gas giants, even if we couldn't get Mercury?

What you are describing is a basic signal processing problem. The doppler shift that one observes is due to the motion of the star in the system around the system's centre of mass. The star will be influenced by the gravitational pull of each of the planets in that system, each of which exerts a gravitational pull that increases with the mass of the planet and decreases with the orbital radius of the planet.

The overall motion of the star will be the sum of the effects of all the planets. Importantly, the effect of each planet will have its own amplitude and will be periodic with a period equal to the orbital period of that planet.

Let's imagine that each planet is in a circular orbit (elliptical orbits are more complicated, but the principle is the same). Each planet would cause a circular motion in the star about the centre of mass of the planet-star pair, leading to an observable doppler signal which has the form of a sine wave with a period equal to the orbital period of the planet. The amplitude of that signal will increase with the mass of the planet and increases with decreasing orbital radius.

The overall signal is the sum over all the planets in the system. Fortunately the decomposition of this signal back into its individual components is a well-trodden problem in physics, electronics and many other fields and is known as Fourier Analysis. Whether you can successfully recover the original signals from each of the planets depends on how long you observed the system (ideally you want to observe for longer than the longest orbital period) and the amplitude of the signals compared with the noise in your observations.

In general it is easier to recover high-mass planets with short orbital periods and more difficult to recover low-mass planets with long orbital periods.

The image below might be helpful. It shows the track of the solar system centre of mass compared with centre of the Sun over a period of several decades. Notice how the Sun executes a complex trajectory (with respect to the solar system centre of mass) that is mainly caused by the orbit of Jupiter, but then there are smaller, superimposed, signals caused by the smaller planets. In principle, if you observed for longer than the period of Neptune and had a detector which gave perfect measurements, you could reconstruct how many planets there were in the solar system, what their orbital periods were (and then from Kepler's 3rd law, what the planet-star sepration was) and what their masses were (multiplied by the inclination of their orbits with respect to the line of sight of observation, which is generally an unknown in doppler measurements).

In terms of what we could currently see if we observed the Sun as a star: basically we would (assuming we observed for 20 years) detect Jupiter quite easily with a doppler amplitude of about 13 m/s. We would also see that there was a drift in Jupiter's signal due to the influence of Saturn, but we would have to observe for >Saturn's orbital period in order to confirm the presence of Saturn, its orbital period and mass. The inner planets produce an amplitude that is too small to be visible using the technology currently available. e.g. The Earth would produce a doppler wobble of amplitude $<8$ cm/s, but the current precision of doppler measurements is limited to about 50 cm/s.

The doppler amplitudes in m/s due to each of the planets (assuming we view them edge on) are:

Mercury <0.01 Venus 0.08 Earth 0.08 Mars <0.01 Jupiter 12.5 Saturn 2.6 Uranus 0.28 Neptune 0.26

Thus with current technology, only Jupiter and Saturn are detectable.

Below I simulate what the doppler signal due to these two planets would be. I hope you can see that the overall signal consists of the superposition of two sinusoidal signals with different periods and amplitudes. Myriad computational tools are available to do the fourier decomposition to establish these.

## Planet Classification: How to Group Exoplanets

With thousands of exoplanet candidates discovered, astronomers are starting to figure out how to group them in order to describe them and understand them better. Many planet classification schemes have been proposed over the years, ranging from science fiction to more scientific ones. But we still know little about exoplanets, and some scientists still debate what the definition of a planet should be.

## How do astronomers detect that a star has a planet orbiting it?

It is amazing what human ingenuity can come up with! In this case, you have thousands of astronomers peering at the heavens every night using a relatively small set of tools (telescopes gathering light or radio waves), so they spend a great deal of time thinking of different ways to use these tools. To be able to sense objects as small as a planet at a distance of trillions of miles using these tools is a major accomplishment.

You can understand what astronomers are doing if you think about our own sun and the planets that orbit it. The largest of these planets is Jupiter. Jupiter weighs about one-thousandth of what the sun weighs, and it orbits the sun every 11.8 years or so at a distance of 5 astronomical units (AU, the average distance between the Earth and the sun, which is 92,955,800 miles or 149,597,870 kilometers).

The sun is not anchored in space with Jupiter orbiting around it. As Jupiter moves around the sun, Jupiter pulls on the sun and moves it. The distance it pulls the sun is proportional to the weight of the two bodies, so the sun moves one-thousandth of the distance that Jupiter does. As Jupiter moves through its orbit, the sun moves through a circle 1,000 times smaller. In other words, the sun moves through a circle about 1,000,000 miles (1.6 million kilometers) in diameter. (The other planets that orbit the sun all have effects on the sun's motion as well -- Jupiter just happens to have the biggest effect, so it is most noticeable.)

In astronomical terms, the 1,000,000 miles that the sun moves is tiny. It also takes a long time to move that distance (11.8 years). However, motions like these are still detectable. There are two ways by which someone could detect them:

• Side-to-side motion - You can detect side-to-side motion by simply looking at the star, plotting the course that you think it should follow and then looking for variations in the path. If the star seems to move side-to-side periodically, then it has a "wobble" that an orbiting planet might produce.
• Front-to-back motion - You can detect front-to-back motion by detecting the Doppler shift in light that the star produces. When anything moves toward you or away from you, the color of its light changes (see How Radar Works for details). By measuring the change in color of a star and looking for a pattern, you can detect front-to-back motion.

See the links below for lots more detail.

One star that has a very noticeable wobble is 51 Pegasi. It wobbles every 4.2 days, implying a planet that orbits it very quickly. That's hard to imagine, but it made detection very easy (compared to 11.8 years. ). Two of the links below go into lots of detail about 51 Pegasi , including a table of its motion data, some nice diagrams and the formulas astronomers use to calculate planet mass and distance.

## How astronomers detected water on a potentially habitable exoplanet for the first time

Artist’s impression of planet K2-18 b, its host star and an accompanying planet in this system. Credit: ESA/Hubble, M. Kornmesser, Author provided

With more than 4,000 exoplanets—planets orbiting stars other than our sun—discovered so far, it may seem like we are on the cusp of finding out whether we are alone in the universe. Sadly though, we don't know much about these planets—in most cases just their mass and their radius.

Understanding whether a planet could host life requires a lot more information. At the moment, one extremely important piece of information that is missing is the presence, composition and structure of their atmospheres. Signs of atmospheric water, oxygen and methane would all be signs that a planet may support life.

Now we have for the first time managed to detect water vapor in the atmosphere of an exoplanet that is potentially habitable. Our results have been published in Nature Astronomy.

A planet's atmosphere plays a vital role in shaping the conditions inside it—or on its surface, if it has one. Its composition, stability and structure all provide important clues about what it is like to be there. Through atmospheric studies, we can therefore learn about the history of the planet, investigate its habitability and, ultimately, discover signs of life.

The primary method that we use when examining exoplanets is transit spectroscopy. This involves looking at starlight as a planet passes in front of its host star. As it transits, stellar light is filtered through the planet's atmosphere—with light being absorbed or deflected based on what compounds the atmosphere consists of.

The atmosphere therefore leaves a characteristic footprint in the stellar light that we try to observe. Further analysis can then help us match this footprint to known elements and molecules, such as water or methane.

At the moment, the study of exoplanets atmospheres is limited, as this kind of measurement requires very high precision, which current instruments were not built to deliver. But molecular signatures from water have been found in the atmospheres of gaseous planets, similar to Jupiter or Neptune. It has never before been seen in smaller planets—until now.

K2-18 b was discovered in 2015 and is one of hundreds of "super-Earths"—planets with a mass between Earth and Neptune—found by NASA's Kepler spacecraft. It is a planet with eight times the mass of the Earth that orbits a so called "red dwarf" star, which is much cooler than the sun.

However, K2-18b is located in the "habitable zone" of its star which means it has the right temperature to support liquid water. Given its mass and radius, K2-18 b is not a gaseous planet, but has a high probability of having a rocky surface.

We developed algorithms to analyze the starlight filtered by this planet using transit spectroscopy, with data provided by the Hubble Space Telescope.

This enabled us to make the first successful detection of an atmosphere with water vapor around a non-gaseous planet, which is also located within the habitable zone of its star.

In order for an exoplanet to be defined as habitable, there is a long list of requirements that need to be satisfied. One is that the planet needs to be in the habitable zone where water can exist in liquid form. It is also necessary that the planet has an atmosphere to protect the planet from any harmful radiation coming from its host star.

Another important element is the presence of water, vital for life as we know it. Although there are many other criteria for habitability, such as the presence of oxygen in the atmosphere, our research has made K2-18b the best candidate to date. It is the only exoplanet to fulfil three requirements for habitability: the right temperatures, an atmosphere and the presence of water.

However, we cannot say, with current data, exactly how likely the planet is to support life. Our data are limited to an area of the spectrum—this shows how light is broken down by wavelength—where water dominates, so other molecules can unfortunately not be confirmed.

With the next generation of telescopes, such as the James Webb Space Telescope and the ARIEL space mission, we will be able to find more information on the chemical composition, cloud coverage and structure of the atmosphere of K2-18 b. This will help us understand just how habitable it is.

These missions could also make it easier to make similar detections for other rocky bodies in the habitable zones of their parent stars.

That would certainly be exciting. With K2-18 b being 110 light years away, it is not really a planet we could visit—even with tiny robotic probes—in the foreseeable future.

Excitingly, it is probably just a matter of time before we find similar planets that are closer. So we may be well on our way to answering the age-old question of whether we are alone in the universe after all.

In the new study, Kipping and Teachey tracked the light signatures of objects around the star Kepler-1625. Following a hunch based on some promising data from the Kepler Space Telescope, they used the Hubble Space Telescope to gather more data on the star Kepler-1625. This is how they figured out that Kepler-1625b, which is about the size of Jupiter and orbits its star at about the same distance that Earth orbits the sun, may be home to a moon.

When searching for exoplanets, astronomers look for dips in the amount of light emanating from a star. By measuring how a planet blocks its home star’s light as it passes in front of it, astronomers can learn a lot about the size, orbit, and even composition of the planet. As a planet transits past the star over and over, the accumulated data allows astronomers nail down the planet’s orbital period pretty precisely.

When observing light from Kepler-1625 in this way, Teachey and Kipping noticed a slight anomaly in the transit data of the planet Kepler-1625b: Each dip in light was accompanied by another small dip — one that couldn’t be explained by just the presence of a planet. With data on only three transits, however, they knew they needed more. Securing 40 hours on the Hubble Space Telescope, they built a much stronger case for their suspicion: The blip in Kepler-1625b’s transit data appeared to be a moon, sometimes trailing the planet, sometimes leading it.

Because of the unique nature of moons, their research required a slightly different approach than most exoplanet hunts. Transit data is a great way to learn about exoplanets, but since moons have slightly irregular orbiting patterns, they are trickier to identify this way.

“Moons are orbiting the planets, so they show up in a different place every time the planet transits, sometimes before the planetary transit, sometimes after,” says Teachey. “So you don’t see that same kind of periodicity, and you can’t really stack the moon transits in the same way to clean up the signal.”

Fortunately, the extra time on the HST allowed Teachey and Kipping to hone in on Kepler-1625b and estimate that the planet and its moon are about the same relative size to one another as Earth and the moon — except they’re about 11 times larger than our home world and its moon.

## How can we tell how many exoplanets a star has? - Astronomy

How do astronomers determine that they've discovered "single" large extrasolar planets and not "multiple" lesser planets?

If planets have different distances to their host stars, as all of the planets in our Solar system and the many known exoplanet systems, the planets will orbit with different periods. This means they go around their host stars at different speeds and will separate themselves spatially and frequency-wise. In all the methods to find exoplanets (microlensing, pulsar timing, radial velocity, transits and direct imaging) these planets are separable. For example, with direct imaging, you can see two different points of light. With radial velocity measurements, you can separate out the frequencies. As a planet revolves around a star, the star too revolves a bit around their common center of mass. A large planet will cause a sinusoidal shift at a single frequency (the time of that planet's year) in the star's apparent motion towards or away from us. Two planets will have two different length years, and cause wobbles in the star's motion of two different frequencies. That's how we know when we're looking only at a single planet.

If there is a binary planet system, where two planets orbit each other's common center of mass which goes around the host star this can be harder to detect with some methods. With direct imaging, the two planets might appear so close as to look like one. With radial velocity measurements, the effect on the star will mostly look the same because the the two planets will basically appear to the host star as one gravitational tug. For the transit method, distinguishing binary planets from single planets is easier though because the transit start times will vary depending on which part of the binary orbit lines up with the star. As of 2015, though, no binary planets have been found around other stars.

Updated on July 18, 2015 by Everett Schlawin.

#### Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

## Examples of Exoplanets

Gamma Cephei Ab: The first exoplanet detected, found in 1998 around the star Gamma Cephei. It was not confirmed until 2003, when better detection techniques were developed.

PSR 1267+12 B and C: The first pulsar planets. These were found in 1992, orbiting the rapidly spinning remains of a massive star that exploded as a supernova. Astronomers found a third planet in this system, and are still trying to figure out when those planets formed and how they survived the supernova explosion. One of the planets is a Super-Earth.

51 Pegasi b: The first planet around a star like the Sun. Astronomers found it using the Observatoire de Haute-Provence in France, a ground-based facility. This planet is also known as a “hot Jupiter” because it appears to be a very warm gas-giant-type world.

Kepler 186f: the first Earth-size planet circling in the habitable zone of its star. Found by the Kepler Mission in 2014.

Kepler 11-f: orbits a Sun-like star and has at least 2.3 times the mass of Earth. It may be a gas dwarf, due to its low density and possible hydrogen-helium atmosphere.

Mu Arae c: a hot Neptune type planet orbiting very close to its parent star, Mu Arae. This is the first hot Neptune discovered.

## Will wobbling stars guide us to ‘100 Earths'?

Debra Fischer, a Yale University professor of astronomy, is a leading expert in the &ldquoradial velocity&rdquo or &ldquowobble&rdquo method for finding exoplanets. She's pictured here with the Cerro Tololo Inter-American Observatory in Chile.

By Pat Brennan,
NASA's Exoplanet Exploration Program

The hunt for planets around other stars so far has been a rousing success: more than 3,400 confirmed in our Milky Way galaxy, where, we now know, hundreds of billions more likely await discovery. Most of these exoplanets were found using the "transit method": looking for a tiny, tell-tale &ldquoshadow&rdquo as the orbiting planet crossed the face of its star.

But in order to know whether a planet is likely to be rocky or gaseous, scientists need to be able to calculate density, and, in many cases, they need another way of getting at that besides the transit method. Debra Fischer, a Yale University professor of astronomy, is a leading expert in the &ldquoradial velocity&rdquo or &ldquowobble&rdquo method for finding exoplanets.

With this method, astronomers track the changing speed of a star as it is tugged around by orbiting planets. The size and nature of the tugs can reveal how massive these planets are, as well as how long it takes them to orbit their stars &ndash critical information in the search for rocky, watery, life-bearing worlds. Future space missions designed to study planetary atmospheres could follow up on these planets, picking apart the spectrum of light to look for gases that might indicate the presence of life, such as oxygen and methane.

Fischer took time recently to chat about a new planet-hunting instrument her team is developing as part of the &ldquo100 Earths&rdquo project, and her hopes for the future of exoplanet astronomy.

#### What do you tell people when they ask what you do?

That I&rsquom a planet finder, a planet hunter, or an astronomer. I tell them I detect planets orbiting nearby stars. And now we&rsquore trying to build more sensitive instruments to detect smaller planets with the Doppler (&ldquowobble&rdquo) technique.

Right now, my team is building a spectrograph called EXPRES, the Extreme Precision Spectrograph, that we will be delivering by the end of this year (to the Lowell Observatory&rsquos Discovery Channel Telescope in Happy Jack, Arizona). I&rsquove put everything I know from 20 years of planet hunting into the design of this spectrograph. The title of our science program is &ldquoThe Search for 100 Earths.&rdquo We&rsquore targeting nearby, bright stars. We know now from Kepler (NASA&rsquos Kepler Space Telescope) that virtually all of these stars will have planetary systems. Perhaps half of them will have small, rocky planets &ndash half or more. We are emboldened, really, by the promise of Kepler.

#### Will these observations be useful to future NASA missions?

Many NASA missions that are being planned today have an exoplanet focus. Kepler, which is flying now in its K2 phase. TESS, the Transiting Exoplanet Survey Satellite. JWST (the James Webb Space Telescope, launching in 2018) is going to be able to look at exoplanets. WFIRST (the Wide Field Infrared Survey Telescope, a mission concept) has this huge exoplanet component. Now I&rsquom a community co-chair for a NASA study of a future observatory concept called LUVOIR (the Large Ultraviolet Optical Infrared Surveyor). This mission will serve astronomers who are studying the origin of the universe and the evolution of galaxies. It will also obtain the spectra of the atmospheres of Earth-like planets around nearby stars to search for biosignatures.

I think the first step &ndash the thing we can to today &ndash is to identify Earth analogs around nearby stars, determine their orbits and measure the masses of the planets. The measurement of planet masses is something that can be uniquely done with the Doppler technique. If you don&rsquot have the mass, then when we have a spectrum of the planet's atmosphere with a NASA observatory, the interpretation will be ambiguous. You need the mass of the planet to understand whether things like oxygen and methane have a geological or biological origin.

Life is driving the crazy chemistry we have here on Earth with oxygen, carbon dioxide, methane, coexisting in our atmosphere.

#### What advice do you have for a young woman interested in a scientific or engineering career?

Go for it. This is an incredibly exciting time in astronomy, in exoplanets. The field is just absolutely booming. And I think it&rsquos very important to have a lot of diversity in the field. I have absolutely no doubt that the way I approach problems is different from the generation of men I have worked with. Because of that, I hope that I am taking the field in a unique and slightly different direction. And that&rsquos what young women coming into the field can do &ndash create something new. This is really important to the vitality of science.

## Life, Here and Beyond

Ask most any American whether life exists on other planets and moons, and the answer you’ll get is a confident “yes!” Going back decades (and in many ways generations), we’ve been introduced to a menagerie of extraterrestrials good and bad. Their presence suffuses our entertainment and culture, and we humans seem to have an almost innate belief-or is it a hope-that we are not alone in the universe.

But that extraterrestrial presence on regular display is, of course, a fiction. No life beyond Earth has ever been found there is no evidence that alien life has ever visited our planet. It’s all a story.

This does not mean, however, that the universe is lifeless. While no clear signs of life have ever been detected, the possibility of extraterrestrial biology – the scientific logic that supports it – has grown increasingly plausible. That is perhaps the single largest achievement of the burgeoning field of astrobiology, the broad-based study of the origins of life here and the search for life beyond Earth.

By exploring and illuminating the world of extreme life on Earth, by experimenting with how life here began, by understanding more about the chemical makeup of the cosmos, by testing for habitability on missions to Mars, Saturn’s moon Titan, and beyond, an enormous body of science has already been assembled to analyze and explain the origins, characteristics and possible extraterrestrial dimensions of life. And unlike the ETs and star-ship invaders of popular culture, these discoveries are real.

Turning Science Fiction into Science Fact

Consider: The rover Curiosity has firmly determined that ancient Mars was significantly more wet and warm, and was an entirely habitable place for microbial life. All the ingredients needed for life as we know it – the proper chemicals, a consistent source of energy, and water that was likely present and stable on the surface for millions of years – were clearly present.

Did microbial life then begin? If so, did it evolve? Those questions remain unanswered, but this much is known: If a second genesis occurred on Mars (or on Jupiter’s moon Europa, Saturn’s moon Enceladus, or anywhere else in our solar system), then the likelihood increases substantially that many other forms of life exist on those billions of exoplanets and exomoons now known to orbit distant stars and planets. One origin of life on Earth could be the result of a remarkable and inexplicable pathway to life. Two origins in one solar system strongly suggests that life is commonplace in the universe.

Consider, too, the revolution in understanding that has taken place since the mid-1990s regarding planets and moons in solar systems well beyond ours. Since ancient times, natural philosophers, then scientists, and untold interested others predicted, assumed even, that many other planets orbited their stars. By now thousands of exoplanets have been officially identified – via NASA missions like Kepler as well as ground-based observations — and billions more await discovery. And that’s just in our Milky Way galaxy.

With advances in the instruments and knowledge that make possible the exoplanet hunt, the focus has been increasing refined to identify planets lying in habitable zones – at distances from their stars that would allow water to remain at least periodically liquid on a planet’s surface. The search for exoplanets was born in the fields of astronomy and astrophysics, but it has always been intertwined with astrobiology as well. As with so many NASA missions, the broad and intense drive to find and understand habitable zone planets and moons both greatly enhances astrobiology and is informed by astrobiology.

Our experience with finding distant planets also makes you wonder: Will the search for the current or past presence of extraterrestrial life some day be viewed as a parallel to the earlier search for exoplanets? Men and women of science, as well as the lay public, intuitively assumed planets existed beyond our solar system, but these planets were identified only when our technology and thinking had sufficiently advanced. Is the discovery of ET life similarly awaiting our coming of scientific age?

The Past As a Guide to the Future

Astrobiology research is taking place because its time has come. Scientists across the country and around the world are diving into origin-of-life and life-beyond-Earth issues and developing exciting and cutting edge work. But NASA also has an astrobiology “strategy” describing where the agency sees promising lines of research – from the highly specific to the wide and broad — that the agency might support. A sampling of examples:

• What were the steps that led inanimate materials – rocks, sediments, organic compounds, water – to come together and build living organisms, with replicating genes, cell walls, and the ability to reproduce?

• What led to the proliferation of new life forms on Earth?

• How do water and essential organic compounds arrive on planets and moons, and how do they interact with the planets and moons they land on?

• Is it possible to learn from chemicals and minerals on the surface of planets whether microbes might live there, including beneath the planet’s surface?

• Is it possible, likely even, that life exists elsewhere based on elements other than carbon and a system different than DNA ? Could such life even exist here on Earth, but is as yet undetected?

## Astronomy and international collaboration

Scientific and technological achievements give a large competitive edge to any nation. Nations pride themselves on having the most efficient new technologies and race to achieve new scientific discoveries. But perhaps more important is the way that science can bring nations together, encouraging collaboration and creating a constant flow as researchers travel around the globe to work in international facilities.

Astronomy is particularly well suited to international collaboration due to the need to have telescopes in different places around the world, in order to see the whole sky. At least as far back as 1887 — when astronomers from around the world pooled their telescope images and made the first map of the whole sky — there have been international collaborations in astronomy and in 1920, the International Astronomical Union became the first international scientific union.

In addition to the need to see the sky from different vantage points on Earth, building astronomical observatories on the ground and in space is extremely expensive. Therefore most of the current and planned observatories are owned by several nations. All of these collaborations have thus far been peaceful and successful. Some of the most notable being:

The Atacama Large Millimeter/submillimeter Array (ALMA), an international partnership of Europe, North America and East Asia in cooperation with the Republic of Chile, is the largest astronomical project in existence.

The European Southern Observatory (ESO) which includes 14 European countries and Brazil, and is located in Chile.

Collaborations on major observatories such as the NASA/ESA Hubble Space Telescope between USA and Europe.

## Astronomers Discover Seventeen New Extrasolar Planets

Using data gathered by NASA’s Kepler space telescope, a team of astronomers in Canada has discovered 17 new exoplanets, including an Earth-sized world. Designated KIC 7340288b, this planet is both rocky and in the habitable zone of its parent star.

Sizes of 17 new planet candidates, compared to Mars, Earth, and Neptune. The planet in green is KIC 7340288b. Image credit: Michelle Kunimoto.

“KIC 7340288b is about 1,000 light-years away, so we’re not getting there anytime soon,” said Michelle Kunimoto, a Ph.D. candidate at the University of British Columbia.

“But this is a really exciting find, since there have only been 15 small, confirmed planets in the habitable zone found in Kepler data so far.”

This planet is just 1.6 times bigger than Earth. It orbits its host star once every 142.5 days at a distance of 0.444 AU (just bigger than the orbit of Mercury in the Solar System) and receives about a third of the light Earth gets from the Sun.

Of the other 16 new planets discovered, the smallest is only two-thirds the size of Earth — one of the smallest planets to be found with Kepler so far. The rest range in size up to 8 times the size of Earth.

In the study, Kunimoto and her colleagues — Dr. Henry Ngo of the NRC Herzberg Astronomy and Astrophysics and University of British Columbia’s Professor Jaymie Matthews — used the transit method to look for planets among the roughly 200,000 stars observed by the Kepler mission.

“Every time a planet passes in front of a star, it blocks a portion of that star’s light and causes a temporary decrease in the star’s brightness,” she said.

“By finding these dips, known as transits, you can start to piece together information about the planet, such as its size and how long it takes to orbit.”

The astronomers also used the Near InfraRed Imager and Spectrometer (NIRI) on the Gemini North 8-m Telescope in Hawaii to capture follow-up images of some of the planet-hosting stars.

“We took images of the stars as if from space, using adaptive optics,” Kunimoto said.

“We’re able to tell if there was a star nearby that could have affected Kepler’s measurements, such as being the cause of the dip itself.”

In addition to the new planets, the team was able to observe thousands of known Kepler planets using the transit method, and will be reanalyzing the exoplanet census as a whole.

“We’ll be estimating how many planets are expected for stars with different temperatures,” Professor Matthews said.

“A particularly important result will be finding a terrestrial habitable zone planet occurrence rate. How many Earth-like planets are there? Stay tuned.”

The team’s paper was published in the Astronomical Journal.

Michelle Kunimoto et al. 2020. Searching the Entirety of Kepler Data. I. 17 New Planet Candidates Including One Habitable Zone World. AJ 159, 124 doi: 10.3847/1538-3881/ab6cf8