What is the Transit Method?

In a series of papers, Professor Loeb and Michael Hippke indicate that conventional rockets would have a hard time escaping from certain kinds of extra-solar planets. Credit: NASA/Tim Pyle

Welcome all to the first in our series on Exoplanet-hunting methods. Today we begin with the most popular and widely-used, known as the Transit Method (aka. Transit Photometry).

For centuries, astronomers have speculated about the existence of planets beyond our Solar System. After all, with between 100 and 400 billion stars in the Milky Way Galaxy alone, it seemed unlikely that ours was the only one to have a system of planets. But it has only been within the past few decades that astronomers have confirmed the existence of extra-solar planets (aka. exoplanets).

Astronomers use various methods to confirm the existence of exoplanets, most of which are indirect in nature. Of these, the most widely-used and effective to date has been Transit Photometry, a method that measures the light curve of distant stars for periodic dips in brightness. These are the result of exoplanets passing in front of the star (i.e. transiting) relative to the observer.


These changes in brightness are characterized by very small dips and for fixed periods of time, usually in the vicinity of 1/10,000th of the star’s overall brightness and only for a matter of hours. These changes are also periodic, causing the same dips in brightness each time and for the same amount of time. Based on the extent to which stars dim, astronomers are also able to obtain vital information about exoplanets.

For all of these reasons, Transit Photometry is considered a very robust and reliable method of exoplanet detection. Of the 3,526 extra-solar planets that have been confirmed to date, the transit method has accounted for 2,771 discoveries – which is more than all the other methods combined.


One of the greatest advantages of Transit Photometry is the way it can provide accurate constraints on the size of detected planets. Obviously, this is based on the extent to which a star’s light curve changes as a result of a transit.  Whereas a small planet will cause a subtle change in brightness, a larger planet will cause a more noticeable change.

When combined with the Radial Velocity method (which can determine the planet’s mass) one can determine the density of the planet. From this, astronomers are able to assess a planet’s physical structure and composition – i.e. determining if it is a gas giant or rocky planet. The planets that have been studied using both of these methods are by far the best-characterized of all known exoplanets.

In addition to revealing the diameter of planets, Transit Photometry can allow for a planet’s atmosphere to be investigated through spectroscopy. As light from the star passes through the planet’s atmosphere, the resulting spectra can be analyzed to determine what elements are present, thus providing clues as to the chemical composition of the atmosphere.

Artist’s impression of an extra-solar planet transiting its star. Credit: QUB Astrophysics Research Center

Last, but not least, the transit method can also reveal things about a planet’s temperature and radiation based on secondary eclipses (when the planet passes behind it’s sun). On this occasion, astronomers measure the star’s photometric intensity and then subtract it from measurements of the star’s intensity before the secondary eclipse. This allows for measurements of the planet’s temperature and can even determine the presence of clouds formations in the planet’s atmosphere.


Transit Photometry also suffers from a few major drawbacks. For one, planetary transits are observable only when the planet’s orbit happens to be perfectly aligned with the astronomers’ line of sight. The probability of a planet’s orbit coinciding with an observer’s vantage point is equivalent to the ratio of the diameter of the star to the diameter of the orbit.

Only about 10% of planets with short orbital periods experience such an alignment, and this decreases for planets with longer orbital periods. As a result, this method cannot guarantee that a particular star being observed does indeed host any planets. For this reason, the transit method is most effective when surveying thousands or hundreds of thousands of stars at a time.

It also suffers from a substantial rate of false positives; in some cases, as high as 40% in single-planet systems (based on a 2012 study of the Kepler mission). This necessitates that follow-up observations be conducted, often relying on another method. However, the rate of false positives drops off for stars where multiple candidates have been detected.

Number of extrasolar planet discoveries per year through September 2014, with colors indicating method of detection – radial velocity (blue), transit (green), timing (yellow), direct imaging (red), microlensing (orange). Credit: Public domain

While transits can reveal much about a planet’s diameter, they cannot place accurate constraints on a planet’s mass. For this, the Radial Velocity method (as noted earlier) is the most reliable, where astronomers look for signs of “wobble” in a star’s orbit to the measure the gravitational forces acting on them (which are caused by planets).

In short, the transit method has some limitations and is most effective when paired with other methods. Nevertheless, it remains the most widely-used means of “primary detection” – detecting candidates which are later confirmed using a different method – and is responsible for more exoplanet discoveries than all other methods combined.

Examples of Transit Photometry Surveys:

Transit Photometry is performed by multiple Earth-based and space-based observatories around the world. The majority, however, are Earth-based, and rely on existing telescopes combined with state-of-the-art photometers. Examples include the Super Wide Angle Search for Planets (SuperWASP) survey, an international exoplanet-hunting survey that relies on the Roque de los Muchachos Observatory and the South African Astronomical Observatory.

There’s also the Hungarian Automated Telescope Network (HATNet), which consists of six small, fully-automated  telescopes and is maintained by the Harvard-Smithsonian Center for Astrophysics. The MEarth Project is another, a National Science Foundation-funded robotic observatory that combines the Fred Lawrence Whipple Observatory (FLWO) in Arizona with the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

The SuperWasp Cameras at the South African Astronomical Observatory. Credit: SuperWASP project & David Anderson

Then there’s the Kilodegree Extremely Little Telescope (KELT), an astronomical survey jointly administered by Ohio State University, Vanderbilt University, Lehigh University, and the South African Astronomical Society (SAAO). This survey consists of two telescopes, the Winer Observatory in southeastern Arizona and the Sutherland Astronomical Observation Station in South Africa.

In terms of space-based observatories, the most notable example is NASA’s Kepler Space Telescope. During its initial mission, which ran from 2009 to 2013, Kepler detected 4,496 planetary candidates and confirmed the existence of 2,337 exoplanets. In November of 2013, after the failure of two of its reaction wheels, the telescope began its K2 mission, during which time an additional 515 planets have been detected and 178 have been confirmed.

The Hubble Space Telescope also conducted transit surveys during its many years in orbit. For instance, the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) – which took place in 2006 – consisted of Hubble observing 180,000 stars in the central bulge of the Milky Way Galaxy. This survey revealed the existence of 16 additional exoplanets.

Other examples include the ESA’s COnvection ROtation et Transits planétaires (COROT) – in English “Convection rotation and planetary transits” – which operated from 2006 to 2012. Then there’s the ESA’s Gaia mission, which launched in 2013 with the purpose of creating the largest 3D catalog ever made, consisting of over 1 billion astronomical objects.

NASA’s Kepler space telescope was the first agency mission capable of detecting Earth-size planets. Credit: NASA/Wendy Stenzel

In March of 2018, the NASA Transiting Exoplanet Survey Satellite (TESS) is scheduled to be launched into orbit. Using the transit method, TESS will detect exoplanets and also select targets for further study by the James Webb Space Telescope (JSWT), which will be deployed in 2019. Between these two missions, the confirmation and characterization or many thousands of exoplanets is anticipated.

Thanks to improvements in terms of technology and methodology, exoplanet discovery has grown by leaps and bounds in recent years. With thousands of exoplanets confirmed, the focus has gradually shifted towards the characterizing of these planets to learn more about their atmospheres and conditions on their surface.

In the coming decades, thanks in part to the deployment of new missions, some very profound discoveries are expected to be made!

We have many interesting articles about exoplanet-hunting here at Universe Today. Here’s What are Extra Solar Planets?, What are Planetary Transits?, What is the Radial Velocity Method?, What is the Direct Imaging Method?, What is the Gravitational Microlensing Method?, and Kepler’s Universe: More Planets in our Galaxy than Stars.

Astronomy Cast also has some interesting episodes on the subject. Here’s Episode 364: The COROT Mission.

For more information, be sure to check out NASA’s page on Exoplanet Exploration, the Planetary Society’s page on Extrasolar Planets, and the NASA/Caltech Exoplanet Archive.


Did We Arrive Early To The Universe’s Life Party?

The Fermi Paradox essentially states that given the age of the Universe, and the sheer number of stars in it, there really ought to be evidence of intelligent life out there. This argument is based in part on the fact that there is a large gap between the age of the Universe (13.8 billion years) and the age of our Solar System (4.5 billion years ago). Surely, in that intervening 9.3 billion years, life has had plenty of time to evolve in other star system!

Continue reading “Did We Arrive Early To The Universe’s Life Party?”

Starshade Prepares To Image New Earths

Artist's concept of the prototype starshade, a giant structure designed to block the glare of stars so that future space telescopes can take pictures of planets. Credit: NASA/JPL

For countless generations, people have looked up at the stars and wondered if life exists somewhere out there, perhaps on planets much like ours. But it has only been in recent decades that we have been able to confirm the existence of extrasolar planets (aka. exoplanets) in other star systems. In fact, between 1988 and April 20th of 2016, astronomers have been able to account for the existence of 2108 planets in 1350 different star systems, including 511 multiple planetary systems.

Most of these discoveries have taken place within just the past three years, thanks to improvements in our detection methods, and the deployment of the Kepler space observatory in 2009. Looking ahead, astronomers hope to improve on these methods even further with the introduction of the Starshade, a giant space structure designed to block the glare of stars, thus making it easier to find planets – and perhaps another Earth!

Continue reading “Starshade Prepares To Image New Earths”

When Will We Find Another Earth?

We hear about discoveries of exoplanets every day. So how long will it take us to find another planet like Earth?

Back in the olden days, astronomers could only guess if there were planets orbiting other stars.

These were the days when we had to wait at the bank to pay our bills, nobody carried computers in their pockets and those computers gave direct connections to everyone else’s pockets because pocket connectivity is highly important, school was uphill both ways, the number 6 was brand new, we recorded images on thin sheets of transparent plastic, 5 bees were worth a quarter and I had an onion tied to my belt, as was the style at the time.

With the discovery of a mega Jupiter-sized world orbiting the star 51 Pegasi in 1995, the floodgates opened up. In the years that followed, dozens more planets were discovered. Then hundreds, and now, we know about thousands orbiting other stars.

The bad news is we can’t get to any of them. The good news is most of these worlds suck. You don’t want any part of them. For starters their wifi is terrible.

Consider Kepler-70b. This world orbits its star 4 times in a 24 hour period. This means it’s super close, and a great place to really quickly win all the human torch cosplay competitions. The surface temperature is a completely unreasonable 7200 Kelvin, hotter than the surface temperature of the Sun.

There’s the planets orbiting pulsar PSR B1257+12, a millisecond pulsar in the constellation of Virgo. As they whip around their exotic host, they’re bathed in intense radiation. Which is generally considered bad for creatures who need functioning organs.

Perhaps HD 106906 b, orbiting its star 650 times more distantly than we orbit the Sun. You’d spend every second of your short life on that planet inventing new words for cold. And then you’d die. Cold.

Imagine a world that orbits a star like our Sun. A world made of about an Earth’s worth of rocky material that you could stand on, at just the right distance from its star that water can exist as a liquid.

This is what astronomers search for, the tri-wizard cup of extrasolar planetary research. Earth 2? Terra Nova? The Gaia part le deux.

Here’s the exciting part. Astronomers have found each of these characteristics in a planet, but never all together. They’ve found plenty of stars similar to our Sun, with planets orbiting them. In fact, the star HD 10180 is incredibly similar to the Sun, and astronomers have discovered 9 planets orbiting it so far. Which does have a familiar ring to it. No word so far on which ones are about to be demoted to dwarf planets.

Sizes and temperatures of Kepler discoveries compared to Earth and Jupiter
Sizes and temperatures of Kepler discoveries compared to Earth and Jupiter

They’ve found planets roughly the same mass as the Earth. Kepler-89, with 98% the mass of the Earth. So close! Sadly, it’s way too close to its parent hydrogen furnace to be habitable.

They’ve found planets in the habitable zone. Here on Earth, the global average temperature is -18 degrees C. Sounds cold, but the wintery nights in Antarctica absolutely wreck our GPA.

The closest analog discovered is Kepler-22b, with a global average temperature of -11C. So, it should feel downright balmy. Except, it’s about 2.4 times bigger than Earth and orbits a nasty red dwarf star.

Astronomers have even matched up two criteria at the same time. Earth-sized world orbiting around a Sun-like star, but it’s hellishly hot. Wrong flavor star but with the right temperature and size, it’s a veritable tic tac toe board of near wins.

So far, there hasn’t been a single extrasolar planet discovered that meets all three criteria. An Earth-sized world, orbiting a Sun-like star inside the habitable zone where liquid water could be present.

Astronomers were hoping that NASA’s Kepler spacecraft would have been the first to discover Earth 2.0. It had already turned up thousands of planets, including many of the ones I’ve already mentioned.

Artist's conception of the Kepler Space Telescope. Credit: NASA/JPL-Caltech
Artist’s conception of the Kepler Space Telescope. Credit: NASA/JPL-Caltech

Sadly, just a few years into the mission, it lost too many reaction wheels, which allow the spacecraft to change direction. It wasn’t able to make enough observations to help confirm a true Earth 2.0. Kepler is still searching for planets, but with a reduced ability to point, it’s only looking at red dwarf stars.

Don’t worry, NASA’s Transiting Exoplanet Survey Satellite will launch in 2017, and will survey a region of the sky 400 times larger than Kepler did. It should turn up thousands of planets, Earth-sized and larger.

Once we actually find New Terra, things get really interesting. Astronomers will search those planets for life. I know it sounds almost impossible to see life from this distance, but astronomers know that if they can analyze the atmosphere of these worlds, they can detect life flourishing there.

They might even be able to detect the pollution from their alien cars and heavy industry, contributing to their CO2 levels, and learn we’re not so different after all. Even if they’re icky bug people.

At the time I’m recording this video, no analog Earth planet has been discovered so far. But it’s just a matter of time. In the next few decades astronomers are going to find that first Earth 2.0, and then dozens, then hundreds, and even figure out which ones have life on them.

It’s a great time to be alive. Place your bets. Predict the date astronomers announce that we’ll find Earth 2.0. Put your guess into the comments below.