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Tight Measurement of Exoplanet Radius

Both the Kepler and Spitzer space telescopes had a role to play in recent work on the planet Kepler-93b, whose size is now known to an uncertainty of a mere 120 kilometers on either side of the planet. What we have here is the most precise measurement of an exoplanet radius yet, a helpful result in the continuing study of ‘super-Earths,’ a kind of world for which we have no analogue in our own Solar System. A third instrument also comes into play, for studies of the planet’s density derived from Keck Observatory data on its mass (about 3.8 times Earth’s mass) and the known radius indicate this is likely an world made of iron and rock.

And that is absolutely the only similarity between Kepler-93b and Earth, for at 0.053 AU, six times closer than Mercury to the Sun, the planet’s surface temperature is estimated to be in the range of 760 degrees Celsius. The planet is 1.481 times the width of Earth. The accuracy of the measurement is the story here, a result so precise that, in the words of Sarah Ballard (University of Washington), lead author of the paper on these findings, “it’s literally like being able to measure the height of a six-foot tall person to within three quarters of an inch — if that person were standing on Jupiter.”


Image: Using data from NASA’s Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist’s conception. The diameter of the exoplanet, dubbed Kepler-93b, is now known with an uncertainty of just one percent. Credit: NASA/JPL-Caltech.

Just how the measurement was made is a story in itself. The Spitzer instrument provided data for seven transits of Kepler-93b between 2010 and 2011, three of them studied with a new observational technique called ‘peak up’ that halved the uncertainty of Spitzer’s own radius measurements. Kepler-93 thus served as a test subject for the new technique, which was developed in 2011 and allows tighter control over how light affects individual pixels in the observatory’s infrared camera. The paper examines all seven light curves in detail.

Meanwhile, we have the Kepler data, which provided light curves as well as the dimming of the star caused by seismic waves in motion in the interior. Now we’re in the realm of asteroseismology, which is a powerful way to probe the makeup of individual stars. Asteroseismic measurements over a long observational baseline can provide useful information about the density of the star (with a precision of 1 percent) as well as its age (within 10%). Such measurements require a long observational baseline at high cadence — cadence refers to the time between observations of the same target — as well has high photometric precision.

When we have both an asteroseismic density measurement of the exoplanet host star as well as a transit light curve, we can improve the precision of our radius measurements. Sara Seager (MIT) and colleagues examined host star densities in relation to planetary orbits and the radius of the star as early as 2003, and later work by a team led by Philip Nutzman (Harvard-Smithsonian CfA) used asteroseismology along with transit light curves to constrain the radius of HD 17156b, highlighting a method that has been found to be relevant to a wide number of recent studies.

From the paper:

The Kepler mission’s long baselines and unprecedented photometric precision make asteroseismic studies of exoplanet hosts possible on large scales… Kepler-93 is a rare example of a sub-solar mass main-sequence dwarf that is bright enough to yield high-quality data for asteroseismology. Intrinsically faint, cool dwarfs show weaker-amplitude oscillations than their more luminous cousins. These targets are scientifically valuable not only as exoplanet hosts, but also as test beds for stellar interior physics in the sub-solar mass regime.

The combination of the Kepler data and Spitzer’s new technique was powerful, and adds luster to the already rich history of Spitzer’s Infrared Array Camera (IRAC) in exoplanetary science. The instrument has been helpful in mapping planetary weather and characterizing super-Earth atmospheres, and has been a major tool in ruling out exoplanet false-positives, because an actual planet will present the same transit depth no matter the wavelength at which it is observed. After losing its coolant in 2009, the telescope, now dubbed ‘warm Spitzer,’ continues to provide key readings that are now enhanced with the development of the ‘peak up’ process.

Kepler-93 is a star of approximately 90 percent of the Sun’s mass and radius, located some 300 light years from Earth. With the Spitzer data corroborating the find and the use of asteroseismology to constrain the result, we wind up with an error bar that is just one percent of the radius of Kepler-93b. A planet thought to be 18,800 kilometers in diameter might be bigger or smaller than that by about 240 kilometers, but no more, an outstanding result for exoplanetary science and a confirmation of the power of asteroseismology in determining stellar radii.

The paper is Ballard et al., “Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode,” The Astrophysical Journal Vol. 790, No. 1 (2014), 12 (abstract / preprint). A JPL news release is also available.


Comments on this entry are closed.

  • Andrew Palfreyman July 28, 2014, 16:43

    Surface gravity is 1.73 gee, so tolerable. Pity about the distance, the temperature, and goodness knows what else.

  • Andrew LePage July 29, 2014, 10:41

    This is an excellent example of the sort of synergism between measurements from various sources (Kepler, Spitzer, Keck-HIRES) but also various astronomical specialties (photometery, spectroscopy, asteroseismology) needed to derive the properties of super-Earth size extrasolar planets. Recent analyses of Kepler data and ground-based radial velocity measurements show that an important transition takes place at planet radii of about 1.5 (or so) times that of the Earth from planets with a predominantly rocky composition (i.e. terrestrial planets) to non-rocky (i.e. mini-Neptunes and gas dwarfs). This not only has implications on how planets form but just how big habitable planets can get. In fact, if recent work on the mass-radii function of planets is correct, most of the planets some people have argued are “potentially habitable” are not rocky planets never mind habitable ones. This is discussed in detail in the following essay:


    More measurements like the one Paul describes here are going to be needed to pin down the characteristics of this important transition in planet composition.

  • Mark Zambelli August 23, 2014, 11:31

    Is the planet tidally locked at that distance? If so, could a temperate though windy belt straddling the terminator offer some harbour for life? I don’t know enough about the possibility of this ‘tidally-locked biosphere’ scenario, sorry if the question sounds absurd.

  • ljk September 8, 2014, 9:11

    One Planet, Two Stars: A System More Common Than Previously Thought

    by SHANNON HALL on SEPTEMBER 4, 2014

    There are few environments more hostile than a planet circling two stars. Powerful tidal forces from the stars can easily destroy the rocky building blocks of planets or grind a newly formed planet to dust. But astronomers have spotted a handful of these hostile worlds.

    A new study is even suggesting that these extreme systems exist in abundance, with roughly half of all exoplanets orbiting binary stars.

    NASA’s crippled Kepler space telescope is arguably the world’s most successful planet hunter, despite the sudden end to its main mission last May. For nearly four years, Kepler continuously monitored 150,000 stars searching for tiny dips in their light when planets crossed in front of them.

    As of today, astronomers have confirmed nearly 1,500 exoplanets using Kepler data alone. But Kepler’s database is immense. And according to the exoplanet archive there are over 7,000 “Kepler Objects of Interest,” dubbed KOIs, that might also be exoplanets.

    There are a seeming endless number of questions waiting to be answered. But one stands out: how many exoplanets circle two stars? Binary stars have long been known to be commonplace — about half of the stars in the Milky Way are thought to exist in binary systems.

    A team of astronomers, led by Elliott Horch from Southern Connecticut State University, has shown that stars with exoplanets are just as likely to have a binary companion. In other words, 40 to 50 percent of the host stars are actually binary stars.

    Full article here:


  • ljk September 25, 2014, 10:56

    Wet exoplanet has clear skies

    Neptune-sized orb is smallest alien world known to have water vapour.

    Alexandra Witze

    24 September 2014

    The smallest exoplanet yet found to contain water is about the size of Neptune — and a rare glimpse at its atmosphere reveals clear conditions. The handful of other small planets whose atmospheres have been studied all have cloudy skies.

    “It’s the smallest planet that we’ve seen anything in the atmosphere besides clouds,” says Jonathan Fraine, an astronomer at the University of Maryland in College Park. “The fact that it’s clear at all is significant.”

    Fraine and his colleagues describe the atmosphere of the planet in the 25 September issue of Nature1. Known as HAT-P-11b, the body is about 38 parsecs (124 light years) away, in the Cygnus constellation.

    Astronomers have been piecing together details on the atmospheres of several alien worlds, trying to find an Earth-like world with an Earth-like atmosphere. So far, however, clouds have generally obscured their view.

    HAT-P-11b is different. Fraine’s team used the Hubble and Spitzer space telescopes to monitor the dimming of its star’s light as the planet passed in front of it, along with spectral details of the light during those transits. The astronomers could briefly glimpse its atmosphere twice, as the planet moved onto the disk of the star and then off it.

    Full article here:


  • ljk September 29, 2014, 8:35

    Using Lasers to Lock Down Exoplanet Hunting

    Posted by Bruce Betts

    2014/09/26 22:59 UTC

    Topics: Planetary Society Projects, explaining technology, extrasolar planets, Exoplanets Laser

    The Planetary Society is launching a new collaboration with Yale exoplanet hunter Debra Fischer and her team, the Exoplanets Laser project. We will support the purchase of an advanced, ultra stable laser to be used in a complex system they are designing to push radial velocity exoplanet hunting to a whole whole new level – a level intended to facilitate the discovery of Earth sized planets around nearby stars. As Debra says:

    “The search for exoplanets is motivated by the question of whether life exists elsewhere. This drives our interest in the detection of planets that are similar to our own world: rocky planets with the potential for liquid surface water and plate tectonics; worlds that might harbor life that we can recognize.”

    Full article here:


  • ljk September 29, 2014, 17:28

    Clear Skies Above: Astronomers Detect Water Vapor On Cloud-free Atmosphere Of A Hot-Neptune

    By Leonidas Papadopoulos

    Sunny and hot all-year-round, with no clouds on the horizon. That’s not a weather forecast only for the Maldive Islands here on Earth, but also for exoplanet HAT-P-11b as well, according to the latest findings by an international team of astronomers. But don’t start packing for that holiday package just yet, for HAT-P-11b is a steamy Neptune-sized world located so close to its host star that average temperatures there reach a scortching 1,120 degrees Fahrenheit.

    Exoplanet research has transitioned in recent years from the simple discovery of planets around other stars to that of their detailed characterization. Following the exciting findings of thousands of extrasolar worlds during the last two decades, which have established that planetary formation is a common occurrence in the galaxy, astronomers around the world are now striving to understand the overall evolution of these distant worlds, by studying their properties, bulk composition and internal structure. To that end, astronomers use one of the best tools at their disposal, which is called transmission spectroscopy.

    More specifically, when an extrasolar planet happens to cross or transit the face of its star as seen by our line of sight here on Earth, it causes a small dip in the star’s brightness which is proportional to the size of the exoplanet itself. If that planet also happens to have an atmosphere, the latter will absorb some of the star’s light in certain wavelengths as it transits, resulting in a wavelength-dependent transit depth, better known as a transmission spectrum. By studying this spectrum of the combined star-planet light, astronomers can extract detailed information about the planet’s atmosphere, like its chemical composition, temperature, density, and overall dynamics.

    Full article here:


    To quote:

    If the results by Fraine’s team are any indication, then the first detailed atmospheric observations of a super-Earth located in a more life-friendly orbit around a distant star somewhere in the Milky Way galaxy, might indeed be a few years in the future.