Tight Measurement of Exoplanet Radius

by Paul Gilster on July 28, 2014

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.”

kepler_93b

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.

tzf_img_post

{ 4 comments… read them below or add one }

Andrew Palfreyman July 28, 2014 at 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 at 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:

http://www.drewexmachina.com/2014/07/24/habitable-planet-reality-check-terrestrial-planet-size-limit/

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 at 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.
Mark.

ljk September 8, 2014 at 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:

http://www.universetoday.com/114286/one-planet-two-stars-a-system-more-common-than-previously-thought/

Leave a Comment