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article imageAstronomy: Most precise measurement of exoplanet to date

By Robert Myles     Jul 24, 2014 in Science
Pasadena - Aided by two NASA telescopes, astronomers have made the most precise calculation yet of the size of an exoplanet — a planet outwith our solar system.
The measurement of the radius of planet, Kepler-93b, a so-called super-Earth orbiting a star 300 light years distant was calculated using NASA’s Kepler and Spitzer Space Telescopes. Scientists reckon they’ve pinned down the planet’s size with an uncertainty factor of just 74 miles (119 kilometers) on either side of the planetary body.
The measurement of a planet lying roughly 60 times further away from us than our closest star system, Alpha Centauri, illustrates just how far astronomical telescopes have developed in less than 100 years.
It was only in 1930 that astronomers first observed the "last planet" of our solar system, Pluto. Since then, as telescopes have developed, Pluto has been demoted to the status of a dwarf planet — a “plutoid” — after it was found to be just one of several large icy bodies in the outer solar system.
The measurement of Kepler-93b confirms it as a super-Earth sized body about 50 percent as big again as the Earth. As more and more exoplanets are catalogued, it’s become clear that super-Earths are common in our galaxy. In our own solar system, however, there are no super-Earths with planets being divided into two categories: small rocky worlds like Earth, Mars, Venus and Mercury and gas giants comprising Jupiter, Saturn, Uranus and Neptune.
Exoplanets like Kepler-93b, therefore, are useful “laboratories” to study this major class of planet. Armed with more refined knowledge of the size and mass of such super-Earths, it’s possible for scientists to theorize about the make-up of these alien worlds.
Scientists already had part of the jig-saw for Kepler-93b since the Hawaii-based Keck Observatory had calculated the planet’s mass to be about 3.8 times that of Earth. Now that an accurate measurement of Kepler-93b’s radius is to hand, putting the two pieces — mass and radius — together indicates the planet, very likely, has an iron and rock constitution, much like Earth.
“With Kepler and Spitzer, we’ve captured the most precise measurement to date of an alien planet’s size, which is critical for understanding these far-off worlds,” commented Sarah Ballard, a NASA Carl Sagan Fellow at the University of Washington in Seattle and lead author of the paper on the Kepler-93b findings.
Putting this feat of measurement in perspective, Ballard added, “The measurement is so precise that 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’s parent star has a mass and radius about 90 percent of that of our Sun. But Kepler-93b is a most inhospitable of worlds since it hugs its parent star close, orbiting at a distance about one-sixth that of Mercury’s from the Sun (57,910,000 km).
Kepler-93b’s orbital distance of less than 10 million kilometers (about 6 million miles) implies scorching surface temperatures of around 760° Celsius (1,400° Fahrenheit) say the scientists. Alien world Kepler-93b may be, but as a home-world for alien life, it’s a non-starter.
Astronomers used the Kepler and Spitzer telescopes in tandem to make the key measurement of Kepler-93b’s radius. Each space telescope watched as Kepler-93b transited across the face of its star, eclipsing a tiny portion of starlight as it did so.
The Kepler telescope simultaneously tracked the dimming of the star caused by seismic waves moving within the star’s interior. These readings are crucial in providing precise information about the star’s interior. The research team then leveraged these measurements to narrowly gauge the star’s radius, an essential prerequisite to measuring Kepler-93b’s planetary radius.
While the Kepler telescope was returning visible-light spectrum observations, Spitzer was doing the same except in the infra-red spectrum. Corroborating data from Spitzer, some of which were gathered in a new, precision observing mode, negated the chance that the Kepler telescope’s readings of the exoplanet were bogus.
Marrying the two data sets meant that Kepler-93b’s radius could be calculated with a margin of error of just one percent. The measurements reveal that Kepler-93b has an estimated diameter of about 11,700 miles (18,800 kilometers), compared with Earth’s mean diameter of 7926 miles (12756 kilometers).
The Kepler-93b measurements could be out by about 150 miles (240 kilometers), but, hey, so what if the astronomers are out by the distance between Washington, D.C., and Philadelphia when taking a measuring tape to a planet 300 light years from us.
Ballard and colleagues findings were published July 20 in the Astrophysical Journal under the title: “Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode.”
More about Exoplanet, Exoplanets, alien worlds, cataloguing exoplanets, Kepler93b
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