Astronomers Determine the Size and Mass of Super-Earth HD 97658b

Astronomers Uncover the Hidden Identity of an Exoplanet

The relative size of the Earth and Sun next to those of HD 97658 (the star) and HD 97658b (the super-Earth exoplanet). Credit: Jason Eastman and Diana Dragomir

Using the Microvariability & Oscillations of Stars (MOST) space telescope, astronomers were able to estimate the true size and mass of super-Earth HD 97658b for the first time.

Santa Barbara, California – Hovering about 70 light-years from Earth – that’s “next door” by astronomical standards – is a star astronomers call HD 97658, which is almost bright enough to see with the naked eye. But the real “star” is the planet HD 97658b, not much more than twice the Earth’s diameter and a little less than eight times its mass. HD 97658b is a super-Earth, a class of planets for which there is no example in our home solar system.

While the discovery of this particular exoplanet is not new, determining its true size and mass is, thanks to Diana Dragomir, a postdoctoral astronomer with UC Santa Barbara’s Las Cumbres Observatory Global Telescope (LCOGT) network. As part of her research, Dragomir looked for transits of this exoplanet with Canada’s Microvariability & Oscillations of Stars (MOST) space telescope. The telescope was launched in 2003 to a pole-over-pole orbit about 510 miles high. Dragomir analyzed the data using code written by LCOGT postdoctoral fellow Jason Eastman. The results were published online today in the Astrophysical Journal Letters.

A super-Earth is an exoplanet with a mass and radius between those of the Earth and Neptune. Don’t be fooled by the moniker though. Super-Earth refers to the planet’s mass and does not imply similar temperature, composition, or environment to Earth. The brightness of HD 97658 means astronomers can study this star and planet in ways not possible for most of the exoplanet systems that have been discovered around fainter stars.

HD 97658b was discovered in 2011 by a team of astronomers using the Keck Observatory and a technique sometimes called Doppler wobble. But only a lower limit could be set on the planet’s mass, and nothing was known about its size.

Transits, such as those observed by Dragomir, occur when a planet’s orbit carries it in front of its parent star and reduces the amount of light we see from the star ever so slightly. Dips in brightness happen every orbit, if the orbit happens to be almost exactly aligned with our line of sight from Earth. For a planet not much bigger than our Earth around a star almost as big as our Sun, the dip in light is tiny but detectable by the ultraprecise MOST space telescope.

The first report of transits in the HD 97658 system in 2011 turned out to be a false alarm. That might have been the end of the story, but Dragomir knew that the ephemeris of the planet’s orbit (a timetable to predict when the planet might pass in front of the star) was not exact. She convinced the MOST team to widen the search parameters, and during the last possible observing window for this star last year, the data showed tantalizing signs of a transit –– tantalizing, but not certain beyond doubt. A year later, MOST revisited HD 97658 and found clear evidence of the planet’s transits, allowing Dragomir and the MOST team to estimate the planet’s true size and mass for the first time.

“Measuring an exoplanet’s size and mass leads to a determination of its density, which in turn allows astronomers to say something about its composition,” Dragomir said. “Measuring the properties of super-Earths in particular tells us whether they are mainly rocky, water-rich, mini gas giants, or something entirely different.”

The average density of HD 97658b is about four grams per cubic centimeter, a third of the density of lead but denser than most rocks. Astronomers see great significance in that value –– about 70 percent of the average density of Earth –– since the surface gravity of HD 97658b could hold onto a thick atmosphere. But there’s unlikely to be alien life breathing those gases. The planet orbits its sun every 9.5 days, at a distance a dozen times closer than we are from our Sun, which is too close to be in the Habitable Zone, nicknamed The Goldilocks Zone. The Goldilocks nickname is apropos: If a planet is too close to its star, it’s too hot; if it’s too far away, it’s too cold, but if it’s in the zone, it’s “just right” for liquid water oceans, one condition that was necessary for life here on Earth.

Over the past few years, systems with massive planets at very small orbital radii have proved to be quite common despite being generally unexpected. The current number of confirmed exoplanets exceeds 600, with the vast majority having been discovered by radial velocity surveys. These are severely biased toward the detection of systems with massive planets (roughly the mass of Jupiter) in small orbits. Bucking that trend is HD 97658b, which orbits its star at a distance farther than many of the currently known exoplanets. HD 97658b is only the second super-Earth known to transit a very bright star.

“This discovery adds to the still small sample of transiting super-Earths around bright stars,” said Dragomir. “In addition, it has a longer period than many known transiting exoplanets around bright stars, including 55 Cnc e, the only other super-Earth in this category. The longer period means it is cooler than many closer-in exoplanets, so studying HD 97658b’s properties is part of the progression toward understanding what exoplanets in the habitable zone might be like.”

Reference: “MOST Detects Transits of HD 97658b, a Warm, Likely Volatile-rich Super-Earth” by Diana Dragomir, Jaymie M. Matthews, Jason D. Eastman, Chris Cameron, Andrew W. Howard, David B. Guenther, Rainer Kuschnig, Anthony F. J. Moffat, Jason F. Rowe, Slavek M. Rucinski, Dimitar Sasselov and Werner W. Weiss, 31 June 2013, The Astrophysical Journal Letters.
DOI: 10.1088/2041-8205/772/1/L2


4 Comments on "Astronomers Determine the Size and Mass of Super-Earth HD 97658b"

  1. Still we fully don’t know about life being liked to Carbon as a building structure for life on a simple scale why not another element or combination of element’s. Here on earth every nook and cranny has something living even under soil and in caves don’t be as narrow minded as water being the only liquid to sustain life. Until we adapt a launch vehicle the can withstand other atmospheres, pressure and temperatures we will be ignorant as what kind of life can live on a exoplanet.

    • The problem here is that there aren’t any other elements that can act as a ‘backbone’ in supporting other elements. Only carbon and silicon have this ability to do it and remain stable. As far as we know, this is a requirement – but not necessarily the last word – yet.

  2. ^ and they are made of carbon. So tell me what the point of your comment is supposed to be about? The theory that there may be life not made from carbon? the last of your post is irrelevant toward the first part of it…

  3. Madanagopal.V.C | July 4, 2013 at 11:30 pm | Reply

    Hello friends! Stop commenting without reading the lines from the article..
    “Super-Earth refers to the planet’s mass and does not imply similar temperature, composition, or environment to Earth” = Thus you people are too greedy to know about the composition of super-earths. They are located by the optometric effect of photo-occlusion of the star whose light they are occluding. They are not like stars to give out light from far off distances so that spectroscopic analysis like what we make with sun ans stars to find out the chemicals of composition. Even detection of exo-planets is itself a more difficult task. How do you arrive at the composition of its atmosphere with just a black dot crossing the star? The photo luminosity waxing and waning plotted in a graph can only tell the presence of circling exo-planet or at best the presence of the thin atmosphere0 The mass can be determined by the same method but assuming its atmosphere is purely hypothetical. Thank You.

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