Thursday, July 24, 2014

Measuring the radius of a planet that is precisely >this< big

Thanks to NASA's Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the radius of a planet outside our solar system. The size of the exoplanet, dubbed Kepler-93b, is now known to an uncertainty of just 74 miles (119 kilometers) on either side of the planetary body (see Ballard et al. 2014). 

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.
Image Credit: 
NASA/JPL-Caltech
The findings confirm Kepler-93b as a "super-Earth" that is about one-and-a-half times the size of our planet. Although super-Earths are common in the galaxy, none exist in our solar system. Exoplanets like Kepler-93b are therefore our only laboratories to study this major class of planet. 

With good limits on the sizes and masses of super-Earths, scientists can finally start to theorize about what makes up these weird worlds. Previous measurements, by the Keck Observatory in Hawaii, had put Kepler-93b's mass at about 3.8 times that of Earth. The density of Kepler-93b, derived from its mass and newly obtained radius, indicates the planet is in fact very likely made of iron and rock, 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," said Sarah Ballard, a NASA Carl Sagan Fellow at the University of Washington in Seattle and lead author of a paper on the findings published in the Astrophysical Journal. 

"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," said Ballard. 

Kepler-93b orbits a star located about 300 light-years away, with approximately 90 percent of the sun's mass and radius. The exoplanet's orbital distance -- only about one-sixth that of Mercury's from the sun -- implies a scorching surface temperature around 1,400 degrees Fahrenheit (760 degrees Celsius). Despite its newfound similarities in composition to Earth, Kepler-93b is far too hot for life. 

To make the key measurement about this toasty exoplanet's radius, the Kepler and Spitzer telescopes each watched Kepler-93b cross, or transit, the face of its star, eclipsing a tiny portion of starlight. Kepler's unflinching gaze also simultaneously tracked the dimming of the star caused by seismic waves moving within its interior. These readings encode precise information about the star's interior. The team leveraged them to narrowly gauge the star's radius, which is crucial for measuring the planetary radius. 

Spitzer, meanwhile, confirmed that the exoplanet's transit looked the same in infrared light as in Kepler's visible-light observations. These corroborating data from Spitzer -- some of which were gathered in a new, precision observing mode -- ruled out the possibility that Kepler's detection of the exoplanet was bogus, or a so-called false positive. 

Taken together, the data boast an error bar of just one percent of the radius of Kepler-93b. The measurements mean that the planet, estimated at about 11,700 miles (18,800 kilometers) in diameter, could be bigger or smaller by about 150 miles (240 kilometers), the approximate distance between Washington, D.C., and Philadelphia. 

Spitzer racked up a total of seven transits of Kepler-93b between 2010 and 2011. Three of the transits were snapped using a "peak-up" observational technique. In 2011, Spitzer engineers repurposed the spacecraft's peak-up camera, originally used to point the telescope precisely, to control where light lands on individual pixels within Spitzer's infrared camera. 

The upshot of this rejiggering: Ballard and her colleagues were able to cut in half the range of uncertainty of the Spitzer measurements of the exoplanet radius, improving the agreement between the Spitzer and Kepler measurements. 

"Ballard and her team have made a major scientific advance while demonstrating the power of Spitzer's new approach to exoplanet observations," said Michael Werner, project scientist for the Spitzer Space Telescope at NASA's Jet Propulsion Laboratory, Pasadena, California.

JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. 

NASA's Ames Research Center in Moffett Field, California, is responsible for Kepler's ground system development, mission operations and science data analysis. JPL managed Kepler mission development. Ball Aerospace & Technologies Corp. in Boulder, Colorado, developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder. The Space Telescope Science Institute in Baltimore archives, hosts and distributes Kepler science data. Kepler is NASA's 10th Discovery Mission and was funded by the agency's Science Mission Directorate. 

For more information about the Kepler mission, visit: 

For more information about Spitzer, visit: 

Whitney Clavin Jet Propulsion Laboratory, Pasadena, Calif. 818-354-4673 whitney.clavin@jpl.nasa.gov

Wednesday, July 16, 2014

Big problems at academic field research sites


Since arriving at Harvard I've become close friends with Katie Hinde, who runs the Comparative Lactation Lab, where she uses evolutionary theory, lab studies and field work to study the relationship between mother's milk and child development. Katie is an active blogger and Tweets as @mammals_suck

As a field researcher, she and her collaborators became all too aware of the problem of sexual harassment and assault at distant field sites where ethical standards and reporting protocols are not often made explicit and bad behavior is often rife. To quantify just how prevalent sexual harassment/assault is at scientific field sites, they conducted a scientific survey of their field. Think of it as an anthropological field study of field anthropologists. However, their respondents weren't limited to just field anthropology, and they ended up having respondents across 31 different social, life, and physical sciences. Their refereed journal article was published in PLOS ONE today. 

Here's the press release:

Survey of Academic Field Experiences (SAFE): Trainees Report Harassment and Assault

For many social, life, and earth science disciplines, conducting research in field settings is an integral component of scholarship. However, anecdotes shared via email or whispered in the corners of hotel conference rooms suggested that sexual harassment and assault were common experiences for many young scientists, especially women. Biological anthropologists Kate Clancy (UIUC), Robin Nelson (Skidmore), Julienne Rutherford (UIC), and Katie Hinde (Harvard) set out to explore more deeply the pervasiveness of these experiences and what the results they published on July 16, 2014 in PLOS ONE are a sobering wake-up call.

Monday, July 14, 2014

Ringing stars that are this big

An illustration of the surface and interior of a giant star. Credit: Paul Beck
Last year I wrote about how some of the results of my PhD thesis were being questioned in the literature. I remarked at the time that, "No scientist enjoys having their results challenged," but since then I've realized that it's actually not that bad. In fact, it's a sign you're doing the right things scientifically. If you aren't doing important work, then no one is going to take much notice, and when people do take notice and ask good questions, it provides an opportunity to do more science! (Well, provided one takes a growth mindset.) So I decided to take the opportunity and last year I began branching out.

As I explained previously, the scientific issue at hand is really quite simple: Either my "retired A stars" really are the evolved counterparts of A-type stars like Vega and Sirius, with masses greater than 1.5 times the mass of the Sun, or they're really not much heavier than the Sun. The test is also straight forward: go out and measure the masses of some of my stars, and compare the direct measurements to the predictions of the models I was using. The difficult part is putting all of this into practice.

One way of directly measuring a star's mass is to see how it rings. Wait, let me step back for a second. The stars that I study have convective outer layers, in which hot gas rises, cools and falls back down toward the star's interior. Kinda like in a bowl of miso soup:

Video credityouareahippo

These rising bubbles of gas bump against the stellar surface and cause it to ring. (The motion of these gas bubbles is what gives rise to stellar "flicker," the phenomenon discovered and studied by Fabienne Bastien) Even though the bubbles do their bumping randomly, the star will vibrate, or ring, at its natural frequencies (see my previous post on resonances). A good analogy once relayed to me by Peter Goldriech is to imagine throwing pebbles at a bell. There will be lots of random strikes, but the bell will still ring at its natural tone. If you know the shape and composition of the bell and you measure its resonant frequencies, in principle you could back out how massive the bell is. The same is true for stars!

One of my target stars, romantically named HD185351, happens to reside in the field of view of the NASA Kepler Mission, and I teamed up with Daniel Huber to apply for director's discretionary time to observe it. I'm glad we asked when we did, because the Kepler space telescope broke down soon thereafter. Fortunately, we received our observations and were able to do our analysis, known as "asteroseismology." Dan, myself and others used a similar technique to measure the mass and spin-orbit alignment of the Kepler-56 planetary system, and Dan (and his collaborators) have used asteroseismology to measure the masses of hundreds of Kepler target stars. 

While I said that one can measure the mass from asteroseismology, one really only measures the stellar density and the surface gravity of the star. The density scales as

$\rho \sim \frac{M}{R^3}$

and the gravity scales as

$g \sim \frac{M}{R^2}$

Two equations, two unknowns, and some algebra yields the mass, $M$. However, we went one step further and directly measured the radius of HD185351 using the CHARA array with another technique called "interferometry." The details of this sort of measurement probably deserve their own blog post, but basically we combined a bunch of little telescopes atop Mt. Wilson and used them as a big, single telescope to measure how big the star appears on the sky. When combined with the known distance to the star, we can get the physical size of the star, or its radius $R$.

Measuring "angular diameters" like we did for HD185351. Image credit UNL Astronomy.
The CHARA Array at Mt. Wilson, which is useful for measuring very small angles. 
We put all of these measurements together and found that HD185351 is, indeed, larger than 1.5 Solar masses, making it a bona fide retired A star (our paper has been accepted to ApJ and our preprint is on the arXiv). Things are a bit complicated because we actually get two different mass measurements. This disagreement has inspired us to make more measurements of this type with the K2 Mission, and additional interferometric observations are underway. But our two mass measurements of HD185351 bracket our model prediction (2.0 and 1.6 Solar compared to the model-based estimate of 1.87 Solar), and both estimates are greater than 1.5 Solar. Further, Jamie Lloyd was kind enough to provide his own model prediction before we completed our analysis. His best estimate of this star's mass was 1.2 Solar, which is inconsistent with all of our estimates. 

However, this is just one measurement and there's plenty more work to be done. But the key is that when two astronomers engage in a theory-based debate, the best thing to do is to go get some data. That's what my group be doing in the months to come, and we'll publish our measurements as we make 'em. Stay tuned as we work to solve the Case of the Retired A Stars!

Sunday, July 6, 2014

The space we are listening to divided as such

A brilliant TED talk by the incomparable Prof. Reggie Watts.
 

Thursday, July 3, 2014

Invisible women: At the intersection of gender, race and sexuality

Today's guest post is by Dr. Chanda Prescod-Weinstein, a Dr. Martin Luther King, Jr. Postdoctoral Fellow at the MIT Department of Physics & Kavli Institute for AstrophysicsShe specializes in theoretical cosmology and has an interest in formal issues in field theory and relativity. She also serves as an informal academic advisor for many of the very few minority women physics majors at M.I.T. This post was originally published on the Women in Astronomy blog (here). 

In 1851, former slave Sojourner Truth asked white feminists, "Ain't I a woman?" when they refused to let her speak at a women's conference because she was Black. One might hope that in 158 years, that speech wouldn't seem so essential and relevant. But at the 2009 Women in Astronomy conference, my first foray into non-race oriented equal opportunity efforts, we were told the news was good: women had made significant gains and equality was on its way. There was no substantive mention of race beyond Peggy McIntosh's talk. But I knew the truth. I have been looking at the NSF and AIP statistics myself for years, and I knew that the news was not good for Black, Latina, and Native American women. Those numbers hadn't changed too significantly in three decades. How could they possibly be telling us that the news was good?

As a queer Black (cis)woman, I live at the intersection of multiple minority statuses. More of my time than I would like has been sucked up in trying to fend off the marginalization that society's structures foist upon anyone who has even one of these identities. More of my time than I would like has been spent thinking about a way out of those structures and trying to convince others to help.

And here's one thing I want out of: the phrase "women and minorities," a phrase I used to use a lot myself. As part of the effort to push for equal opportunities in STEM, I  used it repeatedly for over a decade, thinking that I was advocating not just for strangers but also for myself. At some point however, hearing other people use it began to grate on me. A lot.

Wednesday, July 2, 2014

We want more OK Go!

An NPR Tiny Desk Concert in 223 Takes: