Distant Stars, Giant Planets … And an exhibit on Capitol Hill

by Mary Martialay on April 23, 2013

Emily DeLarme with a graph displaying light from the secondary transit of Kepler 2b

More than 1,000 light years from Earth, a giant gaseous planet is in a tight and fast orbit around a star larger than the Sun, and, between the two of them, there’s something funny going on. The star is known as HAT-P-7 or Kepler 2, the planet is Kepler 2b, and Rensselaer physics and astronomy student Emily DeLarme has the latest clue in the mystery.

DeLarme – whose research will be part of the April 24 Posters on the Hill exhibit on Capitol Hill, sponsored by the Council on Undergraduate Research - is studying data gathered by NASA’s Kepler space telescope. Kepler, launched in 2009 to search for Earth-size planets, gathers data on approximately 150,000 stars. Analysis of Kepler data has led to identification of 2,740 planetary candidates, and 115 confirmed planets.

Working with Jon Morse, Rensselaer professor of physics and associate vice president for research for physical sciences and engineering, DeLarme has been studying data generated on two of the planets – Kepler 2b and Kepler 13b –  which orbit stars Kepler 2 and Kepler 13, respectively. In particular, she is looking at light during the period when the planets circle behind their stars, a phase known as the “secondary eclipse,” or “secondary transit.”

Kepler records light from the planet plus its parent star throughout the orbit, Morse explained. More than 99 percent of the light comes from the parent star, with only a small fraction reflected from the planet. The “exquisite stability” of Kepler makes it possible to see tiny dips in the total brightness caused either as the planet blocks light as it passes in front of the star – known as the primary transit – or as light reflected from the planet surface is lost during the planet’s path behind the star during the secondary transit.

Morse said the characteristics of the primary transit indicate the size of the planet and its orbital distance from the parent star. The secondary eclipse can reveal properties of the planet, as well as gravitational effects that distort the star’s atmosphere for planets with very close orbits, such as Kepler 2b and Kepler 13b. Even more tantalizing, he added, is the prospect of measuring properties of the planet’s atmosphere during different positions in the orbit.

DeLarme, who is enrolled in a co-terminal program in physics and astronomy, said that that measuring the light, or “spectrum,” of the planet, allows researchers like herself to learn about the atmosphere, reflectivity, temperature and other properties of the planet.

Her most interesting results thus far relate to Kepler 2b and Kepler 2, which is already known to be a peculiar star. Kepler 2b, a gas giant about 1.5 times the size of Jupiter, circles Kepler 2 about once every two days in an orbit 26 times closer than the Earth to the sun. Published research has documented that Kepler 2 rotates in the opposite direction of Kepler 2b’s orbit. Also, Kepler 2 appears to have a gravitational dark spot or distortion revealed by previous analysis of the primary transit. Researchers have theorized that the star may be shaped more like a football than a sphere.

DeLarme has used the latest brightness data in the Kepler mission database to measure details of the secondary eclipse, which causes a dip in the total light of only 70 parts-per-million. Typically, the amount and the duration of the dip in light as a planet enters secondary eclipse is similar to the light emitted as it emerges, generating symmetrical readings when plotted on a graph showing the full orbit. But DeLarme found an asymmetry in the so-called “ingress” and “egress” of the Kepler 2b secondary eclipse.

Now that she’s identified the asymetry, DeLarme said she will be working to verify her finding and figure out why it’s happening. Morse said her work adds to the mystery surrounding Kepler 2:

People have seen these dark spots, and gravitational distortions, and now Emily has shown that there is an asymmetry in this hard-to-detect secondary eclipse – something no one else has noted. She has introduced a new anomaly to the system that needs to be explained. Her work will help inform future models of this system.

DeLarme joined Morse’s lab in the fall of 2012, after attending a presentation of his research. She said:

I like the entire idea of figuring things out about planets that we can’t actually see, but we know that they’re there … This is a field that’s growing a lot and it’s fascinating to be a part of this early research.

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