Transits Transform the Quest for Earth-Like Planets

March 2012
Kepler image

Astronomers engaged in the search for planets orbiting stars in the Milky Way galaxy are ecstatic about the results now pouring in from the NASA Kepler mission. The discovery of stars with several planets in orbit has boosted the quest for Earth-like planets in the habitable zone of a parent star.

Kepler is a space observatory, named for the seventeenth-century astronomer Johannes Kepler, who discovered the laws guiding the planets in our solar system. The spacecraft is in orbit round the Sun, trailing behind the Earth. Unlike the Hubble Space Telescope, which circles the Earth once every 96 minutes, Kepler is a rock-steady observing platform designed to lock onto one small patch of sky and observe it continuously. It has about 150,000 stars in its sight.

Although Kepler is an American space observatory, citation analysis shows that its users are truly international.

In June of 2012, Kepler will commence its fourth year of scientific operation. And on June 5-6 there will be a timely coincidence when the planet Venus makes a rare transit of the Sun. This singular event will not be visible again to human eyes until December 11, 2117. To find extrasolar planets, Kepler catches them in action as they glide in front of their parent stars. When a planet transits its parent star, the light detected by Kepler’s photometer dims slightly. When this has happened three times to the same star, it is declared a planetary candidate, on the presumption that the dips are due to a transiting planet. Follow-up observations by a world-wide network of ground-based telescopes are used to confirm a discovery. Because false positives can be caused by natural variations in the light output of a star, such as flares or starspots, there are in fact numerous rejects.

Most-Cited Papers

Now that Kepler has been running for nearly three years, Science Watch made a citation analysis of research papers that give the background to Kepler and announce its findings. Harnessing the Web of Knowledge from Thomson Reuters, the analysis used a sample of 390 papers published between 2009 and 2012 to date. This complete sample has papers on exoplanets, astrobiology, astrophysics, and data reduction.

Our analysis shows a brisk increase in the annual productivity of the Kepler mission: 53 papers in 2009, 138 in 2010, and 195 in 2011. Citations to these reports, meanwhile, increased nearly 40-fold during the same period. The monthly data for 2011 show a rapid acceleration in both publications and citations, and this continues into 2012. Almost all planets discovered were hot Jupiters careering round their parent star in just a few days, or icy Neptunes. However, this did not dismay astrobiologists on the trail of distant Earths. They knew all along that finding the first Earth-like planet in a star’s habitable zone would be a waiting game. That’s because an Earth-like planet going round a solar type star is going to take about one year on its orbit, so three years are needed to track down and then verify Earth-size planets.

Kepler Mission: Highly Cited Authors, 2009-2012
(Listed by citations, based on a sample of 390 papers)
Author Affiliation Papers Citations
William J. Borucki NASA Ames Research Center 70 1,272
Ronald Gilliland Space Science Telescope Institute 63 1,122
Jon Jenkins SETI Institute 45 869
Hans Kjeldsen Aarhus University 70 834
David G. Koch NASA Ames Research Center 37 834
Jorgen Christensen-Dalsgaard Aarhus University 69 756
Timothy M. Brown Las Cumbres Observ. Global Telescope 37 705
David W. Latham Harvard-Smithsonian Center 37 665
William J. Chaplin University of Birmingham 38 404
Saskia Hekker University of Amsterdam 35 353

SOURCE: Web of Knowledge, Thomson Reuters

Although Kepler is an American space observatory, citation analysis shows that its users are truly international. The sample of papers includes guest observers and scientists from 40 countries, including French Polynesia and Antarctica. Table 1 lists the top ten countries by total citations. Clearly, nations in North America and Europe predominate—although, at #10, Australia also manages to make the roster.    

Along with countries, our analysis identified the sample’s most-cited authors, listed in Table 2. William J. Borucki, of the NASA Ames Research Center, Moffett Field, California, tops the list with nearly 1,300 citations. Borucki is the senior scientist responsible for the observatory. A bit farther down the list, Hans Kjeldsen of Aarhus University, Denmark, is the highest-placed author affiliated with a university, with 834 citations.

Kepler Mission: Highly Cited Nations 2009-2012
(Listed by citations, based on a sample of 390 papers)
Country Papers Cites
United States 261 2,268
United Kingdom 128 1,139
Denmark 101 1,050
France 110 943
Belgium 64 600
Austria 56 592
Spain 67 556
Canada 50 546
Germany 61 541
Australia 50 424

SOURCE: Web of Knowledge, Thomson Reuters

Kepler Mission: Selected Papers on Planetary Candidates
(Published between 2010 and 2011, ranked by citations)

Rank Papers Citations
1 W.J. Borucki, et al., “Characteristics of planetary candidates observed by Kepler. II. Analysis of the first four months of data,” Astrophys. J., 736(1): No. 19, 20 July 2011. 39
2 N.M. Batalha, et al., “Kepler’s first rocky planet: Kepler-10b,” Astrophys. J., 729(1): No. 27, 1 March 2011. 38
3 D.W. Latham, et al., “Kepler-7b: A transiting planet with unusually low density,” Astrophys. J. Lett., 713(2): L140-4, 20 April 2010. 34
4 W.J. Borucki, et al., “Kepler-4b: A hot Neptune-like planet of a G0 star near main-sequence turnoff,” Astrophys. J. Lett., 713(2): L126-30, 20 April 2010. 32
5 W.J. Borucki, et al., “Characteristics of Kepler planetary candidates based on the first data set,” Astrophys. J., 728(2): No. 117, 20 February 2011. 30
6 E.W. Dunham, et al., “Kepler-6b: A transiting hot Jupiter orbiting a metal-rich star,” Astrophys. J. Lett., 713(2): L136-9, 20 April 2010. 24
7 W.F. Welsh, et al., “The discovery of ellipsoidal variations in the Kepler light curve of HAT-P-7,” Astrophys. J. Lett., 713(2): L145-9, 20 April 2010. 23
8 J.L. Christiansen, et al., “Studying the atmosphere of the exoplanet HAT-P-7b via secondary eclipse measurements with EPOXI, Spitzer, and Kepler,” Astrophys. J., 710(1): 97-104, 10 February 2010. 20
9 J.H Steffen, et al., “Five Kepler target stars that show multiple transiting exoplanet candidates,” Astrophys. J., 725(1): 1226-41, December 2010. 17
10 J.F. Rowe, et al., “Kepler observations of transiting hot compact objects,” Astrophys. J. Lett., 713(2): L150-4, 20 April 2010. 14

SOURCE: Web of Knowledge, Thomson Reuters

Key Findings Thus Far

Table 3, a listing of selected papers on the exoplanets discovered and investigated by Kepler, is headed by an analysis of the properties of planetary candidates from the first four months of data. This report immediately shows the extent to which Kepler is a game-changer, with the numbers therein saying it all: 1,235 planetary candidates, of which 68 are Earth-sized candidates, 288 are super-Earths, and the majority are Neptunes and Jupiters. During 2011 the number of planetary candidates slowly declined because longer datasets contained fewer false alarms.

Paper #2 describes an important milestone in the mission: the discovery of a rocky planet, Kepler-10b, with a mass almost five times that of Earth. This demonstrated for the first time that Kepler could detect super-Earths. A surprise of a different kind is announced in paper #3. Kepler-7b had the second-lowest reported density of any exoplanet, barely one-sixth the density of water. This finding suggested that the properties of exoplanets may span a very wide range of physical properties, which is of interest to researchers modelling planetary formation.

In #7, observers describe the gravitational interaction between the exoplanet HAT-P-7b and its parent star. The planet is 1.8 times the mass of Jupiter, and it is in a very tight circular orbit only four stellar radii from the host star. Kepler’s photometer detected regular fluctuations in the light curve of the star, and from these variations the shape of the star can be modelled. It is a triaxial ellipsoid with the longest axis pointing at the planet. The variations amount to only 37 parts per million, which demonstrates that Kepler is a high-precision observatory. The hot atmosphere of planet HAT-P-7b is the focus of #8: its atmosphere has no clouds because it is far too hot for condensates or ices to form.

Hot Planets

Hot planets are the subject of #9 reporting the discovery of Kepler planets with surface temperatures of about 13,000 K, some 3,000 K hotter than the host stars. Paper #10 details five stars suspected of having multiple planets. Follow-up observations of one star have now revealed six planets, Kepler-18b, 18c, 18d, 18e, 18f, and 18g. The latest announcements from the Kepler mission indicate that multiple-planet systems are more common than astronomers had expected.

One of the highly cited authors listed in Table 2, David W. Latham, a Kepler investigator at the Harvard-Smithsonian Center for Astrophysics, offered Science Watch this reflection on the first three years of the spacecraft: “I’m delighted to see the many contributions that the Kepler Mission is making to our knowledge of the distribution and characteristics of extrasolar planets and the structures of systems of planets. Exciting discoveries include the observations that the densities of some small planets are too low for them to have a rocky composition, that structures of planetary systems are often very tightly-packed and very flat, and that planets orbiting binary stars are common.”

Scientists working on Kepler will request an extension of the mission, which is currently funded until late 2012. A longer time base is needed for the photometry on the multiple systems, in order to reduce ambiguities, and to learn more of these fascinating cousins of our solar system.

Dr. Simon Mitton is a fellow of St. Edmund’s College, University of Cambridge, U.K.

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.