Jupiter Analogs Research Articles

No X-rays from WASP-18

About 20% out of the $>1000$ known exoplanets are Jupiter analogs orbiting very close to their parent stars. It is still under debate to what detectable level such hot Jupiters possibly affect the activity of the host stars through tidal or magnetic star-planet interaction. In this paper we report on an 87 ks Chandra observation of the hot Jupiter hosting star WASP-18. This system is composed of an F6 type star and a hot Jupiter of mass $10.4 M_{Jup}$ orbiting in less than 20 hr around the parent star. On the basis of an isochrone fitting, WASP-18 is thought to be 600 Myr old and within the range of uncertainty of 0.5-2 Gyr. The star is not detected in X-rays down to a luminosity limit of $4\times10^{26}$ erg/s, more than two orders of magnitude lower than expected for a star of this age and mass. This value proves an unusual lack of activity for a star with estimated age around 600 Myr. We argue that the massive planet can play a crucial role in disrupting the stellar magnetic dynamo created within its thin convective layers. Another additional 212 X-ray sources are detected in the Chandra image. We list them and briefly discuss their nature.

The 55 Cancri planetary system: fully self-consistent N-body constraints and a dynamical analysis

We present an updated study of the planets known to orbit 55 Cancri A using 1,418 high-precision radial velocity observations from four observatories (Lick, Keck, Hobby-Eberly Telescope, Harlan J. Smith Telescope) and transit time/durations for the inner-most planet, 55 Cancri "e" (Winn et al. 2011). We provide the first posterior sample for the masses and orbital parameters based on self-consistent n-body orbital solutions for the 55 Cancri planets, all of which are dynamically stable (for at least $10^8$ years). We apply a GPU version of Radial velocity Using N-body Differential evolution Markov Chain Monte Carlo (RUN DMC; B. Nelson et al. 2014) to perform a Bayesian analysis of the radial velocity and transit observations. Each of the planets in this remarkable system has unique characteristics. Our investigation of high-cadence radial velocities and priors based on space-based photometry yields an updated mass estimate for planet "e" ($8.09\pm0.26$ M$_\oplus$), which affects its density ($5.51\pm^{1.32}_{1.00}$ g cm$^{-3}$) and inferred bulk composition. Dynamical stability dictates that the orbital plane of planet "e" must be aligned to within $60^o$ of the orbital plane of the outer planets (which we assume to be coplanar). The mutual interactions between the planets "b" and "c" may develop an apsidal lock about $180^o$. We find 36-45% of all our model systems librate about the anti-aligned configuration with an amplitude of $51^o\pm^{6^o}_{10^o}$. Other cases showed short-term perturbations in the libration of $\varpi_b-\varpi_c$, circulation, and nodding, but we find the planets are not in a 3:1 mean-motion resonance. A revised orbital period and eccentricity for planet "d" pushes it further toward the closest known Jupiter analog in the exoplanet population.

THE ANGLO-AUSTRALIAN PLANET SEARCH. XXIII. TWO NEW JUPITER ANALOGS

We report the discovery of two long-period giant planets from the Anglo-Australian Planet Search. HD 154857c is in a multiple-planet system, while HD 114613b appears to be solitary. HD 114613b has an orbital period P=10.5 years, and a minimum mass m sin i of 0.48 Jupiter masses; HD 154857c has P=9.5 years and m sin i=2.6 Jupiter masses. These new data confirm the planetary nature of the previously unconstrained long-period object in the HD 154857 system. We have performed detailed dynamical stability simulations which show that the HD 154857 two-planet system is stable on timescales of at least 100 million years. These results highlight the continued importance of "legacy" surveys with long observational baselines; these ongoing campaigns are critical for determining the population of Jupiter analogs, and hence of those planetary systems with architectures most like our own Solar system.

THE ANGLO-AUSTRALIAN PLANET SEARCH. XXII. TWO NEW MULTI-PLANET SYSTEMS

We report the detection of two new planets from the Anglo-Australian Planet Search. These planets orbit two stars each previously known to host one planet. The new planet orbiting HD 142 has a period of 6005\pm427 days, and a minimum mass of 5.3M_Jup. HD142c is thus a new Jupiter analog: a gas-giant planet with a long period and low eccentricity (e = 0.21 \pm 0.07). The second planet in the HD 159868 system has a period of 352.3\pm1.3 days, and m sin i=0.73\pm0.05 M_Jup. In both of these systems, including the additional planets in the fitting process significantly reduced the eccentricity of the original planet. These systems are thus examples of how multiple-planet systems can masquerade as moderately eccentric single-planet systems.

Detecting the signatures of Uranus and Neptune

With more than 15 years since the first radial velocity discovery of a planet orbiting a Sun-like star, the time baseline for radial velocity surveys is now extending out beyond the orbit of Jupiter analogs. The sensitivity to exoplanet orbital periods beyond that of Saturn orbital radii however is still beyond our reach such that very few clues regarding the prevalence of ice giants orbiting solar analogs are available to us. Here we simulate the radial velocity, transit, and photometric phase amplitude signatures of the Solar System giant planets, in particular Uranus and Neptune, and assess their detectability. We scale these results for application to monitoring low-mass stars and compare the relative detection prospects with other potential methods, such as astrometry and imaging. These results quantitatively show how many of the existing techniques are suitable for the detection of ice giants beyond the snow line for late-type stars and the challenges that lie ahead for the detection true Uranus/Neptune analogs around solar-type stars.

ON THE FREQUENCY OF JUPITER ANALOGS

The Anglo-Australian Planet Search has now accumulated 12 years of radial-velocity data with long-term instrumental precision better than 3 m s−1. In this paper, we expand on earlier simulation work, to probe the frequency of near-circular, long-period gas-giant planets residing at orbital distances of 3–6 AU—the so-called Jupiter analogs. We present the first comprehensive analysis of the frequency of these objects based on radial-velocity data. We find that 3.3% of stars in our sample host Jupiter analogs; detailed, star-by-star simulations show that no more than 37% of stars host a giant planet between 3 and 6 AU.

EXOPLANET ALBEDO SPECTRA AND COLORS AS A FUNCTION OF PLANET PHASE, SEPARATION, AND METALLICITY

First generation optical coronagraphic telescopes will obtain images of cool gas and ice giant exoplanets around nearby stars. The albedo spectra of exoplanets at planet-star separations larger than about 1 AU are dominated by reflected light to beyond 1 {\mu}m and are punctuated by molecular absorption features. We consider how exoplanet albedo spectra and colors vary as a function of planet-star separation, metallicity, mass, and observed phase for Jupiter and Neptune analogs from 0.35 to 1 {\mu}m. We model Jupiter analogs with 1x and 3x the solar abundance of heavy elements, and Neptune analogs with 10x and 30x. Our model planets orbit a solar analog parent star at separations of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative-convective model to compute temperature-pressure profiles. The giant exoplanets are cloud-free at 0.8 AU, have H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and 10 AU. For each model planet we compute moderate resolution spectra as a function of phase. The presence and structure of clouds strongly influence the spectra. Since the planet images will be unresolved, their phase may not be obvious, and multiple observations will be needed to discriminate between the effects of planet-star separation, metallicity, and phase. We consider the range of these combined effects on spectra and colors. For example, we find that the spectral influence of clouds depends more on planet-star separation and hence temperature than metallicity, and it is easier to discriminate between cloudy 1x and 3x Jupiters than between 10x and 30x Neptunes. In addition to alkalis and methane, our Jupiter models show H2O absorption features near 0.94 {\mu}m. We also predict that giant exoplanets receiving greater insolation than Jupiter will exhibit higher equator to pole temperature gradients than are found on Jupiter and thus may have differing atmospheric dynamics.

THE CALIFORNIA PLANET SURVEY. I. FOUR NEW GIANT EXOPLANETS

We present precise Doppler measurements of four stars obtained during the past decade at Keck Observatory by the California Planet Survey (CPS). These stars, namely, HD 34445, HD 126614, HD 13931, and Gl 179, all show evidence for a single planet in Keplerian motion. We also present Doppler measurements from the Hobby-Eberly Telescope (HET) for two of the stars, HD 34445 and Gl 179, that confirm the Keck detections and significantly refine the orbital parameters. These planets add to the statistical properties of giant planets orbiting near or beyond the ice line, and merit follow-up by astrometry, imaging, and space-borne spectroscopy. Their orbital parameters span wide ranges of planetary minimum mass (M sin i = 0.38-1.9 M_Jup), orbital period (P = 2.87-11.5 yr), semi-major axis (a = 2.1-5.2 AU), and eccentricity (e = 0.02-0.41). HD 34445b (P = 2.87 yr, M sin i = 0.79 M_Jup, e = 0.27) is a massive planet orbiting an old, G-type star. We announce a planet, HD 126614Ab, and an M dwarf, HD 126614B, orbiting the metal-rich star HD 126614 (which we now refer to as HD 126614A). The planet, HD 126614Ab, has minimum mass M sin i = 0.38 M_Jup and orbits the stellar primary with period P = 3.41 yr and orbital separation a = 2.3 AU. The faint M dwarf companion, HD 126614B, is separated from the stellar primary by 489 mas (33 AU) and was discovered with direct observations using adaptive optics and the PHARO camera at Palomar Observatory. The stellar primary in this new system, HD 126614 A, has the highest measured metallicity (\feh = +0.56) of any known planet-bearing star. HD 13931b (P = 11.5 yr, M sin i = 1.88 M_Jup, e = 0.02) is a Jupiter analog orbiting a near solar twin. Gl 179b (P = 6.3 yr, M sin i = 0.82 M_Jup, e = 0.21) is a massive planet orbiting a faint M dwarf. The high metallicity of Gl 179 is consistent with the planet-metallicity correlation among M dwarfs, as documented recently by Johnson and Apps.

Three Low‐Mass Planets from the Anglo‐Australian Planet Search

We report the detection of three new low-mass planets from the Anglo-Australian Planet Search. The three parent stars of these planets are chromospherically quiet main-sequence G dwarfs with metallicities ranging from roughly solar (HD 117618 and HD 208487) to metal enriched (HD 102117). The orbital periods range from 20.8 to 130 days, the minimum masses from roughly 0.5MSat to 0.5MJup, and the eccentricities from 0.08 to 0.37, with the planet in the smallest orbit (HD 102117) having the smallest eccentricity. With semiamplitudes of 10.6-19 m s-1, these planets induce Doppler amplitudes similar to those of Jupiter analogs, albeit with shorter periods. Many of the most interesting future Doppler planets will be detected at these semiamplitude levels, placing a premium on measurement precision. The detection of such amplitudes in data extending back 6 yr gives confidence in the Anglo-Australian Planet Search's ability to detect Jupiter analogs as our time baseline extends to 12 yr. We discuss the criticality of such detections for the design of the next generation of extremely large telescopes and also highlight prospects for suitable observing strategies to push to below 1 m s-1 precisions for bright stars in a search for sub-Neptunian planets.

Detectability of extrasolar planets in radial velocity surveys

Radial velocity surveys are beginning to reach the time baselines required to detect Jupiter analogs, as well as sub-Saturn mass planets in close orbits. Therefore it is important to understand the sensitivity of these surveys at long periods and low amplitudes. In this paper, I derive analytic expressions for the detectability of planets at both short and long periods, for circular and eccentric orbits. I suggest an extension of the Lomb-Scargle periodogram for Keplerian orbits, and describe how to estimate the false alarm probability associated with a Keplerian fit. Using this to investigate the detectability of eccentric orbits shows that there are significant selection effects against eccentric orbits for e>0.6, and the small number of highly eccentric planets discovered so far may reflect this. Finally, I present a Bayesian approach to the periodogram, which emphasises the equivalence of least squares and periodogram techniques.

A Planet at 5 AU around 55 Cancri

We report precise Doppler shift measurements of 55 Cancri (G8V) obtained from 1989 to 2002 at Lick Observatory. The velocities reveal evidence for an outer planetary companion to 55 Cancri orbiting at 5.5 AU. The velocities also confirm a second, inner planet at 0.11 AU. The outer planet is the first extrasolar planet found that orbits near or beyond the orbit of Jupiter. It was drawn from a sample of ~50 stars observed with sufficient duration and quality to detect a giant planet at 5 AU, implying that such planets are not rare. The properties of this jupiter analog may be compared directly to those of the Jovian planets in our Solar System. Its eccentricity is modest, e=0.16, compared with e=0.05 for both Jupiter and Saturn. Its mass is at least 4.0 jupiter masses (M sin i). The two planets do not perturb each other significantly. Moreover, a third planet of sub-Jupiter mass could easily survive in between these two known planets. Indeed a third periodicity remains in the velocity measurements with P = 44.3 d and a semi-amplitude of 13 m/s. This periodicity is caused either by a third planet at a=0.24 AU or by inhomogeneities on the stellar surface that rotates with period 42 d. The planet interpretation is more likely, as the stellar surface is quiet, exhibiting log(R'_{HK}) = -5.0 and brightness variations less than 1 millimag, and any hypothetical surface inhomogeneity would have to persist in longitude for 14 yr. Even with all three planets, an additional planet of terrestrial--mass could orbit stably at ~1 AU. The star 55 Cancri is apparently a normal, middle-aged main sequence star with a mass of 0.95 solar masses, rich in heavy elements ([Fe/H] = +0.27). This high metallicity raises the issue of the relationship between its age, rotation, and chromosphere.