BD+20594b is a Neptune-Sized Mega-Earth

Espinoza et al. (2016) present the discovery of BD+20594b, a Neptune-sized mega-Earth in orbit around a Sun-like star with ~96 percent the mass, ~93 percent the size and ~88 percent the luminosity of the Sun. The planet’s density is measured to be high enough for it to be consistant with a pure rock composition. BD+20594b circles its host star every 41.7 days. It was first detected by NASA’s K2 mission, which looks for planets that transit their host stars. The amount of light BD+20594b blocks when it transits its host star indicates that it is ~2.23 times the size of Earth.

Figure 1: Artist’s impression of a rocky planet.

Subsequent high precision radial velocity measurements obtained from the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the 3.6 m telescope at La Silla Observatory in Chile show that gravitational perturbations from BD+20594b causes its host star to wobble by ~3.1 m/s. This allows the mass of BD+20594b to be estimated at ~16.3 times the mass of Earth. With both the size and mass known, the density of BD+20594b is found to be ~7.89 g/cm³. For comparison, the mean density of Earth is 5.514 g/cm³. Such a high density indicates that BD+20594b has a high probability of being a rocky planet even when considering uncertainties of a few Earth-masses in its mass measurement.

BD+20594b is a rare kind of planet because planets of that size tend to have low densities and are composed of large amounts of volatiles. In terms of mass, size and orbital period, the planet that is most similar to BD+20594b is Kepler-10c. Kepler-10c has ~17.2 times the mass of Earth, ~2.35 times the size of Earth and a 45.3 day orbital period. Being a rocky world with many times the mass of Earth, the surface gravity of BD+20594b is expected to be relatively high. On the surface of BD+20594b, the gravitational attraction is more than 3 times that on Earth, and the escape velocity is ~30 km/s. BD+20594b is also quite close to its host star. The estimated temperature on BD+20594b is ~546 K assuming the planet has zero reflectivity, and ~386 K assuming a 75 percent reflectivity.

Figure 2: Phase-folded K2 transit light curve indicating the presence of BD+20594b. Espinoza et al. (2016)

Figure 3: Phase-folded HARPS radial velocity curve indicating the presence of BD+20594b. Espinoza et al. (2016)

Reference:
Espinoza et al. (2016), “A Neptune-sized Exoplanet Consistent with a Pure Rock Composition”, arXiv:1601.07608 [astro-ph.EP]

The Properties of Two Bloated Hot-Jupiters

Figure 1: Artist’s impression of a gas giant planet.

Lillo-Box et al. (2016) present the discovery of two bloated hot-Jupiters in close-in orbits around stars similar to the Sun. The two hot-Jupiters are identified as EPIC210957318b and EPIC212110888b, hereafter referred to as EPIC318b and EPIC888b. Both planets were first detected using data from NASA's K2 mission which hunts for planets around other stars via the transit method. Based on how much light from the host star each planet obscures as it transits, EPIC318b and EPIC888b were found to have, respectively, 1.196 ± 0.060 and 0.0905 ± 0.0017 times the radius of Jupiter. Subsequent ground-based radial velocity measurements with SOPHIE, HARPS-N and CAFE indicate that EPIC318b and EPIC888b have 0.623 ± 0.031 and 1.76 ± 0.13 times the mass of Jupiter, respectively.

With the sizes and masses known, the densities of EPIC318b and EPIC888b are 0.364 ± 0.058 and 0.71 ± 0.11 times the average density of Jupiter, respectively. The relatively low densities indicate that both planets are somewhat inflated. Both EPIC318b and EPIC888b orbit very close to their host stars. The orbital period of EPIC318b is 4.099 days, while the orbital period of EPIC888b is 2.996 days. The close proximity to their host stars means that EPIC318b is heated to almost ~1200 K, while EPIC888b is heated to temperatures exceeding ~1700 K. The inflated radii of EPIC318b and EPIC888b is probably due to the high amount of insolation they receive from their host stars, although tidal heating can also play a significant role.

Figure 2: Transit light curve (top) and radial velocity curve (bottom) indicating the presence of EPIC318b. Lillo-Box et al. (2016)

Figure 3: Transit light curve (top) and radial velocity curve (bottom) indicating the presence of EPIC888b. Lillo-Box et al. (2016)

Reference:
Lillo-Box et al. (2016), "EPIC210957318b and EPIC212110888b: two inflated hot-Jupiters around Solar-type stars", arXiv:1601.07635 [astro-ph.EP]

Discovery of a Large and Diffuse Dwarf Galaxy

Torrealba et al. (2016) present the discovery of a dwarf galaxy identified as Crater 2. It is a satellite galaxy of the Milky Way that is estimated to lie ~380,000 light years from the Sun. The half-light radius of Crater 2 is ~3500 light years in size. A dwarf galaxy does not have a clear boundary since the density of stars gradually decreases towards the outer regions. For this reason, the size of a dwarf galaxy is measured by its half-light radius, which is basically the distance from the center of the dwarf galaxy, whereby within this radius, half the total brightness of the galaxy is emitted. In fact, the sizes of other types of stellar systems such as globular clusters are also denoted by the half-light radius.

Crater 2 has an extremely low surface brightness of only ~31 mag/arcsec². The discovery of Crater 2 indicates that a substantial number of extremely low surface brightness dwarf galaxies have yet to be detected. Crater 2 is a remarkable dwarf galaxy because it is the largest known ultra-faint satellite galaxy of the Milky Way and the fourth largest satellite galaxy, surpassed only by the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) and the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph).

Crater 2 appears to be in alignment with the globular cluster Crater, the pair of ultra-faint dwarf galaxies Leo IV and Leo V, and the dwarf galaxy Leo II. The alignment seems to be statistically significant enough to suggest that the Leo-Crater group was once a more cohesive stellar system that has since dissipated into a stream of multiple stellar systems due to tidal disruption from the massive Milky Way galaxy.

This diagram shows absolute magnitude versus half-light radius, whereby dwarf galaxies that are satellites of the Milky Way are denoted by red open circles, dwarf galaxies that are satellites of the Andromeda galaxy are denoted by black unfilled triangles, and other nearby galaxies are denoted by gray crosses. Black dots indicate globular clusters of the Milky Way and grey dots indicate extended objects with half-light radii smaller than ~325 light years. Crater 2 is marked with a filled red circle. The black solid line and the black dashed line, respectively, correspond to the surface brightness levels of 31 and 30 mag/arcsec². Torrealba et al. (2016)

Reference:
Torrealba et al. (2016), “The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater”, arXiv:1601.07178 [astro-ph.GA]

Red Giant Stars Hosting Close-In Jupiter-Like Planets

As a planet orbits its host star, it indices gravitational perturbations on its host star, causing its host star to wobble back and forth. If the planet is massive enough and/or if the planet is sufficiently close to its host star, the wobbling motion of the planet’s host star can be detected through radial velocity measurements. Radial velocity measurements of two red giant stars indicate that they harbour Jupiter-like planetary companions. Red giant stars are basically stars that have reached the end stages of their stellar evolution. Jupiter-like planets in close-in orbits seem to be very rare around red giant stars. One explanation is that such planets are ingested by their host stars due to tidal interactions.

HD 5583 is a red giant star with ~1.01 times the Sun’s mass, ~9.09 times the Sun’s radius, ~41 times the Sun’s luminosity, and it has an effective temperature of roughly 4830 K. It is located ~720 light years away and it hosts a gas giant planet identified as HD 5583 b. The planet has 5.78 ± 0.53 times the mass of Jupiter and it orbits in a near-circular orbit around its host star at 0.53 ± 0.02 AU. Its orbital period around its host star is ~139 days and the amount of wobbling it induces on its host star is ~226 m/s.

BD+15 2375 is a red giant star with ~1.08 times the Sun’s mass, ~8.95 times the Sun’s radius, ~37 times the Sun’s luminosity, and it has an effective temperature of roughly 4649 K. The star is located ~2520 light years away and circling it is a gas giant planet identified as BD+15 2375 b. Radial velocity measurements indicate the planet has ~1.06 times the mass of Jupiter and it is in an almost circular orbit around its host star at ~0.58 AU. The planet’s orbital period is ~153 days and it causes its host star to wobble by ~38.3 m/s. BD+15 2375 b is currently the lightest known planet around a red giant star.

Reference:
Niedzielski et al. (2016), “Tracking Advanced Planetary Systems (TAPAS) with HARPS-N. III. HD 5583 and BD+15 2375 - two cool giants with warm companions”, arXiv:1601.06832 [astro-ph.EP]

The Most Widely-Separated Star-Planet System

TYC9486-927-1 and 2MASS J2126-8140 are two previously known objects in the Sun’s stellar neighbourhood. TYC9486-927-1 is a red dwarf star and 2MASS J2126-8140 is a low-gravity substellar object. Both objects are relatively young. A new study shows that the motion of both objects through space reveals that they are likely bound. New observations also indicate that TYC9486-927-1 is 10 to 45 million years old. Assuming that both objects formed together, at the same time, the mass of 2MASS J2126-8140 is estimated to be 11.6 to 15.0 times the mass of Jupiter. This places 2MASS J2126-8140 in the planetary-mass regime.

The low-gravity of 2MASS J2126-8140 indicates that the object is somewhat inflated, and still in the process of cooling and contracting in size. The separation of 2MASS J2126-8140 from its host star, TYC9486-927-1, is estimated to be over 4500 AU. This gives 2MASS J2126-8140 the wides orbit known for any planetary-mass object. So far, only a handful of such objects have been discovered. Some of these objects include WD 0806-661B, a planetary-mass object with 6 to 9 times the mass of Jupiter at 2500 AU from its host star; and GU Psc b, a planetary-mass object with 9 to 12 times the mass of Jupiter at 2000 AU from its host star.

Deacon et al. (2016), “A nearby young M dwarf with a wide, possibly planetary-mass companion”, arXiv:1601.06162 [astro-ph.EP]

The Free-Floating Planetary-Mass Object PSO J318.5-22

As object with less than ~75 times the mass of Jupiter is referred to as a substellar object as it is not massive enough to burn hydrogen in its core and shine as a star. Since substellar objects cool as they age, it sets up a mass-age-luminosity degeneracy whereby a massive and old brown dwarf can have the same luminosity as a young planetary-mass object. In order to determine the mass for a substellar object of a given luminosity, the object’s age has to be constrained.

One way to determine the age of a substellar object is if the object is associated with a co-moving group of stars that were once part of the same natal cluster. PSO J318.5-22 is a planetary-mass object that has been identified to be a member of the β Pictoris Moving Group, a co-moving group of stars estimated to be 23 ± 3 million years old. Allers et al. (2016) present new observations on the physical properties of PSO J318.5-22. Measurements of the motion of PSO J318.5-22 through space indicate that it has a 99.98 percent probability of being a member of the β Pictoris Moving Group.

Assuming an age of 23 ± 3 million years, PSO J318.5-22 is estimated to have 8.3 ± 0.5 times the mass of Jupiter, an effective temperature of about 1127 K, and ~1.46 times the size of Jupiter. Interestingly, the gravity on PSO J318.5-22 is only slightly stronger than the surface gravity on Earth. Observations also indicate that the rotation period of PSO J318.5-22 is 5 to 10.2 hours, somewhat longer than the typical ~3 hour rotation periods of substellar objects that are similar in temperature to PSO J318.5-22. This is expected as PSO J318.5-22 is a young object and is still in the process of contracting.

Reference:
Allers et al. (2016), “The Radial and Rotational Velocities of PSO J318.5338-22.8603, a Newly Confirmed Planetary-Mass Member of the β Pictoris Moving Group”, arXiv:1601.04717 [astro-ph.SR]

A Hierarchical-Triple System of Brown Dwarfs

VHS 1256-1257 is a hierarchical-triple system whose three components are all substellar in nature. This means all three components are below the minimum mass needed to sustain hydrogen fusion to shine as stars. In VHS 1256-1257, the “A” and “B” components are being orbited by the more distant “b” component.

If VHS 1256-1257 is assumed to be ~41 light years away, then the “A” and “B” components each have ~64.6 times the mass of Jupiter and orbit one another every 5.87 ± 2.7 years, while the more distant “b” component has ~11.2 times the mass of Jupiter. If the system is assumed to be ~56 light years away, then the “A” and “B” components each have ~73 times the mass of Jupiter and orbit one another every 8.7 ± 4.3 years, while the “b” component has ~35 times the mass of Jupiter

The configuration of VHS 1256-1257 resembles hierarchical-triple star systems. This suggests VHS 1256-1257 formed via the extension of the star formation process down to lower masses. VHS 1256-1257 is the third known triple system whose components are all substellar in nature.

Reference:
Stone et al. (2016), “Adaptive Optics imaging of VHS 1256-1257: A Low Mass Companion to a Brown Dwarf Binary System”, arXiv:1601.03377 [astro-ph.SR]

Gas Giant Planets in Orbital Resonance

Figure 1: Artist’s impression of a gas giant planet.

HD 33844 is a metal-rich, evolved giant star with almost twice the Sun’s metallicity, 1.78 times the Sun’s mass and 5.29 times the Sun’s radius. The effective temperature of HD 33844 is 4861 K, much cooler than the Sun. Nevertheless, due to the large physical size of the star, HD 33844 has 14 times the Sun’s luminosity. HD 33844 hosts a planetary system consisting of two gas giant plants that are each more massive than Jupiter. Both planets were found using the radial velocity method which involves measuring the “wobbling” motion of the host star due to the presence of the two planets.

The two planets are identified as HD 33844b and HD 33844c, and they induce radial velocity amplitude of 33.5 ± 2.0 m/s and 25.4 ± 2.9 m/s on their host star, respectively. HD 33844b has at least 1.96 ± 0.12 times the mass of Jupiter and it orbits its host star at 1.60 ± 0.02 AU. HD 33844c has at least 1.75 ± 0.18 times the mass of Jupiter and it orbits its host star at 2.24 ± 0.05 AU. The orbital periods of HD 33844b and HD 33844c are 551.4 days and 916.0 days, respectively. Both HD 33844b and HD 33844c are likely trapped in a 3:5 orbital resonance. For every 3 times HD 33844c goes around its host star, HD 33844b goes around 5 times.

Figure 2: Radial velocity curve indicating the presence of the inner planet HD 33844b. Wittenmyer et al. (2015)

Figure 3: Radial velocity curve indicating the presence of the outer planet HD 33844c. Wittenmyer et al. (2015)

Reference:
Wittenmyer et al. (2015), “The Pan-Pacific Planet Search. IV. Two super-Jupiters in a 3:5 resonance orbiting the giant star HD 33844”, arXiv:1512.07316 [astro-ph.EP]

Brown Dwarf with a Co-Moving Group of Stars

Brown dwarfs are objects with masses that span the mass regime between planets and stars - roughly 13 to 75 times the mass of Jupiter. Since brown dwarfs do not have sufficient mass to sustain hydrogen fusion, they slowly cool and dim from the moment they are born. W2319+7645 is a newly discovered brown dwarf located ~85 light years away and its mass could be in the planetary-mass regime. There is a 79 percent probability W2319+7645 is part of the Argus co-moving group of stars and a 21 percent probability it is just a field object. Since a more massive brown dwarf takes a longer time to cool (i.e. becomes less luminous) than a less massive brown dwarf, knowing a brown dwarf’s age and luminosity can allow its mass to be estimated.

Young brown dwarfs associated with co-moving groups of stars can have their ages estimated more accurately than young brown dwarfs in the field. Assuming W2319+7645 is part of the Argus co-moving group of stars, then its age is somewhere between 30 to 50 million years. Furthermore, the distance and observed brightness of W2319+7645 allow its luminosity to be determined and the effective temperature of W2319+7645 is estimated to be 1800 ± 200 K. Based on its age and luminosity, the mass of W2319+7645 is estimated to be 12.1 ± 0.4 times the mass of Jupiter, placing it in the planetary-mass regime.

Reference:
Castro & Gizis (2015), “Discovery of an L4β Candidate Member of Argus in the Planetary Mass Regime: WISE J231921.92+764544.4”, arXiv:1512.06524 [astro-ph.SR]

Oxygen Abundance of the Dwarf Galaxy Leo P

Regions of star formation are usually identified by the presence of massive stars. The reason is because massive stars live bright and die young, which means they do not travel far from their place of birth. Heavy elements such as oxygen are created via nuclear fusion in the cores of massive stars. When these stars end their lives in supernova explosions, the heavy elements get ejected out into interstellar space. Unlike a large galaxy like the Milky Way, a dwarf galaxy has a weaker gravitational field. As a result, heavy elements ejected from supernova explosions tend to get pushed out of a dwarf galaxy. This makes dwarf galaxies less efficient at retaining heavy elements, resulting in dwarf galaxies having lower metallicities (i.e. low abundance of elements heavier than hydrogen and helium) compared to more massive galaxies.

Figure 1: Artist’s impression of a galaxy.

However, external processes can also remove heavy elements from a dwarf galaxy. For example, a dwarf galaxy can become tidally-disrupted when it passes too close to a massive galaxy. A dwarf galaxy can also be stripped of its store of heavy elements via ram pressure stripping when it moves through the intracluster medium in the vicinity of a massive galaxy. These external processes make it difficult to study the feedback star formation has on the expulsion of heavy elements from a dwarf galaxy. Fortunately, Leo P is an isolated, gas-rich dwarf galaxy located ~5 million light years away, far from the influence of any massive galaxy. Leo P is estimated to contain ~560,000 times the mass of the Sun in the form of stars.

Using oxygen as a tracer for heavy elements, observations of Leo P show that it has lost ~95 percent of its oxygen. Of the ~5 percent of oxygen that remains, one quarter is locked in stars and the remaining three-quarters is distributed in the gas phase through the interstellar medium of the dwarf galaxy. For comparison, non-isolated dwarf galaxies similar in mass to Leo P have lost ~99 percent of their heavy elements. This shows that feedback from star formation, in the form of supernova explosions, can effectively remove most of the heavy elements in a dwarf galaxy, and external processes such as tidal-stripping or ram pressure stripping remove only a few addition percent of heavy elements. Besides, if only the oxygen locked in stars is counted for Leo P, then its abundance of oxygen is similar to the abundance of heavy elements for the non-isolated dwarf galaxies. 

Figure 2: The percentage of oxygen lost in Leo P compared to the percentage of heavy elements lost in three satellite dwarf galaxies of the Milky Way. Leo P has retained its gas content and thus a larger fraction of oxygen. If the oxygen atoms in the gas phase are ignored, the percentage of oxygen lost is comparable to the three other dwarf galaxies.

Reference:
McQuinn et al. (2015), “Leo P: How Many Metals can a Very Low-Mass, Isolated Galaxy Retain”, arXiv:1512.00459 [astro-ph.GA]

Off-Center Supermassive Black Hole in the Heart of M87

M87 is a giant elliptical galaxy hosting a supermassive black hole (SMBH) with a few billion times the mass of the Sun. Using data from the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST), Batcheldor et al. (2010) found that the SMBH in the heart of M87 is displaced by ~22 light years from the photo-center of the galaxy. The two most likely explanations for the displacement are acceleration of the SMBH by an asymmetric jet, or the gravitational “kick” resulting from the merger of a SMBH binary.

If the present SMBH is the outcome of a merger between two black holes, the merger process can cause the resulting SMBH to receive a “kick” due to the emission of gravitational waves. The “kick” can displace the SMBH from the center of M87. Depending on the merger process, the SMBH can remain displaced from the center of the galaxy for millions to billions of years.

The nucleus of M87 has a highly-collimated, relativistic jet of matter extending outwards for thousands of light years. This jet is made up of material ejected from the galaxy by the SMBH. Since the displacement of the SMBH from the center of M87 is in the counter-jet direction, acceleration of the SMBH by an asymmetric jet (i.e. jet is stronger in one direction) can result in the observed displacement if the jet is long-lasting and if the restoring force exerted by the galaxy on the SMBH is small.

Reference:
Batcheldor et al. (2010), “A Displaced Supermassive Black Hole in M87”, arXiv:1005.2173 [astro-ph.CO]

Lakes on Titan’s Southern Mid-Latitudes

Observations of Titan by the Visual and Infrared Mapping Spectrometer (VIMS) on NASA’s Cassini spacecraft reveal the presence of albedo-dark features on the southern mid-latitudes that could potentially be temperate lakes. If the albedo-dark features are indeed temperate lakes, they will be the first known persistent bodies of surface liquid on Titan that lie outside the areas southwards of 70°S latitude and northwards of 53°N latitude. The albedo-dark features are located near 40°S latitude and they are consistent with the presence of two potential lakes identified as Sionascaig Lacus and Urmia Lacus.

Figure 1: Image of Titan taken by NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/Space Science Institute.

Both Sionascaig Lacus and Urmia Lacus have lacustrine (i.e. associated with lakes) morphologies. For example, the surface of a lake is usually darker than its surrounding terrain. This is seen for both Sionascaig Lacus and Urmia Lacus, whereby the light-dark boundary outlining each potential lake is continuous, and appears quite similar to previously confirmed lakes on Titan, such as Ontario Lacus.

At the cold cryogenic temperatures on Titan, the seas and lakes on Titan are comprised of liquid hydrocarbons, mainly methane and ethane. When observed in the infrared, the surfaces of these seas and lakes have very low albedos (i.e. they appear very dark) because methane and ethane are strong absorbers in the 5 μm waveband. Sionascaig Lacus and Urmia Lacus are consistent with being bodies of liquid hydrocarbons due to their very low albedos. The surface albedos of Sionascaig Lacus and Urmia Lacus are 0.0070 and 0.0081, respectively. For comparison, the surface albedos of Kraken Mare and Ligeia Mare, two large northern seas on Titan, are 0.0114 and 0.0050 to 0.0089, respectively.

Figure 2: Artist’s impression of a lake on Titan.

The terrain surrounding Sionascaig Lacus and Urmia Lacus also exhibit low surface albedos, albeit not as low as the surfaces of the two potential lakes. It appears that the surrounding terrain could be interpreted as a wetland, with the ground soaked in liquid hydrocarbons. Sionascaig Lacus is by far the larger of the two potential lakes. Its surface area is estimated to be ~5000 km² and its average depth is estimated to be around 40 to 70 m. Sionascaig Lacus is also estimated to hold roughly 100 billion tons of liquid methane. Clouds have been observed on Titan’s southern mid-latitudes. The presence of temperate lakes may be driving cloud formation or be the result of cloud activity (i.e. precipitation); most likely both.

Reference:
G. Vixie et al. (2015), “Possible temperate lakes on Titan”, Icarus Volume 257, 1 September 2015, Pages 313 to 323

The Diverse Densities of Ten Kepler Exoplanets

Figure 1: Artist’s impression of a pair of exoplanets.

Measuring a planet’s size and mass is essential in determining the planet’s density and hence its bulk composition. The masses of most of the small planets detected by NASA’s Kepler space telescope are beyond the sensitivity of radial velocity (RV) measurements. As a result, another technique known as transit timing variations (TTV) is required to determine the masses of these small planets. TTV involves precisely measuring the gravitational perturbation one planet has on another in the same planetary system with two or more planets. The gravitational perturbation shows up as a change in the transit timing of the planet (i.e. the planet transits its host star slightly earlier or slightly later).

From the analysis of TTV data, Jontof-Hutter et al. (2015) present mass measurements of 10 exoplanets that range from super-Earth-size to Neptune-size. All 10 planets are below 8 Mᴇ, whereby Mᴇ denotes the mass of Earth. Although their radii range from 1.31 (Kepler-105c) to 3.35 (Kepler-29b) times the radius of Earth, they span over an order of magnitude in density, indicating a huge compositional diversity. The densest of the 10 planets, Kepler-105c, has a density that is consistent with an Earth-like rocky composition. The other planets have lower densities that are consistent with mixtures of rock and ice, or rock and gas. None of the 10 planets are habitable as they receive between a few times to a few hundred times the amount of flux Earth gets from the Sun.

Figure 2: Parameters of the 10 planets. The mass, radius, and flux are in units of the mass and radius of Earth, and in units of the amount of flux Earth gets from the Sun. Jontof-Hutter et al. (2015)

Figure 3: Mass-radius diagram of super-Earth-mass exoplanets from 1 to 4 times the radius of Earth, compared to theoretical curves of pure water ice, silicate rock and iron. The 10 well-characterized exoplanets below 8 Mᴇ are new additions to the planetary mass-radius diagram, and they are represented by open circles and orange error bars. Despite a small range in mass, they span a wide range in density and hence bulk composition. Jontof-Hutter et al. (2015)

Reference:
Jontof-Hutter et al. (2015), “Robust TTV Mass Measurements: Ten Kepler Exoplanets between 3 and 8 Earth Masses with Diverse Densities and Incident Fluxes”, arXiv:1512.02003 [astro-ph.EP]

HATS-17b is a Warm-Jupiter Denser than Aluminium

Figure 1: Artist’s impression of a gas giant planet.

HATS-17b is a warm Jupiter-mass planet in a circular orbit around a Sun-like star. Every 16.255 days, HATS-17b transits its host star. The host star of HATS-17b has ~1.13 times the mass and ~1.09 times the radius of the Sun. Additionally, the host star’s surface temperature is estimated to be 5846 ± 78 K and the luminosity of the host star is ~25 percent grater than the Sun’s. By measuring how much light the plant blocks when it transits its host star, the radius of HATS-17b is estimated to be 0.777 ± 0.056 times the radius of Jupiter.

Subsequent radial velocity measurements indicate that HATS-17b has 1.338 ± 0.065 times the mass of Jupiter. With the size and mass known, the density of HATS-17b is found to be ~3.50 g/cm³, roughly 30 percent denser than aluminium. The density of HATS-17b is remarkably high for an object with its mass. For comparison, the mean density of Jupiter is 1.326 g/cm³.

Figure 2: Transit light curve indicating the presence of HATS-17b. Brahm et al. (2015)

Figure 3: Radial velocity curve indicating the presence of HATS-17b. The lower panel shows the residuals from the best-fit model. Brahm et al. (2015)

Gas giant planets are typically comprised almost entirely of hydrogen and helium, with heavier elements making up just a small proportion of the mass. This is not the case for HATS-17b. Among gas giant planets with less than twice the mass of Jupiter, HATS-17b is currently the densest known. The compact nature of HATS-17b means that the gravity on HATS-17b is almost 6 times the surface gravity on Earth.

Interior models of HATS-17b suggest that ~50 percent of the planet’s mass is comprised of a massive core of heavier elements. This works out to be ~200 times the mass of Earth. The massive core of HATS-17b is consistent with the high metallicity of its parent star. A star’s metallicity basically refers to its abundance of elements heavier than hydrogen and helium. The host star of HATS-17b has roughly twice the metallicity of the Sun and a more metal-rich protoplanetary disk can form massive cores more efficiently.

HATS-17b is the first warm Jupiter-mass planet detected by the HATSouth network - a network of 6 telescopes in South America, Africa, and Australia. The orbital period of HATS-17b is currently the longest known for any transiting planet detected from the ground. HATS-17b orbits its host star at a distance of 0.131 AU (i.e. 19.6 million km). At that distance, the level of irradiation HATS-17b receives from its host star is over 70 times more than what Earth receives from the Sun, and the temperature on HATS-17b is estimated to be 814 ± 25 K. HATS-17b belongs to the category of gas giant planets known as warm-Jupiters since it is still not hot enough to be considered a hot-Jupiter.

Figure 4: Left panel: mass-radius diagram of the known transiting gas giant planets. HATS-17b is the one of the smallest known gas giant planet for its mass. Right panel: density of gas giant planets as function of the planetary mass. HATS-17b lies at the upper envelope of this distribution. Black circles indicate gas giant planets with irradiation levels high enough for them to be classified as hot-Jupiters, while orange circles correspond to planets receiving lower levels of irradiation. Brahm et al. (2015)

Reference:
Brahm et al. (2015), “HATS-17b: A Transiting Compact Warm Jupiter in a 16.3 Days Circular Orbit”, arXiv:1510.05758 [astro-ph.EP]

HD 85512b is a Potentially Habitable Super-Earth

HD 85512b is a potentially habitable super-Earth with at least 3.6 times the mass of Earth. Its host star is a K5V star that is less massive and much less luminous than the Sun. A planet around such a star has to orbit much closer in than Earth is from the Sun in order to receive a similar level of insolation as what Earth gets from the Sun. HD 85512b orbits its host star at 0.26 AU and its orbital period is 58.4 days.

Figure 1: Artist’s impression of a potentially habitable planet.

The distance of HD 85512b from its host star places it within the habitable zone. The habitable zone is a range of distances from a star where it is neither too hot nor too cold for a planet to sustain liquid water on its surface, and possibly even life. The estimated equilibrium temperature on HD 85512b is 298 K, or 25°C. Although HD 85512b is within the habitable zone of its host star, it is closer to the warm inner edge of the habitable zone.

HD 85512b was detected through radial velocity measurements by the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph. The planet gravitationally tugs its host star, causing its host star to wobble by as much as 0.769 ± 0.090 m/s. The strength and periodicity of this wobbling allows the mass and orbital period of the planet to be determined.

Figure 2: Phase-folded radial velocity curve indicating the presence of the potentially habitable super-Earth HD 85512b. F. Pepe et al. (2011)

Reference:
F. Pepe et al. (2011), “The HARPS search for Earth-like planets in the habitable zone: I -- Very low-mass planets around HD 20794, HD 85512 and HD 192310”, arXiv:1108.3447 [astro-ph.EP]

HD 20794 is Host to Three Rocky Super-Earths

HD 20794 is a Sun-like star located ~20 light years away. It has 70 percent the Sun’s mass and 0.656 ± 0.003 times the Sun’s luminosity. Its surface temperature is estimated to be 5401 ± 17 K and it is a middle-age star about 5.76 ± 0.66 billion years old. High precision radial velocity observations of HD 20794 by the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph reveal the presence of three planets orbiting the star. The three planets are identified as HD 20794b, HD 20794c, and HD 20794d; hereafter referred to as planets “b”, “c”, and “d”.

Figure 1: Artist’s impression of a rocky planet.

Gravitation tugging from planets “b”, “c”, and “d” cause their host star to wobble by 0.83 ± 0.09 m/s, 0.56 ± 0.10 m/s, and 0.85 ± 0.10 m/s, respectively. The strength and periodicity of the wobbling indicates that planets “b”, “c”, and “d” have 2.68, 2.38, and 4.73 times the mass of Earth, and their orbital periods are 18.3, 40.1, and 90.3 days, respectively. Furthermore, planets “b”, “c”, and “d” orbit 0.12 AU, 0.20 AU, and 0.35 AU from their host star, and their estimated equilibrium temperatures are 660 K, 508 K, and 388 K, respectively. All three planets around HD 20794 have masses in the super-Earth regime, and they are probably rocky worlds.

Figure 2: Phase-folded radial velocity curve for the three planetary components “b”, “c”, and “d” around HD 20794. F. Pepe et al. (2011)

Reference:
F. Pepe et al. (2011), “The HARPS search for Earth-like planets in the habitable zone: I -- Very low-mass planets around HD 20794, HD 85512 and HD 192310”, arXiv:1108.3447 [astro-ph.EP]

Forming the Five Small Rocky Worlds of Kepler-444

Kepler-444 is a planetary system hosting five sub-Earth-sized planets in orbit around a metal-poor Sun-like star. The five planets are in an extremely compact configuration, with the outermost planet orbiting at a distance of only 0.081 AU. For comparison, the average distance of Mercury from the Sun is 0.387 AU. The host star of the Kepler-444 planetary system is estimated to have 0.758 ± 0.043 times the mass and 0.752 ± 0.014 times the radius of the Sun.

Figure 1: Artist’s impression of a rocky planet.

Not far from Kepler-444, at a projected separation of 66 AU is a tightly-bound pair of red dwarf stars identified as Kepler-444BC. Both red dwarf stars are separated from each other by no more than ~0.3 AU and the red dwarf stars are estimated to have 0.29 ± 0.03 and 0.25 ± 0.03 times the Sun’s mass, respectively. If Kepler-444BC orbits Kepler-444 in a circular orbit at 66 AU, then the orbital motion of Kepler-444BC is expected to be 4.2 km/s. In terms of astrometric motion, this is 25 milli-arcseconds per year. An arcsecond is a unit of measurement for angular separation, whereby 60 arcseconds constitute an arcminute and 60 arcminutes constitute a degree.

However, measurements of the astrometric motion of Kepler-444BC indicate a value of only 1.0 ± 0.3 milli-arcseconds per year. This implies that either Kepler-444BC is near the furthest end of a highly eccentric orbit around Kepler-444, or the actual distance of Kepler-444BC from Kepler-444 is much further than the projected separation of 66 AU. Radial velocity measurements show that Kepler-444 is gravitationally “tugged” too strongly by Kepler-444BC and this makes the large separation scenario unlikely.

Kepler-444BC is in a highly eccentric orbit around Kepler-444. The orbital eccentricity is estimated to be 0.864 ± 0.023 and the orbital period is roughly 200 years. The highly eccentric orbit of Kepler-444BC brings it as close as ~5 AU to the planetary system around Kepler-444. Dynamical analysis indicates that the close passage of Kepler-444BC will not disrupt the planetary system around Kepler-444.

The orbit of Kepler-444BC is expected to be primordial, already in place before or during the epoch of planet formation. As a result, the protoplanetary disk around Kepler-444 is expected to have been truncated to within ~2AU due to the close passage of Kepler-444. Furthermore, Kepler-444 is a metal-poor star with a low abundance of heavy elements. These factors severely deplete the amount of solid material available for planet formation and can explain why the five planets around Kepler-444 are less massive than Earth.

All five planets around Kepler-444 have a total mass of only ~1.5 times the mass of Earth. Kepler-444 appears to be a hostile environment for the formation of planets given the low metallicity of its host star and the disrupting presence of the nearby Kepler-444BC. Yet the presence of five sub-Earth-sized planets seems to suggest that the formation process of such planets is quite robust.

Figure 2: The orbit of Kepler-444BC in the frame of the host star (black star) of the Kepler-444 planetary system. The best-fit orbit is shown in black, and 100 randomly drawn orbits from the analysis are shown in gray. Orbit locations that correspond to the range of the observation epochs are shown in red. Left: the orbit in plane of the sky, which is consistent with being seen edge on. Right: the same orbit shown in a top down view of the orbital plane. Kepler-444BC is currently close to the furthest end of its highly eccentric orbit, with almost no motion in the plane of the sky. Dupuy et al. (2015)

Reference:
Dupuy et al. (2015), “Orbital Architectures of Planet-Hosting Binaries: I. Forming Five Small Planets in the Truncated Disk of Kepler-444A”, arXiv:1512.03428 [astro-ph.EP]

Five Small Rocky Planets Circling an Ancient Star

Figure 2: Artist’s impression of a rocky exoplanet.

Over the years, observations have shown that small planets are common around stars, regardless of whether the star is metal-poor or metal-rich. In contrast, gas giant planets tend to be more abundant only around metal-rich stars. Kepler-444 is a planetary system that is home to five small rocky planets that orbit around a metal-poor Sun-like star. The host star of this planetary system is estimated to have formed 11.2 ± 1.0 billion years ago, when the Universe was less than 20 percent its current age. This makes Kepler-444 the oldest known planetary system hosting rocky planets and it is a good indication that planets, especially small rocky planets, can readily form throughout most of the Universe’s 13.8 billion year history. Interestingly, at the time when Earth formed, Kepler-444 and its system of rocky planets were already older than Earth is today.

Kepler-444 is located at a distance of approximately 115 light years away and it is a highly compact planetary system consisting of five transiting, sub-Earth-size planets. The orbit of the outermost planet is only one-fifth the size of Mercury’s orbit around the Sun. All five rocky planets orbit their host star in less than 10 days, and have sizes between that of Mercury and Venus. The radii of the five rocky planets are 0.403, 0.497, 0.530, 0.546, and 0.741 times the radius of Earth; and their orbital periods are 3.600, 4.546, 6.189, 7.743, and 9.740 days; respectively. The host star of this planetary system has 0.758 ± 0.043 times the mass and 0.752 ± 0.014 times the radius of the Sun. Additionally, the star’s effective surface temperature is 5046 ± 74 K and it is a metal-poor star with less than one-third the Sun’s metallicity.

Figure 2: Semi-major axes of planets belonging to the highly-compact multiple-planet systems Kepler-444, Kepler-11, Kepler-32, Kepler-33, and Kepler-80. Semi-major axes of planets in the Solar System are shown for comparison. The vertical dotted line marks the semi-major axis of Mercury. Symbol size is proportional to planetary radius. Note that all planets in the Kepler-444 system are interior to the orbit of the innermost planet in the Kepler-11 system, the prototype of this class of highly-compact multiple-planet systems. Campante et al. (2015)

Figure 3: Sizes of the five rocky planets of Kepler-444 in comparison with the Moon, Mercury, Mars and Earth. Image credit: NASA.

Reference:
Campante et al. (2015), “An ancient extrasolar system with five sub-Earth-size planets”, arXiv:1501.06227 [astro-ph.EP]

Multi-Year Observations Reveal Two Jupiter-Like Planets

Figure 1: Artist’s impression of a Jupiter-like planet.

If a star has a planet orbiting around it, the gravitational pull from the planet can cause the star to wobble. A sufficiently massive planet can induce a large enough “wobbling” that can be detectable through radial velocity measurements. Detecting Jupiter-like planets will require multi-year observations as these planets have long orbital periods. Using radial velocity measurements over a long time baseline of more than 10 to 15 years, Endl et al. (2015) present the discovery of two Jupiter-like planets identified as HD 95872b and HD 162004b.

HD 95872b is estimated to have at least ~4.6 times the mass of Jupiter. It orbits a K0V host star at ~5.2 AU, in an almost circular orbit, taking ~4375 days to complete an orbit. The host star of HD 95872b is estimated to be 10.0 ± 3.7 billion years old. It has 0.95 ± 0.04 times the mass of the Sun, an effective surface temperature around 5312 ± 100 K, and it is located ~25 light years away. The amplitude of the “wobbling” induced by HD 95872b on its host star is ~59 m/s. Since HD 95872b is at least a few times more massive the Jupiter, it may host Mars-sized moons.

HD 162004b is estimated to have at least ~1.53 times the mass of Jupiter. It orbits a G0V host star at ~4.43 AU, in an elongated orbit with eccentricity ~0.40, taking ~3117 days to complete an orbit. The host star of HD 162004b is estimated to be 3.3 ± 1.0 billion years old. It has 1.19 ± 0.07 times the mass of the Sun, an effective surface temperature of 6212 ± 75 K, and it is located ~70 light years away. The amplitude of the “wobbling” induced by HD 162004b on its host star is ~21 m/s.

Figure 2: Radial velocity curve indicating the presence of HD 95872b. Endl et al. (2015)

Figure 3: Radial velocity curve indicating the presence of HD 162004b. Endl et al. (2015)

Reference:
Endl et al. (2015), “Two New Long-Period Giant Planets from the McDonald Observatory Planet Search and Two Stars with Long-Period Radial Velocity Signals Related to Stellar Activity Cycles”, arXiv:1512.02965 [astro-ph.EP]

Signature of an Intermediate-Mass Black Hole

CO-0.40-0.22 is a cloud of gas located very near the center of the Milky Way galaxy. It is estimated to contain ~4000 times the Sun’s mass. Observations of CO-0.40-0.22 show that the gas in the gas cloud is moving too rapidly for the self-gravity of the gas cloud to hold itself together. Furthermore, the gas cloud itself is also moving very fast. These kinematic signatures suggest that CO-0.40-0.22 is being “gravitationally kicked” by an invisible compact object with ~100,000 times the Sun’s mass.

The lack of counterparts at other wavelengths indicates that the motion of CO-0.40-0.22 cannot be due to expansion driven by the death of massive stars in supernova explosions, or by the gravitational acceleration from a dense and massive cluster of stars. As a result, an intermediate-mass black hole (IMBH) is the most likely explanation for an invisible point-like mass with ~100,000 times the Sun’s mass. Such an IMBH could have formed through the runaway merger of massive stars at the center of a dense cluster of stars, or it was once the central black hole of a dwarf galaxy that got ingested by the Milky Way galaxy.

Reference:
Tomoharu et al. (2015), “Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy”, arXiv:1512.04661 [astro-ph.GA]

Seven Long-Period Planet Candidates

Using archival data from NASA’s Kepler space telescope, Wang et al. (2015) present the validation of seven long-period planet candidates that have planet confidence over 99.7 percent. These long-period planet candidates include 3 single-transit planets (KIC-3558849c, KIC-5951458b, and KIC-8540376d), 3 planets with double transits (KIC-8540376c, KIC-9663113c, and KIC-10525077b), and 1 planet with 4 transits (KIC-5437945c). All seven long-period planet candidates orbit stars hotter than the Sun in orbits with periods ranging from ~100 to over ~1000 days.

KIC-3558849c was observed to transit its host star once. Based on its transit duration and the properties of its host star, the planet’s orbital period is estimated to be between 1311 to 1708 days and the radius of the planet is between 6.0 to 7.9 Rᴇ, where Rᴇ denotes the Earth’s radius. The host star of KIC-3558849c has 0.87 to 1.09 times the mass and 0.90 to 1.11 times the radius of the Sun, and the star’s surface temperature is ~6175 K. KIC-3558849c is validated with a planet confidence of 99.7 percent. This planetary system has another planet candidate identified as KIC-3558849b. Its orbital period is 160.85 days and it measures ~2.35 Rᴇ.

KIC-5951458b is a long-period planet that was observed to transit its host star once in what was probably a grazing transit. As a result, its properties are only weakly constrained. Nevertheless, the planet is validated with a planet confidence of 99.8 percent. The orbital period of KIC-5951458b is estimated to be between 1168 to 13722 days and its radius can assume a wide range of values centred on ~6.6 Rᴇ. The host star of KIC-5951458b has 0.77 to 1.19 times the mass and 0.70 to 2.34 times the radius of the Sun, and the star’s surface temperature is ~6260 K.

KIC-8540376d (one observed transit) and KIC-8540376c (two observed transits) belong to the same planetary system which also consists of a third planet candidate whose orbital period is 10.7 days and whose radius is ~1.0 Rᴇ. KIC-8540376d is estimated to have an orbital period of between 74 to 114 days and its radius is ~2.4 Rᴇ. KIC-8540376c orbits its host star interior to KIC-8540376d. The orbital period of KIC-8540376c is 31.8 days and its radius is ~4.1 Rᴇ. Both KIC-8540376d and KIC-8540376c are validated with planet confidence levels of 99.9 percent. The host star of this planetary system has 0.84 to 1.23 times the mass and 0.70 to 1.82 times the radius of the Sun, and the star’s surface temperature is ~6475 K.

KIC-9663113c was observed to transit its host star twice. The orbital period of this planet is 572.4 days and the radius of the planet is between 3.9 to 5.2 Rᴇ. KIC-9663113c is probably a Neptune-like planet whose estimated equilibrium temperature is ~232 K, and the planet is validated with a planet confidence of 99.9 percent. The host star of KIC-9663113c has 0.85 to 1.10 times the mass and 0.91 to 1.15 times the radius of the Sun, and the star’s surface temperature is ~6065 K. This planetary system also hosts a second planet identified as KIC-9663113b. Its orbital period is 20.74 days and its radius is ~3.3 Rᴇ. The estimated equilibrium temperature on KIC-9663113b is ~702 K.

KIC-10525077b was observed to transit its host star on two occasions. However, there is a data gap between the two transits and this makes it impossible to tell whether the planet’s orbital period is 854.08 days or half the value at 427.04 days. KIC-9663113b is estimated to be ~5.5 Rᴇ in radius and it is validated with a planet confidence of 99.8 percent. The host star of KIC-10525077b has 0.89 to 1.13 times the mass and 0.91 to 1.11 times the radius of the Sun, and the star’s surface temperature is ~6090 K. This planetary system also hosts another planet candidate whose orbital period is 11.01 days and whose radius is ~1.36 Rᴇ.

KIC-5437945c was observed to transit its host star 4 times. Its orbital period is 440.8 days and its radius is ~6.4 Rᴇ. At that distance from its host star, the planet’s equilibrium temperature is ~308 K. KIC-5437945c is validated with a planet confidence of 99.9 percent. The host star of KIC-5437945c has 0.90 to 1.24 times the mass and 0.95 to 1.53 times the radius of the Sun, and the star’s surface temperature is ~6340 K. This planetary system also has another planet candidate identified as KIC-5437945b. Its orbital period is 220.13 days and its radius is ~6.1 Rᴇ. The equilibrium temperature on KIC-5437945b is ~389 K.

Reference:
Wang et al. (2015), “Planet Hunters. VIII. Characterization of 41 Long-Period Exoplanet Candidates from Kepler Archival Data”, arXiv:1512.02559 [astro-ph.EP]

Red Dwarf Star Exciting a Massive Planet

Figure 1: Artist’s impression of a gas giant planet.

HD 7449A is a Sun-like star with a massive planet identified as HD 7449Ab in orbit around it. Radial velocity measurements indicate that HD 7449Ab has at least ~7.8 times the mass of Jupiter and the plant is in a highly-elongated orbit around its host star. The minimum and maximum distance of HD 7449Ab from its host star is ~0.47 AU and ~4.19 AU, respectively. Furthermore, the orbital period of HD 7449Ab is ~1270 days.

More recent radial velocity measurements and dynamical analysis indicate that HD 7449A has a companion star identified as HD 7449B. HD 7449B is most likely a low-mass red dwarf star with ~0.2 times the Sun’s mass and it orbits HD 7449A at ~18 AU, with an orbital period of roughly 65 years. The orbit of HD 7449B is expected to be only mildly elongated. Dynamical studies show that HD 7449B may be exciting the orbit of HD 7449Ab, resulting in the planet’s highly-elongated orbit.

Figure 2: Radial velocity curve indicating the presence of HD 7449Ab. Rodigas et al. (2015)

Figure 3: Radial velocity curve indicating the presence of HD 7449B. Rodigas et al. (2015)

Reference:
Rodigas et al. (2015), “MagAO Imaging of Long-period Objects (MILO). I. A Benchmark M Dwarf Companion Exciting a Massive Planet around the Sun-like Star HD 7449”, arXiv:1512.04540 [astro-ph.EP]