Reflected Light from a Giant Planet’s Periastron Passage

HD 20782b is a giant planet with at least twice the mass of Jupiter. What makes HD 20782b bizarre is that its orbit around its host star, a Sun-like star, is the most eccentric orbit known for any exoplanet. HD 20782b takes 597 days to go around its host star once in an extremely elongated orbit with eccentricity 0.956. Once per orbit, HD 20782b swings in for a brief fiery encounter with its parent star. At closest approach (periastron), HD 20782b is only 0.061 AU from its host star, and the planet is 2.73 AU from its host star at its furthest (apastron). 

The highly elongated orbit of HD 20782b means that the intensity of flux it receives from its host star varies from 3.6 to over 7000 times the intensity of flux Jupiter receives from the Sun. Stephen R. Kane et al. (2015) present evidence for reflected light from HD 20782b during the planet’s periastron passage. The signature of reflected light is indicated as a tiny increase in the brightness of the planet’s host star as reflected light from the planet adds to the overall observed brightness of the host star. 

Short-period giant planets tends to have relatively low albedos (i.e. low reflectivity) while long-period giant planets tend to have relatively high albedos (i.e. high reflectivity). Because HD 20782b spends the vast majority of its time far from its host star, it will have a high albedo similar to a long-period giant planet. HD 20782b spends only a very small amount of time around periastron and the planet’s high albedo atmosphere does not have enough time to react to the intense flux from its host star. As a consequence, HD 20782b, with its high albedo atmosphere, is able to reflect a larger amount of flux from its host star than a typical short-period giant planet. This makes HD 20782b easier to detect in reflected light when it is around periastron. Planets with highly eccentric orbits like HD 20782b can provide a good opportunity for the study of planetary atmosphere in reflected light.

Reference:
Stephen R. Kane et al. (2015), “Evidence for Reflected Light from the Most Eccentric Exoplanet Known”, arXiv:1511.08679 [astro-ph.EP]

Warm Super-Earth Circling a Red Dwarf Star

Circling a red dwarf star located 173 ± 26 light years from Earth is a super-Earth with 2.38 ± 0.25 times the size of Earth. This planet is identified as EPIC 206318379b and its discovery was announced by Teruyuki et al. (2015) using data from NASA’s K2 mission together with follow-up observations. EPIC 206318379b is in a close-in orbit around its host star, circling around its host star once every 2.26 days.

Figure 1: Artist’s impression of a super-Earth.

Being a red dwarf star, the host star of EPIC 206318379b has only 29.3 ± 3.0 percent the diameter and 26.4 ± 5.0 percent the mass of the Sun. The host star of EPIC 206318379b also has an effective temperature of 3214 ± 60 K and it is comparatively metal-rich, with approximately twice the Sun’s metallicity. A star’s metallicity indicates the abundance of elements that are heavier than hydrogen and helium.

EPIC 206318379b is relatively warm as it orbits close to its host star. A reflectivity of zero percent gives the planet an equilibrium temperature of 570 ± 36 K and a reflectivity of 40 percent gives the planet an equilibrium temperature of 502 ± 32 K. EPIC 206318379b is similar in size and receives a similar amount of insolation from its host star as the well-studied GJ 1214b. Follow-up observations to compare the atmospheric properties of these two planets can provide new insights to the atmospheres of such worlds.

Figure 2: Transiting planets around red dwarf stars cooler than 3400 K. Teruyuki et al. (2015)

Reference:

Teruyuki et al. (2015). “The K2-ESPRINT Project III: A Close-in Super-Earth around a Metal-rich Mid-M Dwarf”, arXiv:1511.08508 [astro-ph.EP]

Detection of Five Brown Dwarfs around Sun-Like Stars

Figure 1: Artist’s impression of a giant planet/brown dwarf.

Brown dwarfs are substellar objects with roughly 13 to 80 times the mass of Jupiter. These objects are not massive enough to burn hydrogen in their cores and they span the gap between giant planets and low-mass stars. Bouchy et al. (2015) present the detection of five brown dwarfs with minimum masses between 32 and 83 times the mass of Jupiter. These brown dwarfs orbit Sun-like stars with orbital periods longer than 10 years, and they were detected through radial velocity measurements of Sun-like stars over a relatively long period of roughly 20 years. This discovery doubles the number of known brown dwarfs with orbital periods longer than 10 years.

Figure 2: Radial velocity curve of HD10844, a F8V star with 0.98 times the mass of the Sun, located about 170 light years away. The brown dwarf orbiting it has at least 83 times the mass of Jupiter, and its orbit has a period of 32 years and an eccentricity of 0.57. With a minimum mass close to the boundary between the most massive brown dwarfs and the least massive stars, the brown dwarf around HD10844 is quite likely a star and not a brown dwarf at all. Bouchy et al. (2015)

Figure 3: Radial velocity of HD14348, a F5V star with 1.20 times the mass of the Sun, located about 185 light years away. The brown dwarf orbiting it has at least 49 times the mass of Jupiter, and its orbit has a period of 13.0 years and an eccentricity of 0.46. Bouchy et al. (2015)

Figure 4: Radial velocity of HD18757, a G4V star with 0.88 times the mass of the Sun, located only about 80 light years away. The brown dwarf orbiting it has at least 35 times the mass of Jupiter, and its orbit has a period of 109 years and a large eccentricity of 0.94. Its highly elongated orbit brings the brown dwarf from as close as 1.27 AU to as far as 43.1 AU from its host star. At closest approach, the brown dwarf receives close to the same intensity of insolation from its host star as what Earth receives from the Sun. Bouchy et al. (2015)

Figure 5: Radial velocity of HD72946, a G5V star with 0.96 times the mass of the Sun, located about 85 light years away. The brown dwarf orbiting it has at least 60 times the mass of Jupiter, and its orbit has a period of 15.9 years and an eccentricity of 0.50. Bouchy et al. (2015)

Figure 6: Radial velocity of HD209262, a G5V star with 1.02 times the mass of the Sun, located about 160 light years away. The brown dwarf orbiting it has at least 32 times the mass of Jupiter, and its orbit has a period of 14.9 years and an eccentricity of 0.35. Bouchy et al. (2015)

Reference:
Bouchy et al. (2015), “The SOPHIE search for northern extrasolar planets VIII. Follow-up of ELODIE candidates: long-period brown-dwarf companions”, arXiv:1511.08397 [astro-ph.SR]

Perturbations from a Neptune-Mass Planet

Figure 1: Artist’s impression of a Neptune-mass planet.

When a foreground star crosses the line-of-sight to a background star, the gravitational field of the foreground star can act as a lens and magnify light from the background star. This phenomenon is known as gravitational microlensing and the presence of a planet around the foreground star can induce perturbations in the resulting gravitational microlensing light curve. The core-accretion theory of planet formation predicts that gas giant planets and Neptune-mass planets form beyond the “snow-line” of their host stars due to the much greater abundance of solid material. In the hunt for planets, gravitational microlensing has proven to be sensitive to planets located beyond the “snow-line” of their host stars.

Skowron et al. (2015) present the discovery of a Neptune-mass planet identified as MOA 2011-BLG-028Lb. The planet is located ~20,000 light years away, near the center of the galaxy, and it was detected during a gravitational microlensing event that lasted from December 2010 until September 2011. The presence of MOA 2011-BLG-028Lb was inferred from a perturbation in the gravitational microlensing light curve that was observed from 12 to 14 May 2011. MOA 2011-BLG-028Lb is estimated to have ~30 times the mass of Earth and it orbits a host star with ~0.75 times the mass of the Sun at a projected separation of between 3.1 to 5.2 AU. This places MOA 2011-BLG-028Lb beyond the “snow-line” of its host star, which is at ~2.0 AU.

Figure 2: Gravitational microlensing light curve indicating the presence of MOA 2011-BLG-028Lb. Skowron et al. (2015)

Reference:
Skowron et al. (2015), “MOA 2011-BLG-028Lb: a Neptune-mass Microlensing Planet in the Galactic Bulge”, arXiv:1512.03422 [astro-ph.EP]

Detection of Weather on a Young Planetary Mass Object

PSO J318.5-22 is a young free-floating planetary mass object with roughly 8.3 times the mass of Jupiter, an effective temperature of about 1160 K and a relatively young age of only 23 ± 3 million years. This planetary mass object is still glowing hot from heat acquired during its formation and it is in the process of cooling down, contracting in size as it cools. PSO J318.5-22 is a good candidate for high precision characterisation because it is free-floating, and unlike exoplanets, it is not “lost” in the glare of a host star.

Observations of PSO J318.5-22 in the infrared reveal the presence of variability in its luminosity. The first observation was on 9 October 2014 in the J band (1.1 to 1.4 μm) over 5 hours and a variability of 10 ± 1.3 percent was measured. The second observation was on 9 November 2014 in the J band and a variability of 7 ± 1 percent was measured. The third observation was on 11 November 2014 in the K band (2.0 to 2.4 μm) and a variability of up to 3 percent was measured.

Although variability is common for cool brown dwarfs, this is the first time variability has been observed for such a planetary mass object. The most likely cause for the variability is inhomogeneous cloud cover on PSO J318.5-22, which makes this the first detection of weather on a free-floating planetary mass object. The observed variability also indicates that PSO J318.5-22 is a fast rotator, with a rotation period of several hours.

Reference:
Biller et al. (2015), “Variability in a Young, L/T Transition Planetary-Mass Object”, arXiv:1510.07625 [astro-ph.EP]

Blue Skies on the Warm Neptune-Size Planet GJ 3470b

GJ 3470b is a warm Neptune-size planet in orbit around a red dwarf star. The orbit of the planet around its host star is oriented such that the planet periodically passes in front of its host star every 3.34 days. GJ 3470b has 3.9 times the size and 13.7 times the mass of Earth, and an estimated equilibrium temperature of roughly 700 K. Observations of how light from the planet's host star filters through the planet's atmosphere when the planet passes in front of its host star indicate the presence of Rayleigh scattering in the atmosphere of GJ 3470b.

Figure 1: Artist’s impression of a warm Neptune-size planet.

The size of GJ 3470b is determined by measuring how much light the planet obscures each time it passes in front of its host star. At certain wavelengths of light, a planet's atmosphere can appear more opaque. This means that if the planet is observed to pass in front of its host star at these wavelengths, the planet can obscure more light from its host star and appear larger.

For the case of GJ 3470b, the effect of Rayleigh scattering makes the planet appear larger when observed at shorter (blue) wavelengths of light than at longer (red) wavelengths of light. This is because the planet's atmosphere scatters shorter (blue) wavelengths of light more strongly than longer (red) wavelengths of light. Such an effect also occurs in Earth's atmosphere and is the reason why the sky can appear blue on Earth.

Observations of the Rayleigh scattering in the atmosphere of GJ 3470b also reveal that the planet's atmosphere has a low mean molecular weight. The atmosphere of GJ 3470b is most likely a hydrogen-helium atmosphere covered by high-altitude clouds and hazes. GJ 3470b is currently the smallest known exoplanet with a detection of Rayleigh scattering.

Figure 2: The transmission spectrum of GJ 3470b. The effect of Rayleigh scattering makes the planet appear larger when observed at shorter (blue) wavelengths of light than at longer (red) wavelengths of light. Dragomir et al. (2015)

Reference:
Dragomir et al. (2015), “Rayleigh Scattering in the Atmosphere of the Warm Exo-Neptune GJ 3470b”, arXiv:1511.05601 [astro-ph.EP]

Ageing Hypergiant Star Expelling Large Dust Grains

VY Canis Majoris is an ageing red hypergiant star located approximately 3840 light years from Earth. In terms of size, VY Canis Majoris is one of the largest stars known, with approximately 1420 times the radius of the Sun. If placed in the Solar System, the surface of VY Canis Majoris would extend well beyond the orbit of Jupiter. VY Canis Majoris is estimated to contain up to 35 times the Sun’s mass and about 300,000 times the Sun’s luminosity.

Observations of VY Canis Majoris using the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument at the Very Large Telescope (VLT) at the European Southern Observatory’s Paranal Observatory in Chile show the presence of large dust grains in the thick stellar wind of VY Canis Majoris. Giant stars like VY Canis Majoris expel huge amounts of gas and dust. VY Canis Majoris expels ~30 times the mass of Earth in the form of gas and dust every year.

Measurements of how light from VY Canis Majoris is polarised by the surrounding dust grains show that the dust grains around VY Canis Majoris are relatively large, about 0.5 micrometers in size. Although that may appear small, it is approximately 50 times larger than the average size of dust grains in interstellar space. These dust grains are sufficiently large to be accelerated from VY Canis Majoris by the star’s own radiation, thereby contributing to the star’s prodigious rate of mass loss.

VY Canis Majoris will eventually explode as a supernova. The large size of the dust grains allow them to be accelerated sufficiently far from VY Canis Majoris, making them resistant to sublimation in the ensuring supernova explosion. The large dust grains, together with the heavy elements created during the supernova explosion will go on to form subsequent generations of stars and planets.

Reference:
Scicluna et al. (2015), “Large dust grains in the wind of VY Canis Majoris”, arXiv:1511.07624 [astro-ph.SR]

2400 m/s Winds on the Hot-Jupiter HD189733b

HD189733b is an exoplanet that is slightly more massive and slightly larger in size than Jupiter. However, HD189733b orbits very close to its host star, going around it once every 2.22 days. During each orbit, HD189733b will transit in front of its host star. The proximity of HD189733b to its host star means that the planet is expected to be tidally-locked. As a result, the planet has a permanent substellar point on its day side where the planet’s host star is always directly overhead.

Figure 1: Artist’s impression of the planet HD189733b in front of its host star. A belt of wind races around the equator of the planet at over 2000 m/s from the heated day side to the night side. The day side of the planet appears blue due to scattering of light from silicate haze in the atmosphere. The night side of the planet glows a deep red due to its high temperature from winds that deliver heat from the hotter day side. Image credit: Mark A. Garlick/University of Warwick.

By studying data from a transit observation of HD189733b made with the High Accuracy Radial Velocity Planet Searcher (HARPS) on the European Southern Observatory’s 3.6 m telescope at La Silla, Chile; Louden & Wheatley (2015) present wind velocity measurements on the leading limb (90 degrees west of the substellar point) and trailing limb (90 degrees east of the substellar point) of HD189733b.

As parts of the atmosphere of HD189733b move towards or away from Earth, the Doppler Effect causes light from the planet’s host star to be red or blue shifted as it passes through the planet’s atmosphere. By measuring the wavelength of the sodium absorption feature in the planet’s atmosphere, the planet’s leading limb is observed to be moving away from the observer (i.e. red shifted) at a velocity of roughly 2.3 km/s and the planet’s trailing limb is observed to be moving towards the observer (i.e. blue shifted) at a velocity of roughly 5.3 km/s.

Since HD189733b is tidally-locked, its rotation period is the same as its orbital period of 2.22 days. As a result, based on rotation alone, both leading and trailing limbs will have symmetric red and blue shifts of 2.9 km/s. When these rotational velocities are subtracted from the measured velocities, the planet’s leading limb has a westward excess velocity of roughly 0.6 km/s and the planet’s trailing limb has an eastward excess velocity of roughly 2.4 km/s.

This study shows that the atmosphere of HD189733b has a strong eastward motion, with winds racing at over 2000 m/s from the intensely heated day side to the night side. Such an eastward motion also explains why the region of peak temperature on HD189733b is displaced 30 degrees east of the substellar point.

Figure 2: The posterior distributions of atmospheric velocities from the analysis. The leading limb (left) is red shifted by roughly 2.3 km/s and the trailing limb is blue shifted by roughly 5.3 km/s. The average velocity (middle) is found to be blue shifted with a velocity of roughly 1.9 km/s. The strength and direction of the velocity sets are consistent with a combination of tidally-locked rotation and an eastward equatorial jet that is seen crossing from the day side to the night side of the planet on the trailing limb. Louden & Wheatley (2015)

Reference:
Louden & Wheatley (2015), “Spatially resolved eastward winds and rotation of HD189733b”, arXiv:1511.03689 [astro-ph.EP]

Massive White Dwarf with a Dark Spot on its Surface

Kilic et al. (2015) present the discovery of eclipse-like events around the massive white dwarf J1529+2928. The eclipse-like events occur every 38 minutes and they are believed to be caused by the presence of a dark spot on the surface of the white dwarf that comes into view every 38 minutes due to the rotation of the white dwarf. J1529+2928 is modelled to have a temperature of 11600 K, a radius of 5500 km (i.e. slightly smaller than Earth) and have about the same mass as the Sun. The eclipse-like events cannot be due to a transiting planet because a planet in a 38 minute orbit around the white dwarf would be tidally disrupted as it is too close-in.

Radial velocity measurements of J1529+2928 also show that there is no companion star or companion brown dwarf in orbit around it that could be responsible for the eclipse-like events. The dark spot on J1529+2928 is predicted to have a temperature of roughly 10000 K and covers ~14 percent of the surface area of the white dwarf. The presence of such a dark spot is most likely due to channelling of accreted heavy elements onto a spot on the surface of the white dwarf by a magnetic field. Because heavy elements are more opaque to shorter wavelengths of light, this can explain why the depth of the eclipse-like events is shallower when J1529+2928 is observed at longer wavelengths.

Reference:
Kilic et al. (2015), “A Dark Spot on a Massive White Dwarf”, arXiv:1511.07320 [astro-ph.SR]

An Extreme Dark Matter Dominated Galaxy

A cluster of stars can be considered a galaxy if it shows evidence for dark matter, where the stars in the cluster are moving too fast for the gravitational field from just the stars alone to hold the cluster together. Additionally, a cluster of stars can also be considered a galaxy if the stars exhibit different metallicities (i.e. different abundances of elements heavier than hydrogen and helium), which indicates multiple episodes of star formation. Dwarf galaxies tend to contain very few stars but a lot of dark matter. Triangulum II is a dwarf galaxy located just beyond the edge of the Milky Way. It contains only ~1000 stars and it is a satellite galaxy of the Milky Way.

By observing the motion of six stars in Triangulum II, the gravitational force acting on the stars can be measured. This technique allows the mass of the galaxy to be estimated. The total mass of Triangulum II is found to be much greater than the mass of all its stars. Although the galaxy contains ~2 million times the Sun’s mass, its total luminosity is only ~450 times the Sun’s luminosity. In fact, Triangulum II has one of the largest mass-to-light ratios of any galaxy and it is the most dark matter dominated galaxy currently known.

Furthermore, the stars in Triangulum II are observed to have different metallicities, indicating multiple episodes of star formation. This shows that the galaxy has to be many times more massive than the total mass of all its stars in order for its gravity to keep in all the gas and dust that were dispersed from past episodes of star formation to form new generations of stars.

Although dark matter is few times more abundant than ordinary matter in the Universe, it has never been directly observed. The existence of dark matter is inferred by its gravitational influence in galaxies and clusters of galaxies. It is believed that the particles that make up dark matter can annihilate one another when they collide to produce gamma rays that can be detected. However, detecting these gamma ray signals is challenging because other astrophysical objects and phenomena also generate gamma rays.

Triangulum II is a very quiet galaxy and it is not forming any new stars. As a result, Triangulum II, with its high concentration of dark matter, may be a good and pristine place to search for gamma ray signals from annihilating dark matter particles, hopefully shedding more light on the nature of dark matter.

Reference:
Kirby et al. (2015), “Triangulum II: Possibly a Very Dense Ultra-Faint Dwarf Galaxy”, arXiv:1510.03856 [astro-ph.GA]