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]

Metal-Poor Star Hosting a Sub-Neptune Planet

HD 175607 is an old G6 main-sequence star with a metallicity that is less than 25 percent of the Sun’s. Basically, the metallicity of a star is the fraction of a star’s mass that is comprised of elements heavier than hydrogen and helium. It is known that stars with a higher metallicity are more likely to have Jupiter-like planets compared to stars with a lower metallicity. Such a correlation can be expected as stars with a higher metallicity have more heavy elements, making it more conducive for the formation of Jupiter-like planets. However, such a correlation is not observed for lower-mass planets (i.e. planets with less than ~0.1 times the mass of Jupiter) such as Neptune-mass planets and super-Earths. The occurrence rate of such planets does not depend on the metallicity of the host star.

As part of a search for Neptune-mass planets and super-Earths around a sample of 109 low-metallicity stars with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph, Mortier et al. (2015) present the discovery of a sub-Neptune-mass planet around HD 175607. As a result of this discovery, HD 175607 is currently the most metal-poor FGK main-sequence star with a low-mass planet. The planet around HD 175607 is identified as HD 175607b, with the suffix “b” denoting its planetary nature. This sub-Neptune-mass planet has at least 8.98 ± 1.10 times the mass of Earth and it goes around its host star in a slightly eccentric orbit with an orbital period of 29.01 ± 0.02 days.

Reference:
Mortier et al. (2015), “The HARPS search for southern extra-solar planets. XXXIX. HD175607 b, the most metal-poor G dwarf with an orbiting sub-Neptune”, arXiv:1511.03941 [astro-ph.EP]

Five Billion Solar Mass Supermassive Black Hole

NGC 1277 is a nearby lenticular galaxy that is thought to host one of the largest supermassive black holes known in the Universe. This black hole was first reported to contain a whopping 17 billion solar mass. Using more recent measurements of the motion of stars within ~1500 light years of the galaxy’s nucleus, the mass of the supermassive black hole in the centre of NGC 1277 has been revised downwards to 4.9 ± 1.6 billion solar mass, ~3 times smaller than previously claimed. Still, the supermassive black hole is an order of magnitude more massive than expected for a galaxy with the luminosity of NGC 1277.

Figure 1: Artist’s impression of a black hole.

NGC 1277 hosts an over-massive black hole in its center and it joins several other galaxies that are known to be positive outliers on the correlation between a supermassive black hole’s mass and the luminosity of its host galaxy’s central bulge. NGC 1277 appears similar in morphology to galaxies in the early Universe. If over-massive black holes are common in the centres of galaxies in the early Universe, then NGC 1277 could be a relic from the early Universe where the present black hole scaling relations did not apply. It can also imply that the growth of a supermassive black hole precedes that of its host galaxy.

Figure 2: The location of NGC 1277 (red square) and other similar galaxies with over-massive black holes (red asterisks) on the black hole to host galaxy correlations. The left panel shows the relationship between a black hole’s mass and the velocity dispersion of stars in its host galaxy. The right panel shows the relationship between a black hole’s mass and the luminosity of its host galaxy’s central bulge. Walsh et al. (2015)

Reference:
Walsh et al. (2015), “A 5x10^9 Solar Mass Black Hole in NGC 1277 from Adaptive Optics Spectroscopy”, arXiv:1511.04455 [astro-ph.GA]

A Jupiter-Like Circumbinary Planet

Kostov et al. (2015) present the discovery of the largest and longest-period transiting circumbinary planet. This planet is identified as KOI-2939b and it was detected using data from NASA’s Kepler space telescope. The planet orbits a pair of stars that revolve around each other every 11.26 days. The primary star has 1.2207 ± 0.0112 times the mass and 1.7903 ± 0.0055 times the radius of the Sun, and it has a temperature of 6210 ± 100 K. The secondary star has 0.9678 ± 0.0039 times the mass and 0.9663 ± 0.0057 times the radius of the Sun, and it has a temperature of 5770 ± 125 K. The primary star is ~5 times more luminous than the secondary star. This binary system is estimated to be 4.4 ± 0.25 billion years old.

Figure 1: Artist’s impression of a gas giant planet with a large terrestrial-size moon.

KOI-2939b is a gas giant planet similar to Jupiter. It has 1.52 ± 0.65 times the mass and 1.06 ± 0.01 times the radius of Jupiter (i.e. 483 ± 206 times the mass and 11.9 ± 0.1 times the radius of Earth). The next largest transiting circumbinary planet is Kepler-16b, with 0.75 times the radius of Jupiter. Observations of KOI-2939b by Kepler include two transits across the secondary star and one heavily blended transit across the primary star whereby the secondary star and the planet simultaneously pass in front of the disk of the primary star. These transit observations allow the size of KOI-2939b to be determined.

As for the mass of KOI-2939b, it was determined by measuring perturbations in the timing of the mutual eclipses of the central pair of stars that the planet orbits around. These perturbations are caused by the planet’s gravitational influence. The orbital period of KOI-2939b is ~1100 days. Despite having an orbital period ~3 times longer than Earth’s KOI-2939b is in the conservative habitable zone throughout its entire orbit. On average, KOI-2939b receives 0.71 ± 0.06 times the insolation that Earth gets from the Sun. Although KOI-2939b is not habitable as it is a gas giant planet, it can harbour large terrestrial-size moons that are capable of supporting life.

Figure 2: Three transit light curves indicating the presence of KOI-2939b. The left panel shows long-cadence data, the middle and right panels show short-cadence data. The first and third transits are across the secondary star. The second transit is heavily blended whereby the secondary star and the planet simultaneously pass in front of the disk of the primary star. Kostov et al. (2015).

Reference:
Kostov et al. (2015), “KOI-2939b: the largest and longest-period Kepler transiting circumbinary planet”, arXiv:1512.00189 [astro-ph.EP]

The Coldest Exoplanet Found to Date

When a foreground object passes in front of a background star, the gravitational field of the foreground object can act as a lens, magnifying light from the background star. This phenomenon is known as gravitational microlensing and it can be observed in the form of a light curve. As the foreground object moves into alignment, the brightness of the background star increases and reaches a peak, before decreasing as the foreground object moves out of alignment. If the foreground object is comprised of a star-planet system, the presence of the planet can show up as a “bump” in the light curve.

Using the technique of gravitational microlensing, Sumi et al. (2015) present the discovery of the coldest low-mass planet ever found. This planet is identified as MOA-2013-BLG-605Lb. Analysis of the gravitational microlensing light curve reveals three physical solutions. At present, it is impossible to distinguish which physical solution is the more likely one. Regardless, MOA-2013-BLG-605Lb has a mass similar to that of Neptune or a super-Earth. The planet orbits its host star/brown dwarf at a distance that is roughly 9 to 14 times the expected position of the snowline around its host star/brown dwarf. The snowline is basically the distance from a young star or brown dwarf where the temperature becomes cool enough for water to condense into solid ice grains. For the Sun, the snowline is located at a distance of 2.7 AU.

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

Figure 2: Gravitational microlensing light curve indicating the presence of MOA-2013-BLG-605Lb. The middle and bottom panels show the detail of the planetary signal and the residuals from the best fit model. Sumi et al. (2015)

The three physical solutions to the gravitational microlensing lightcurve depend on the distance of MOA-2013-BLG-605Lb from Earth. For the small parallax model, the distance is 11.7 [-2.6, +2.0] thousand light years; for the medium parallax model, the distance is 5.9 [-0.7, +1.3] thousand light years; and for the large parallax model, the distance is 2.8 [-0.3, +0.4] thousand light years.

In the small parallax model, MOA-2013-BLG-605Lb has 21 [-7, +6] times the mass of Earth and it orbits a low-mass red dwarf star with 0.19 [-0.06, +0.05] times the mass of the Sun at a projected separation of 4.2 [-0.9, +0.7] AU, which is 8.9 [-1.4, +10.5] times the expected position of the snowline around its host star. Basically for this model, MOA-2013-BLG-605Lb is a Neptune-mass planet at a wide separation from a low-mass red dwarf star. 

In the medium parallax model, MOA-2013-BLG-605Lb has 7.9 [-1.2, +1.8] times the mass of Earth and it orbits a high-mass brown dwarf with 0.068 [-0.011, +0.019] times the mass of the Sun at a projected separation of 2.1 [-0.2, +0.4] AU, which is 12 [-1, +7] times the expected position of the snowline around its host brown dwarf. Basically for this model, MOA-2013-BLG-605Lb is a mini-Neptune in a distant orbit around a high-mass brown dwarf.

In the large parallax model, MOA-2013-BLG-605Lb has 3.2 [-0.3, +0.5] times the mass of Earth and it orbits a low-mass brown dwarf with 0.025 [-0.004, +0.005] times the mass of the Sun at a projected separation of 0.94 [-0.09, +0.12] AU, which is 14 [-1, +11] times the expected position of the snowline around its host brown dwarf. Basically for this model, MOA-2013-BLG-605Lb is a super-Earth in a distant orbit around a low-mass brown dwarf.

Figure 3: Artist’s impression of a cold Neptune-mass planet.

For the small, medium and large parallax models, the estimated temperature on MOA-2013-BLG-605Lb is ∼26 K, ∼13 K and ∼7 K, respectively. This makes MOA-2013-BLG-605Lb the coldest low-mass planet found to date and it could be the first known example of a Neptune-like planet in terms of mass and temperature. Future observations can resolve which of the three models are more likely. For example, direct imaging may be able to detect the system if it is a low-mass red dwarf star since a brown dwarf will be too dim to be detectable.

Reference:
Sumi et al. (2015), “The First Cold Neptune Analog Exoplanet: MOA-2013-BLG-605Lb”, arXiv:1512.00134 [astro-ph.EP]

Highly Irradiated Hot-Jupiter Circling a Sun-Like Star

Figure 1: Artist’s impression of a hot-Jupiter. Image credit: NASA/Goddard Space Flight Center/Conceptual Image Lab.

A new transiting planet identified as WASP-135b has been discovered from the WASP survey. WASP-135b is a hot-Jupiter with 1.30 ± 0.09 times the radius of Jupiter, 1.90 ± 0.08 times the mass of Jupiter and it has a short orbital period of only 1.401 days. Its parent star is a Sun-like star with 0.98 ± 0.06 times the mass and 0.96 ± 0.05 times the radius of the Sun. WASP-135b orbits very close to its parent star. As a consequence, the planet is highly irradiated; receiving insolation that is ~1500 times more intense than what Earth receives from the Sun.

With a radius ~30 percent larger than Jupiter’s, WASP-135b appears to be somewhat inflated in size. The high levels of insolation that the planet receives may be the cause of its inflated radius. There is also some evidence that WASP-135b may be transferring angular momentum to its parent star, causing the star to rotate faster than normal.

Figure 2: Transit light curve indicating the presence of WASP-135b. The dip in the middle is due to the passage of WASP-135b in front of its host star. The size of the dip is consistant with a planet that has 1.30 ± 0.09 times the radius of Jupiter. Spake at al. (2015)

Figure 3: Radial velocity curve indicating the presence of WASP-135b. The amplitude of the curve depends on the amount of gravitational “tugging” WASP-135b exerts on its host star. The radial velocity curve is consistant with a planet that has 1.90 ± 0.08 times the mass of Jupiter. Spake at al. (2015)

Reference:
Spake at al. (2015), “WASP-135b: a highly irradiated, inflated hot Jupiter orbiting a G5V star”, arXiv:1511.05954 [astro-ph.EP]

Sun-Like Star with Two Low Density Sub-Saturns

Using data collected by the repurposed Kepler Space Telescope, now known as K2, Petigura et al. (2015) present the discovery of two low density sub-Saturn planets orbiting a Sun-like star identified as EPIC-203771098. The planets are designated with the suffixes “b” and “c”. Planet “b” is the inner planet and planet “c” is the other planet. EPIC-203771098 was observed by K2 from 23 August to 14 November 2014. The planets were detected by K2 as they periodically transit EPIC-203771098. The transit depths from the transit light curves indicate that planet “b” has 5.68 ± 0.56 times the radius of Earth and planet “c” has 7.82 ± 0.72 times the radius of Earth.

Figure 1: Artist’s impression of a gaseous exoplanet.

Figure 2: Transit light curves indicating the presence of planet “b” (red) and planet “c” (blue). Petigura et al. (2015)

Subsequent radial velocity measurements of EPIC-203771098 with the High Resolution Echelle Spectrometer (HIRES) on the Keck Telescope in Hawaii allow the masses of the two planets to be determined. Planet “b” has 21.0 ± 5.4 times the mass of Earth and planet “c” has 27.0 ± 6.9 times the mass of Earth. With the sizes and masses of both planets known, the densities of planets “b” and “c” are 0.63 ± 0.25 g/cm³ and 0.31 ± 0.12 g/cm³, respectively.

The low densities of both planets indicate that they have thick hydrogen-helium envelopes. Furthermore, interior models suggest both planets have fairly massive cores. The estimated core masses for planets “b” and “c” are 17.6 ± 4.3 (~75 percent of the planet’s total mass) and 16.1 ± 4.2 (~50 percent of the planet’s total mass) times the mass of Earth, respectively.

Both planets are relatively hot as they orbit quite close to EPIC-203771098. Planet “b” has an orbital period of 20.9 days and it receives 60 ± 14 times the intensity of insolation Earth gets from the Sun, resulting in an estimated equilibrium temperature of 767 ± 177 K. Planet “c” has an orbital period of 42.4 days and it receives 24 ± 5 times the intensity of insolation Earth gets from the Sun, resulting in an estimated equilibrium temperature of 606 ± 139 K.

Reference:
Petigura et al. (2015), “Two Transiting Low Density Sub-Saturns from K2”, arXiv:1511.04497 [astro-ph.EP]

A System of Five Super-Earths and a Saturn-Mass Planet

Vogt et al. (2015) present the discovery of a six-planet system orbiting the nearby K3V star HD 219134. The planetary system consists of at least 5 super-Earth mass planets in a compact configuration and a Saturn-mass planet orbiting much further out. The six planets have orbital periods of 3.1, 6.8, 22.8, 46.7, 94.2 and 2247 days, spanning 3.8, 3.5, 8.9, 21.3, 10.8 and 108 times the Earth’s mass, respectively. All 5 inner planets orbit HD 219134 closer than Mercury’s average distance from the Sun.

The six planets around HD 219134 were detected by precisely measuring the tiny wobbles the planets induce on their host star. The 5 inner planets are orbiting too close, and hence, too hot to be habitable. Nevertheless, HD 219134 is 0.31 times as luminous as the Sun and a planet orbiting it at 0.56 AU, corresponding to an orbital period of 167 days, would receive the same intensity of stellar radiation Earth receives from the Sun. If this hypothetical planet has a mass equal to that of Earth, the amount of wobbling it induces on its host star would be challenging but not impossible to detect.

Reference:
Vogt et al. (2015), “A Six-Planet System Orbiting HD 219134”, arXiv:1509.07912 [astro-ph.EP]

Giant Planet Transiting a Rapidly Rotating A-Type Star

HAT-P-57b is a giant planet in a close-in orbit around a rapidly rotating A8V star. A-type stars are more massive and much more luminous than F, G and K-type stars. To date, only a handful of planets have been found around A-type stars. HAT-P-57b was detectable because it transits its host star every 2.465 days, which is also the orbital period of the planet around its host star. Such a short orbital period implies HAT-P-57b is in an extremely close-in orbit, only about 5 stellar radii from the surface of its host star. HAT-P-57b is estimated to have an equilibrium temperature of roughly 2200 K.

The host star of is estimated to have 1.47 ± 0.12 times the mass, 1.500 ± 0.050 times the radius and 6.4 ± 1.1 times the luminosity of the Sun. With an equatorial rotation velocity of at least 102 km/s, the host star of HAT-P-57b has the highest rotation velocity of any star currently known to host a transiting planet. The next most rapidly rotating stars to host transiting planets are KOI-89 with a rotation velocity of at least 90 km/s and WASP-33 with a rotation velocity of at least 86 km/s. Finally, the mass of HAT-P-57b is estimated to be no more than 1.85 times the mass of Jupiter.

Transit light curve indicating the presence of HAT-P-57b. Hartman et al. (2015)

Reference:
Hartman et al. (2015), “HAT-P-57b: A Short-Period Giant Planet Transiting A Bright Rapidly Rotating A8V Star Confirmed Via Doppler Tomography”, arXiv:1510.08839 [astro-ph.EP]