Two Eccentric Giant Planets around a Sun-Like Star

L. Mancini et al. (2015) report on the discovery of KOI-372, a planetary system consisting of two giant planets on wide and eccentric orbits around a Sun-like star. The two giant planets are identified as KOI-372 b and KOI-372 c, with the suffixes “b” and “c” indicating their planetary nature. KOI-372 b is a transiting giant planet that was first detected by NASA’s Kepler space telescope. Follow-up radial velocity measurements were then made using a ground-based telescope to determine the mass of KOI-372 b. The radial velocity measurements and observations by Kepler indicate that KOI-372 b is a dense Jupiter-like planet with 3.25 ± 0.20 times the mass and 0.882 ± 0.088 times the diameter of Jupiter.

Figure 1: Artist’s impression of a giant planet with a hypothetical moon in orbit around it.

KOI-372 b orbits its host star every 125.6 days in a fairly elongated orbit. The orbit of KOI-372 b brings the planet as close as 61 million km (0.41 AU) from its host star and out as far as 87 million km (0.58 AU). KOI-372 b is just interior to the habitable zone around its host star and any large Earth-size moon around KOI-372 b will be too hot to be habitable. Nevertheless, Kepler observed 12 transit events of KOI-372 b and analysis of the mid-transit times shows a clear variation in transit timing. This variation is due to the presence of a second planet in a wider orbit that is perturbing the orbit of KOI-372 b.

The second planet is identified as KOI-372 c and it has a mass between 0.13 to 0.31 times the mass of Jupiter. KOI-372 c also orbits its host star in an elongated orbit with an orbital period of roughly 460 days, at an average distance of about 1.2 AU from its host star. The orbit of KOI-372 c is right within the habitable zone. Any large Earth-size moon in orbit around KOI-372 c could potentially be habitable. Unlike its more massive sibling KOI-372 b, KOI-372 c does not transit its host star. Gyrochronological analysis shows that the planetary system of KOI-372 is relatively young, with an estimated age of 1.0 ± 0.3 billion years. In comparison, the Solar System is 4.57 billion years old.

Figure 2: When the known transiting planets are plotted in a semi-major axis versus planetary-mass diagram, it can be seen that KOI-372 b and KOI-372 c occupy sparsely-populated regions of the plot. The black circles indicate the positions of KOI-372 b and KOI-372 c (the size of the circle that denotes KOI-372 c is arbitrary since the planet does not transit its host star, so its size remains unknown). L. Mancini et al. (2015).

Reference:
L. Mancini et al. (2015), “KOI-372: a young extrasolar system with two giant planets on wide and eccentric orbits”, arXiv:1504.04625 [astro-ph.EP]

Using Supernovae to Detect the Faintest Galaxies

Low-mass dwarf galaxies with stellar masses totalling less than a million times the Sun’s mass are expected to exist in huge numbers. However, these dwarf galaxies are very difficult to detect due to their low luminosities and low surface brightness. Only several of these dwarf galaxies have been found so far. They appear as ultra-diffuse extended sources and detecting them requires imaging large areas of the sky. With the exception of the nearest ones, these dwarf galaxies will remain beyond detection limits even with upcoming wide-field ground-based survey telescopes such as the Large Synoptic Survey Telescope (LSST).

Surface brightness of known dwarf galaxies in the Local Group from data compiled by McConnachie (2012). The dotted line marks the surface brightness limit that can be detected by LSST. Conroy & Bullock (2015).

Galaxies contain stars, and stars are the progenitors of nova and supernova explosions. A study by Conroy & Bullock (2015) investigates the use of nova and supernova explosions to detect the faintest dwarf galaxies in the universe. The detection of such a luminous transient with no apparent host galaxy could indicate the presence of a faint dwarf galaxy. LSST will be able to detect nova explosions out to ~100 million light years and supernova explosions out to billions of light years.

References:
- Conroy & Bullock (2015), “Beacons In the Dark: Using Novae and Supernovae to Detect Dwarf Galaxies in the Local Universe”, arXiv:1504.04015 [astro-ph.GA]
- McConnachie (2012), “The Observed Properties of Dwarf Galaxies in and around the Local Group”, AJ, 144, 4

A Compact Galaxy and its Supermassive Black Hole

An ultra-compact dwarf (UCD) is a type of galaxy whose size and mass is between that of globular clusters and compact elliptical galaxies. Two mechanisms have been proposed to explain the formation of UCDs. In the first scenario, UCDs are actually the most massive globular clusters. In the second scenario, UCDs are either extremely compact galaxies that formed in dense dark matter halos or the leftover compact cores of more massive galaxies that have been tidally-stripped.

M60-UCD1 is an extraordinary UCD located near the massive elliptical galaxy M60, about 54 million light years from Earth. It was discovered from imagery taken by NASA’s Hubble Space Telescope, with follow-up observations by NASA’s Chandra X-ray Observatory and ground-based telescopes. M60-UCD1 is the most luminous UCD known and also one of the most massive. What is remarkable about M60-UCD1 is its compactness. M60-UCD1 contains 200 million times the Sun’s mass. Half of this mass is concentrated within a central sphere only 80 light years in radius.

Figure 1: An artist impression of the supermassive black hole located at the center of M60-UCD1. Credit: NASA, ESA, D. Coe, G. Bacon (STScI).

Figure 2: An image taken by NASA’s Hubble Space Telescope showing M60-UCD1 near the giant elliptical galaxy M60. Credit: NASA/Space Telescope Science Institute/ESA.

The density of stars in M60-UCD1 is ~20,000 times greater than that found in the Sun’s stellar neighbourhood, which implies stars in this UCD are spaced ~30 times closer to one another. M60-UCD1 is the densest galaxy known in the local Universe and is believed to be the leftover core of a galaxy that underwent tidal stripping. The progenitor galaxy of M60-UCD1 was a much larger galaxy. A couple of billion years ago, it came too close to M60 and got tidally torn apart, leaving behind only its dense core behind.

M60-UCD1 also hosts a supermassive black hole at its core. Its presence is inferred from observations of the velocities of stars at the center of the galaxy. These stars are moving with such high velocities that only a supermassive black hole in the vicinity can keep these stars bound. From dynamical modelling, the supermassive black hole at the center of M60-UCD1 is estimated to have 21 million times the Sun’s mass. This is a remarkable ~15 percent of the total mass of the galaxy’s stars. The large mass of its supermassive black hole is consistant with M60-UCD1 being the tidally-stripped nucleus of what was once a much bigger galaxy.

References:
- Strader et al. (2014), “The Densest Galaxy”, arXiv:1307.7707 [astro-ph.CO]
- Seth et al. (2014), “A Supermassive Black Hole in an Ultracompact Dwarf Galaxy”, arXiv:1409.4769 [astro-ph.GA]

A Low-Density Mid-Size KBO

2002 UX25 is a mid-size Kuiper Belt Object (KBO) with an estimated diameter of about 650 km. It orbits the Sun with an orbital period of roughly 280 years. 2002 UX25 has a tiny moon in orbit around it. Measuring the moon’s orbit allows the mass of 2002 UX25 to be estimated. With the size and mass known, 2002 UX25 is found to have a remarkably low density of only 0.82 ± 0.11 g/cm³. This makes 2002 UX25 the largest known object in the Kuiper Belt with a measured density below that of pure water-ice (~1.0 g/cm³). Its surface gravity is only 1/125th the gravity on Earth.

Small KBOs with diameters less than ~350 km have densities below that of pure water-ice due to their high porosities. However, an object as large as 2002 UX25 is unlikely to have a porosity of more than ~20 percent because ice is more compressed due to the higher pressure in such a large object. Basically, smaller objects tend to have lower densities, while larger objects tend to have higher densities. The low density of 2002 UX25 and its inability to support a high porosity suggest that a very small fraction of its mass is in the form of rocky material.

Figure 1: Artist’s impression of a Kuiper Belt Object.

Figure 2: Densities of objects in and from the Kuiper Belt. Michael E. Brown (2013).

KBOs larger than ~1,000 km tend to have 70 percent or more of their mass in the form of rocky material. The low density of 2002 UX25 poses a problem for the formation of large KBOs if these objects formed from the merger of smaller objects with rock mass fractions as low as what is inferred for 2002 UX25. For example, Eris is one of the largest objects in the Kuiper Belt. To form an object with the volume of Eris would require ~40 objects the size of 2002 UX25. Nevertheless, such an assembled object, even with the additional compression from its greater mass, is expected to have a density of only ~1.0 g/cm³, much less than the measured density of 2.5 g/cm³ for Eris.

A number of explanations have been presented, but none appear likely. One explanation is that the porosity of 2002 UX25 is severely underestimated. 2002 UX25 could have porosity as high as 50 percent, thereby allowing it to have a higher rock mass fraction. Another possibility is that large KBOs have high densities because they have lost much of their icy mantles through the effects of giant impacts. 2002 UX25 challenges the leading model for the formation of large KBOs. Even if 2002 UX25 turns out to be an oddball, its exceptionally low density cannot simply be ignored.

Reference:
Michael E. Brown (2013), “The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets”, arXiv:1311.0553 [astro-ph.EP]

Measuring the Spins of Trans-Neptunian Objects

Figure 1: Artist’s impression of a trans-Neptunian object. Credit: NASA/JPL-Caltech/T. Pyle (SSC).

Using the exquisite photometric precision of NASA’s repurposed Kepler space telescope as part of the K2 mission, A. Pál et al. (2015) present the first K2 observations of two trans-Neptunian objects (TNOs). As an object rotates, its shape and surface albedo variations can cause its observed brightness to vary. This variation in brightness is known as the rotational light curve and its periodicity gives a direct measurement of the object’s rotational period.

The two TNOs observed by K2 are 2007 JJ43 and 2002 GV31. From the observed rotational lightcurves, 2007 JJ43 and 2002 GV31 are found to have rotational periods of 12.097 hours and 29.2 hours, respectively. 2007 JJ43 is a relatively large object located near the outer edge of the Kuiper belt. Its diameter is estimated to be ~600 km. At this size, its self-gravity is certainly strong enough for it to be a round object. The other TNO, 2002 GV31, is much smaller with an estimated diameter of less than 200 km.

Figure 2: Rotational light curve of 2007 JJ43 (red points). The bold symbols with the error bars are binned values. A. Pál et al. (2015).

Figure 3: Rotational light curve of 2002 GV31 (red points). The bold symbols with the error bars are binned values. A. Pál et al. (2015).

Reference:
A. Pál et al. (2015), “Pushing the limits: K2 observations of the trans-Neptunian objects 2002 GV31 and (278331) 2007 JJ43”, arXiv:1504.03671 [astro-ph.EP]

Oblate & Prolate Neutron Stars

Neutron stars are the ultra-dense collapsed cores of massive stars. A typical neutron star measures only several kilometres across, but can pack more mass than the Sun. Neutron stars are generally assumed to be perfectly spherical. However, the presence of extremely strong magnetic fields and/or anisotropic pressure gradients in their cores can deform neutron stars. The deformation can be either oblate or prolate. An oblate object bulges around the equator, whereas a prolate object is elongated along the polar axis.

Figure 1: Artist’s impression of a neutron star.

The oblate or prolate shape of a neutron star is characterised by the deformation parameter “γ”, where γ = 1 denotes a perfect sphere. An oblate neutron star has γ < 1 and a prolate neutron star has γ > 1. Basically, the maximum mass of a neutron star is how massive a neutron star can get before it becomes too massive and collapses into a black hole. The ability to deform results in a range of maximum masses a neutron star can have.

The maximum mass of a neutron star increases with increasing oblateness, but decreases with increasing prolateness. Assuming a perfectly spherical neutron star has a maximum mass of 2.3 times the Sun’s mass. If instead the neutron star has oblateness γ = 0.8, its maximum mass increases to 3.02 times the Sun’s mass. On the contrary, if the neutron star has prolateness γ = 1.2, its maximum mass decreases to 1.81 times the Sun’s mass. 

Figure 2: An oblate neutron star with γ = 0.8 and a maximum mass of 3.02 times the Sun’s mass. Weber et al. (2015).

Figure 3: A prolate neutron star with γ = 1.2 and a maximum mass of 1.81 times the Sun’s mass. Weber et al. (2015).

Reference:
Weber et al. (2015), “Non-Spherical Models of Neutron Stars”, arXiv:1504.03006 [astro-ph.SR]

A T-Dwarf Binary Candidate

Brown dwarfs are objects that span the gap between the least massive stars and planetary-mass objects. Analysis of observational data from NASA’s Wide-field Infrared Survey Explorer (WISE), together with follow-up observations by ESO’s Very Large Telescope (VLT) in Chile, has led to the discovery of a candidate brown dwarf binary system. Identified as WISE J0612-3036, the binary system consists of a pair of T6 brown dwarfs. T-dwarfs and Y-dwarfs are among the coolest and least luminous brown dwarfs. Assuming WISE J0612-3036 is located ~100 light-years away, the projected separation of the pair of T6 brown dwarfs is about 11 AU.

Figure 1: Artist’s impression of a binary system consisting of two brown dwarfs. Credit: NASA/JPL-Caltech.

Multiplicity is common among stars and it also appears to be common for brown dwarfs. Several T-dwarf binary systems have already been discovered. Most have separations less than 5 AU. The projected separation of about 11 AU makes WISE J0612-3036 an unusually wide system. Another T-dwarf binary system with a similarly large separation is WISEJ1711+3500, whose components are separated by 8 to 15 AU.

If the age of WISE J0612-3036 is ~1 billion years (Gyr), each T6 brown dwarf should have ~30 times the mass of Jupiter and should go around one another with an orbital period ~150 years, assuming a circular orbit. If an older age of ~5 Gyr is assumed, each T6 brown dwarf should have ~60 times the mass of Jupiter and should go around one another with an orbital period ~105 years, again assuming a circular orbit. WISE J0612-3036 is only a candidate brown dwarf binary system because its binarity is based on just a single observation. Additional observations will be needed to confirm if the two T6 brown dwarfs are indeed bound in orbit around one another.

Figure 2: Observations of WISE 0612-3036 clearly showing the two components of the binary system. Huelamo et al. (2015).

Reference:
Huelamo et al. (2015), “WISE J061213.85-303612.5: a new T-dwarf binary candidate”, arXiv:1504.03150 [astro-ph.SR]

White Dwarf Accreted a Water-Rich Dwarf Planet

White dwarfs are the dense remnant cores of low to intermediate mass stars. Planetary systems are ubiquitous around stars and when a star evolves into a white dwarf, its planetary system can become destabilised. Planets can get ejected; spiral into the star or even collide with one another. Nevertheless, a significant fraction of white dwarfs are expected to have retained at least parts of their planetary systems.

Packing as much mass as the Sun into a volume as small as the Earth, white dwarfs are very compact objects with intensely strong gravities. This causes heavier elements to sink and lighter elements to stay on top. Elements heavier than hydrogen and helium tend to quickly sink out of the outermost layer of the white dwarf. As a result, heavy elements detected on the surface of a white dwarf must have come recently from an external source, such as a planet that has accreted onto the white dwarf.

SDSS J1242 is a cool white dwarf with a helium dominated atmosphere and an effective temperature of 13,000 K. Observations of SDSS J1242 show that its outermost layer contains a significant amount of rock-forming elements, almost all in the form of oxygen, magnesium, silicon and iron. The rock-forming elements total ~1.5 times the mass of the dwarf planet Ceres, indicating that SDSS J1242 quite recently accreted a planetary object with the mass of a dwarf planet.

Additionally, the abundance of oxygen appears too high to have just come from the accretion of rocky material. Since water is comprised of hydrogen and oxygen, the oxygen excess implies SDSS J1242 accreted a dwarf planet with water content ~38 percent by mass. The abundance of heavy elements in the outermost layer of the white dwarf is expected to decrease exponentially with time. Based on what is observed, the accreted water-rich dwarf planet is estimated to have at least 7 times the mass of Ceres (1/10th the mass of Moon) if the accretion took place ~3 million years ago and 1.5 times the mass of Pluto (1/4th the mass of Moon) if the accretion took place ~5 million years ago. Such discoveries can offer insight into the ubiquity of water-rich worlds.

Reference:
R. Raddi et al. (2015), “Likely detection of water-rich asteroid debris in a metal-polluted white dwarf”, arXiv:1503.07864 [astro-ph.SR]a

Giant Planet near the Edge of Destruction

WTS-2b is a hot Jupiter in an unusually close-in 1.02-day orbit around a K-type star. The planet has 1.12 times the mass and 1.363 times the diameter of Jupiter. Like Jupiter, WTS-2b is a gas giant planet comprised mostly of hydrogen and helium. WTS-2b orbits so close to its host star that the separation between the planet and its host star is only 1.5 times the distance where the planet will start to get torn apart by strong tidal forces.

Figure 1: Artist’s impression of a hot Jupiter in a close-in orbit around its host star.

Figure 2: Transit light curve of WTS-2b. Birkby et al. (2014).

WTS-2b raises tides on its host star. The tides exert a strong torque that transfers energy from the planet’s orbit to the star’s spin. This causes the planet’s orbit to shrink and the star to spin up. WTS-2b is estimated to have another ~40 million years before its in-spiralling orbit brings it close enough to its host star to be tidally destroyed. Because WTS-2b transits in front of its host star every 1.02 days, the planet’s shrinking orbit can be directly measured. The in-spiralling is estimated to create a ~17 seconds decrease in the duration between consecutive transits over a period of 15 years.

Most hot Jupiters have orbital periods around 3 to 4 days. There appears to be a lack of hot Jupiters with orbital periods less than 2 days. This could mean that either it is very difficult to get hot Jupiters into very close-in orbits, or that they are quickly destroyed by tidal forces once they are in such orbits. Being so close to its host star, the amount of insolation WTS-2b gets is ~1,000 times more intense than what Earth gets from the Sun. As a result, the day side of WTS-2b is heated to an estimated ~2,000 K.

Reference:
Birkby et al. (2014), “WTS-2 b: a hot Jupiter orbiting near its tidal destruction radius around a K-dwarf”, arXiv:1402.5416 [astro-ph.EP]

A Mini-Neptune with Earth-Like Insolation

Observations by K2, an extension of the Kepler mission using the repurposed Kepler space telescope, has led to the detection of a mini-Neptune that receives a similar amount of insolation from its host star as Earth gets from the Sun. The planet is identified as EPIC 201912552.01. Its host star, being a red dwarf star, is smaller and much less luminous than the Sun. EPIC 201912552.01 receives an Earth-like insolation because it orbits its host star much closer than Earth is from the Sun. In fact, the planet has an orbital period of only 33 days. Being so close to its host star, the planet is most likely tidally-locked, with the same hemisphere perpetually facing its host star, resulting in permanent day and night sides.

By measuring the dip in brightness when EPIC 201912552.01 transits in front of its host star, the size of the planet is estimated to be 2.24 ± 0.25 times the diameter of Earth. With such a size, EPIC 201912552.01 is too large to be a rocky planet like Earth. Instead, the planet is most probably a mini-Neptune. The equilibrium temperature of the planet is estimated to be 271 ± 16 K. Both the planet and its host star are located at a distance of approximately 110 light-years. The proximity and relative brightness of the system makes EPIC 201912552.01 a good target for its atmosphere to be characterised by future space-based telescopes such as the James Webb Space Telescope (JWST).

Reference:
Montet et al. (2015), “Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 18 Systems, Including a Planet Receiving Earth-like Insolation”, arXiv:1503.07866 [astro-ph.EP]

Nebulae from Destructed Giant Planets

A white dwarf is basically the leftover core of a star that has shed its outer layers. The large amount of mass that is lost as a star evolves into a white dwarf can destabilise a planetary system around the star. This can potentially send planets towards the star where they either collide with the star or become tidally disrupted. If a gas giant planet meets such a fate, it can drive the formation of what is known as a “real planetary nebula”.

The planetary nebula Sharpless 2-71, as imaged by the Gemini Multi-Object Spectrograph on Gemini North in Hawaii. Image credit: Gemini Observatory/AURA.

A gas giant planet is comprised mostly of hydrogen and helium, with a dense solid core in the middle. If it is orbiting a white dwarf and happens to be perturbed into an orbit which brings it too close to the white dwarf, it can become tidally disrupted or even collide with the white dwarf. Such an event strips away the hydrogen and helium envelope of the planet. Roughly half of the stripped material forms an accretion disk around the white dwarf and the other half gets flung out of the system.

The accreted hydrogen undergoes nuclear burning on the surface of the white dwarf. This causes part of the accreted hydrogen to re-inflate into a red giant envelope ~100 times the Sun’s radius around the white dwarf. For a newly formed white dwarf with 0.6 times the Sun’s mass, a red giant envelope can be inflated with an accreted mass of only ~0.001 times the Sun’s mass (i.e. about the mass of Jupiter). For an old and cool white dwarf of the same mass, a red giant envelope can be inflated with an even lower accreted mass of only ~0.0001 times the Sun’s mass (i.e. ~1/10th the mass of Jupiter).

Subsequently, part of the red giant envelope is blown away in the form of a wind to form a nebula. The central white dwarf, still very hot from all the accretion and nuclear burning, heats up and ionizes the nebula. This causes the nebula to glow. As a result, such a nebula whose material came from the destruction of a planet, is known as a “real planetary nebula”. By contrast, typical planetary nebulae are formed from ionized gas ejected from old red giant stars at the last stages of their evolution.

References:
- Bear & Soker (2015), “Planetary systems and real planetary nebulae from planets destruction near white dwarfs”, arXiv:1502.07513 [astro-ph.SR]
- Corradi et al. (2015), “Binarity and the abundance discrepancy problem in planetary nebulae”, arXiv:1502.05182 [astro-ph.SR]