Measuring the Core Mass of the Hot-Jupiter HAT-P-13b

HAT-P-13b is a transiting hot-Jupiter in orbit around a star with ~1.3 times the mass and ~1.8 times the radius of the Sun. The planet’s orbit around its host star is mildly eccentric due to tidal dissipation, and perturbations from a much more massive outer planet that is in a highly eccentric orbit. HAT-P-13b has ~0.9 times the mass and ~1.5 times the radius of Jupiter. The orbital eccentricity of HAT-P-13b is determined by its Love number, which in turn depends on how mass is distributed in the interior of HAT-P-13b. As a result, the orbital eccentricity of HAT-P-13b can be used to predict the planet’s interior structure.

Using new observations of HAT-P-13b with the Infrared Array Camera on the Spitzer Space Telescope, Buhler et al. (2016) show that the orbital eccentricity of HAT-P-13b is 0.007 ± 0.001, and this corresponds to an estimated Love number of approximately 0.31. Based on this Love number and the measured radius of HAT-P-13b, interior structure models of HAT-P-13b show that the most probable core mass for HAT-P-13b is 11 times the mass of Earth. Also, the planet’s core mass has a 68 percent probability of being less than 25 times the mass of Earth, and a 95 percent probability of being less than 47 times the mass of Earth. This represents the best measurement to date on the core mass of a hot-Jupiter. 

Reference:
Buhler et al. (2016), “Dynamical Constraints on the Core Mass of Hot Jupiter HAT-P-13b”, arXiv:1602.03895 [astro-ph.EP]

J031300 Could be a Direct Descendant of the First Stars

Figure 1: Artist’s impression of a supernova.

A star's metallicity is basically a measure of the star's abundance of elements heavier than hydrogen and helium. J031300 is the most metal-poor star known to date. It also has an unusually low iron-to-hydrogen abundance ratio of only ~0.0000001. J031300 is suspected to have formed from the debris of the first supernovae explosions in the early universe. The first stars in the universe are comprised entirely of hydrogen and helium. These stars are known as Population III stars. They are predicted to be very massive and end their lives in supernovae explosions, seeding the early universe with the first nucleosynthetic yields of heavy elements.

The most metal-poor stars known in the present universe might be the direct descendents of Population III stars. Simulations performed by Chen et al. (2016) of supernovae explosions of Population III stars with 12 and 60 times the mass of the Sun indicate that they produce nucleosynthetic elemental yields that fit quite well with the abundance of metals measured for J031300. Furthermore, the simulations suggest that the fall back of iron-group elements into the newly formed black hole or neutron star following a supernova explosion can explain the remarkably low iron-to-hydrogen abundance ratio of J031300. More observations of such extremely metal-poor stars in the present universe might shed light on the properties of the first stars in the universe.

Figure 2: Simulated nucleosynthetic elemental yields for supernovae explosions of Population III stars with 12 (Z12) and 60 (Z60) times the mass of the Sun in comparison with the abundance of metals measured for J031300. Chen et al. (2016)

Reference:
Chen et al. (2016), “Low Energy Population III Supernovae and the Origin of the Extremely Metal-Poor Star SMSS J031300.36-670839.3”, arXiv:1601.06896 [astro-ph.HE]

The First Known Multi-Planet System in an Open Cluster

The Beehive Cluster is an open cluster containing ~1000 stars and it is one of the nearest open clusters. Pr 0211 is a Sun-like star located in the Beehive Cluster. It has a hot-Jupiter identified as Pr 0211b in orbit around it. Pr 0211b has at least 1.88 ± 0.03 times the mass of Jupiter and its orbital period is 2.15 days. Using new radial velocity measurements obtained with HARPS-N and TRES, Malavolta et al. (2016) present the discovery of a second planet in orbit around Pr 0211. The discovery of this second planet means that the planetary system around Pr 0211 is the first known multi-planet system in an open cluster.

The newly discovered planet around Pr 0211 is identified as Pr 0211c. It is a massive Jupiter-like planet with at least 7.9 ± 0.2 times the mass of Jupiter, and its orbital period around its host star is greater than 3500 days. Furthermore, Pr 0211c is in a highly eccentric orbit around its host star, with an orbital eccentricity greater than 0.6. The highly eccentric orbit of Pr 0211c could be caused by disruption of the planetary system from close encounters with passing stars as Pr 0211 is a member of an open cluster.

Reference:
Malavolta et al. (2016), “The GAPS programme with HARPS-N at TNG XI. Pr~0211 in M~44: the first multi-planet system in an open cluster”, arXiv:1602.00009 [astro-ph.EP]

The Orbit of WASP-43b is Probably Decaying

WASP-43b is a hot-Jupiter with twice the mass of Jupiter, and a radius similar to Jupiter’s. It orbits its host star in an extremely close-in orbit. The orbital period of WASP-43b is only 0.8 days. WASP-43b is one of the most massive planets known to have an extremely short orbital period. Because WASP-43b is so close to its host star, the planet is expected to experience orbital decay due to tidal interactions with its host star.

Jiang et al. (2015) present eight new transit light curves of WASP-43b, and by combining it with previously obtained data, the team found that the orbit of WASP-43b is likely to be decaying. The variations in the transit timings appear to indicate that the orbital period of WASP-43b is decaying at a rate of roughly 3 seconds per hundred years. At this rate, WASP-43b is expected to be tidally disrupted by its host star in just a few million years.

Reference:
Jiang et al. (2015), “The Possible Orbital Decay and Transit Timing Variations of the Planet WASP-43b”, arXiv:1511.00768 [astro-ph.EP]

Keplerian-Like Disk around the O-Type Star AFGL 4176

Massive stars with more than 8 times the mass of the Sun form so rapidly that even after formation, they can still remain deeply embedded in their natal envelopes of gas and dust. This makes it difficult to observe whether high-mass stars that are still in the process of forming can have stable Keplerian disks of material. Using the Atacama Large Millimeter/submillimeter Array (ALMA) to peer through the gas and dust, Johnston et al. (2015) present the discovery of a Keplerian-like disk around the still-forming O-type star AFGL 4176.

The disk of material around AFGL 4176 is estimated to have a radius of roughly 2000 AU. The inner radius of the disk is predicted to be 31.3 AU, because any closer to the massive protostar, temperatures become high enough for dust grains to sublimate. The total mass of gas in the disk is predicted to be ~12 times the mass of the Sun.

At the center of the Keplerian-like disk lies the still-forming O-type star AFGL 4176. The mass of AFGL 4176 is estimated to be ~25 times the mass of the Sun. Also, AFGL 4176 is predicted to have an effective temperature of roughly 36,900 K and almost 7 times the Sun’s radius. The disk of material around AFGL 4176 is currently the best example of a disk in Keplerian-like rotation around a forming O-type star.

Reference:
Johnston et al. (2015), “A Keplerian-like disk around the forming O-type star AFGL 4176”, arXiv:1509.08469 [astro-ph.SR]

Globular Clusters are Perfect for Advanced Civilisations

Globular clusters are dense, spherical clusters of stars that are typically found in the halo of galaxies. The Milky Way galaxy is estimated to have over 150 globular clusters. A globular cluster can contain anywhere from a hundred thousand to several million stars packed in a relatively small region of space measuring just ~100 light years across. Other galaxies, such as the Andromeda galaxy, have as many as 500 globular clusters, while giant elliptical galaxies can have thousands of globular clusters.

Globular clusters are ancient objects that formed ~10 billion years ago. The oldest globular clusters can be as old as ~13 billion years. As a result, globular clusters have a low abundance of heavy elements such as iron and silicon that are necessary for the formation of rocky worlds like the Earth. Nevertheless, data from NASA’s Kepler space telescope has shown that rocky planets can readily form around stars with abundances of heavy elements as low as those found in globular clusters.

In contrast, Jupiter-like planets require a higher abundance of heavy elements to form their massive cores. For this reason, Jupiter-like planets are expected to be very rare in globular clusters, while rocky Earth-like worlds might be quite common. The compact nature of globular clusters means that the stars in globular clusters are much closer to each other than the stars in the Sun’s stellar neighbourhood. Distances between stars in a globular cluster can be as small as hundreds to thousands of AU, whereby one AU is the average Earth-Sun distance.

It turns out that globular clusters might be the perfect place for advanced civilisations to inhabit. The small distances between stars means that interstellar communication between stars is expected to take only days, weeks, or months. For comparison, a signal sent from Earth to Alpha Centauri, the nearest star system, would take over 4 years. An advanced civilisation in a globular cluster that has the same level of technology as the one currently on Earth would have already known a great deal about the nearest ~100,000 stars. Most of the planets around these stars would have already been detected and even characterised. Sending exploratory probes to these planets would be much easier since signals sent from the home planet takes take only days, weeks, or months to reach them.

However, planets in globular clusters can have their orbits disrupted by close encounters with passing stars. This is due to the dense stellar environments of globular clusters. Fortunately, globular clusters are ancient objects and so they only contain low-mass stars as more massive stars have shorter lifespans. The majority of stars in globular clusters are expected to be low-mass red dwarf stars.

Because the luminosities of stars drop steeply with decreasing mass, the habitable zone around low-mass stars is expected to be much closer-in than Earth is from the Sun. This means that planets orbiting within the habitable zone of stars in globular clusters are sufficiently close-in that their orbits are unlikely to be disrupted due to interactions with passing stars. Basically, the more compact a planetary system is, the less likely it will be disrupted by passing stars.

Red dwarf stars can live for hundreds of billions to trillions of years. As a result, planets in the habitable zone of red dwarf stars can support the development and evolution of life for extremely long periods of time. If an advanced civilisation developed on a planet around a red dwarf star in a globular cluster, it would find interstellar space travel far more feasible. Travelling at just one percent the speed of light, it could reach the nearest stars in only a few years.

Given that the stars in a globular cluster are billions of years older than the Sun, an advanced civilisation residing in a globular cluster could be far older than the one on Earth and could have already colonised the entire cluster. An advanced civilisation with colonies around many different stars would be immune to many existential threats. If an apocalyptic event occurs on one planet, the civilisation would still continue on other worlds. Finally, the view from the surface of a rocky planet in a globular cluster would be breathtaking. The entire night sky would be densely packed with stars.

Reference:
Stefano & Ray (2016), “Globular Clusters as Cradles of Life and Advanced Civilizations”, arXiv:1601.03455 [astro-ph.EP]

Changing Gas-Rich Planets into Habitable Worlds

Over the years, discoveries from NASA’s Kepler space telescope have shown that low-mass planets are very common. However, a significant fraction of these low-mass planets are enshrouded by voluminous hydrogen-helium envelopes. For such a planet that resides in the habitable zone of its host star, the removal of its hydrogen-helium envelope is necessary in order for the planet to become habitable.

If such a planet is in orbit within the habitable zone of a Sun-like star, the X-ray/UV flux from the planet’s host star over billion-year timescales is too low to remove the planet’s hydrogen-helium envelope. However, things are different if such a planet is residing in the habitable zone of a red dwarf star. Since the luminosity of a star declines sharply with decreasing mass, a red dwarf star is many times less luminous than a Sun-like star. As a result, the habitable zone around a red dwarf star is situated much closer in, causing a planet in the habitable zone of a red dwarf star to be much nearer to its host star. Furthermore, red dwarf stars tend to be active and subject planets within their habitable zones to much higher X-ray/UV flux over billion year timescales.

Owen & Mohanty (2016) present a study using a rocky planet orbiting in the habitable zone of a red dwarf star. The planet is assumed to be made of 2/3 rock and 1/3 iron. The study shows that if the planet resides on the inner edge of the habitable zone and has less than 90 percent of the Earth’s mass, it can lose enough hydrogen and helium to be subsequently habitable if its initial hydrogen-helium mass fraction is ~1 percent or less. 

The same process can remove an initial hydrogen-helium envelope comprising up to ~1 percent of the planet’s mass for a rocky planet with less than ~80 percent of the Earth’s mass residing at the outer edge of the habitable zone. Basically, a rocky planet in the habitable zone of a red dwarf star that is more massive than the Earth and has a hydrogen-helium envelope that makes up more than one percent of the planet’s mass cannot lose enough of its hydrogen-helium envelope to be habitable.

Low-mass planets are abundant around red dwarf stars, and red dwarf stars are the most common stars in the galaxy. If the evaporation of hydrogen-helium envelopes occurs readily for low-mass planets in the habitable zones around red dwarf stars, then red dwarf stars hosting planetary systems with habitable worlds could be a common phenomenon.

Reference:
Owen & Mohanty (2016), “Habitability of Terrestrial-Mass Planets in the HZ of M Dwarfs. I. H/He-Dominated Atmospheres”, arXiv:1601.05143 [astro-ph.EP]

Metallicity and the Evolution of the Habitable Zone

The habitable zone around a star is defined as a region around the star where temperatures are just right for liquid water to exist on a planet’s surface. During the course of a star’s life, the location and width of the habitable zone changes as the luminosity of the star changes. As a consequence, a planet that is currently in the habitable zone of its host star only spends a finite amount of time there. The longer a planet spends in the habitable zone, the more time it has for life to develop and evolve.

Danchi & Lopez (2013) present a study on the evolution of the habitable zone around stars with 1.0, 1.5 and 2.0 times the mass of the Sun, for metallicities ranging from 0.0001 to 0.070. A star’s metallicity is basically the fraction of a star’s mass that is comprised of elements heavier than hydrogen and helium. For comparison, the Sun’s metallicity is 0.017. The study shows that the metallicity of a star strongly affects the amount of time a planet spends in the star’s habitable zone.

A star like the Sun spends the majority of its life producing energy by fusing hydrogen into helium in its core. This period of hydrogen-burning is known as the main sequence phase. The Sun is estimated to spend 11.4 billion years in the main sequence phase. Currently, the Sun is 4.6 billion years into its main sequence life. As the Sun evolves through its main sequence phase, its luminosity will gradually increase. At the start of its main sequence phase, the Sun had only 70 percent of its present luminosity; and at the end of its main sequence phase, the Sun is predicted to have about three times its present luminosity.

A planet located between 1.2 to 2.0 AU from the Sun will spend over 10 billion years in the habitable zone. For the Earth, which orbits the Sun at 1.0 AU, its time within the habitable zone will be somewhat less. The main sequence phase of the Sun comes to an end when it stars to burn helium in its core. This helium-burning phase occurs when the Sun is between 11.4 and 12.8 billion years old. During this period, the Sun will swell in size to become a red giant star and its luminosity will increase to many times its current value. After that, the Sun will shed its outer layers and leave behind its core as a white dwarf star which will cool into oblivion.

In the course of the Sun’s helium-burning phase, there is a period of stable helium-burning which lasts for ~200 million years when the Sun is between 12.5 to 12.7 billion years old. During this period, the habitable zone stays at a distance of between 6 to 15 AU from the Sun. For stars with 1.5 and 2.0 times the Sun’s mass that are in such a period of stable helium-burning, the habitable zone stays at distances of between 7 to 16 AU and 12 to 15 AU, respectively, for durations exceeding ~100 million years.

The metallicity of a star strongly affects the duration a planet can stay in the star’s habitable zone. For a high metallicity star whose metallicity is 0.070 and whose mass is identical to the Sun, a planet circling it between 0.7 to 1.8 AU is expected to spend over 20 billion years in the habitable zone. For a low metallicity star whose metallicity is 0.0001 and whose mass is identical to the Sun, a planet circling it only spends about 4 billion years in the habitable zone.

Stars with 1.5 and 2.0 times the Sun’s mass have shorter lifespans, resulting in a short period a planet can remain in the habitable zone. For a star with 1.5 times the Sun’s mass and whose metallicity is identical to the Sun’s metallicity, it can support a stable habitable zone for over 3 billion years. Finally, stars with 2.0 times the Sun’s mass do not host habitable zones for longer than 3 billion years.

Reference:
Danchi & Lopez (2013), “Effect of Metallicity on the Evolution of the Habitable Zone from the Pre-Main Sequence to the Asymptotic Giant Branch and the Search for Life”, arXiv:1304.1464 [astro-ph.SR]

Temperate Planets Circling Low-Mass Stars

K2-26b and K2-9b are two small, temperate planets orbiting relatively nearby, low-mass stars. Both planets are larger than Earth but smaller than Neptune. Their sizes place them near the transition between Earth-like rocky planets and Neptune-like planets with thick gaseous envelopes. Both K2-26b and K2-9b transit their host stars, and were detected by K2, the re-purposed Kepler mission.

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

Figure 2: Transit light curve indicating the presence of K2-26b. Schlieder et al. (2016)

K2-26b is estimated to have 2.67 times the radius of Earth and its orbital period around its host star is 14.57 days. The host star of K2-26b is located ~300 light years from Earth. It is a low-mass star with ~56 percent the Sun’s mass, ~52 percent the Sun’s radius, ~49 percent the Sun’s luminosity, and an effective temperature of around 3785 K.

The time it takes for K2-26b to transit its host star is too long to be consistent with the planet having a circular orbit. Instead, the orbit of K2-26b around its host star is somewhat elongated, with an eccentricity of at least 0.14 with a 95 percent confidence. Due to its eccentric orbit, K2-26b is predicted to experience significant tidal heating, with a tidal heat flux of at least ~600 W/m². This level of tidal heating amounts to ~8 percent of the incident flux K2-26b receives from its host star.

Based on the planet’s distance from its host star, the level of incident flux K2-16b gets from its host star is at least ~5.4 times what Earth gets from the Sun. When tidal heating is factored in, this value becomes ~5.8 times. As a result, the estimated equilibrium temperature of K2-26b is ~430 K. This means that K2-26 b is somewhat too hot to be in the habitable zone of its host star. The composition of K2-16b is unknown as its mass has not yet been measured. Nevertheless, K2-26b has a likelihood of less than ~3 percent of having a bulk composition that is denser than silicate rock. This means that K2-26b has a good chance of having a thick gaseous envelope.

Figure 3: Transit light curve indicating the presence of K2-9b. Schlieder et al. (2016)

K2-9b is another small temperature planet with 2.25 times the radius of Earth. Its orbital period around its host star is 18.45 days. The host star of K2-9b is located ~350 light years from Earth. It is a low-mass star with ~30 percent the Sun’s mass, ~31 percent the Sun’s radius, ~1.2 percent the Sun’s luminosity, and an effective temperature of around 3390 K.

The level of incident flux K2-9b gets from its host star is estimated to be ~1.36 times what Earth gets from the Sun. This gives K2-9b an equilibrium temperature of roughly 315 K, or 42°C. K2-9b has a ~21 percent probability of being a rocky planet with an Earth-like composition. Given these conditions, K2-9b is a potentially habitable planet in the habitable zone of its host star, albeit on the warm side. However, K2-9b receives strong ultraviolet flux from its host star and this may affect the planet’s atmosphere.

Reference:
Schlieder et al. (2016), “Two Small Temperate Planets Transiting Nearby M Dwarfs in K2 Campaigns 0 and 1”, arXiv:1601.02706 [astro-ph.EP]