A Tight Pair of Brown Dwarfs at the T/Y Transition

WISE J0146+4234AB is a substellar binary consisting of a T9 brown dwarf and a slightly cooler Y0 brown dwarf in a tight orbit around one another. Brown dwarfs cool with time and more massive brown dwarfs cool more slowly. Observations show both components of WISE J0146+4234AB are less luminous than any other substellar binaries currently known. This indicates both components must be around planetary mass (~12 to 21 times Jupiter’s mass) and probably very old as well.

The low luminosity also implies that the T9 brown dwarf is unusually cold. Assuming an age ~10 billion years, its temperature is only 345 ± 45 K. Its companion Y0 brown dwarf is slightly cooler at 330 ± 45 K. Up close; these substellar objects will appear utterly black. Only in the infrared do they shine.

Both components of WISE J0146+4234AB have a projected separation ~ 0.9 AU and an estimated orbital period less than ~10 years. This makes WISE J0146+4234AB one of the tightest known ultra-cool substellar binary. Its relatively short orbital period will allow better mass measurements in the coming few years from observing the motion of both components around one another. Other such binaries have orbital periods of several tens to hundreds of years.

Reference:

Trent J. Dupuy, Michael C. Liu, S. K. Leggett, “Discovery of a Low-Luminosity, Tight Substellar Binary at the T/Y Transition”, arXiv:1502.04707 [astro-ph.SR]

Planets around Red Giant Stars

Niedzielski et al. (2015) report the detection of planetary-mass companions around three red giant stars by the ongoing Penn State-Torun Planet Search using the 9.2-meter aperture Hobby-Eberly Telescope (HET) at McDonald Observatory in Texas. The red giant stars are identified as BD+49 828, HD 95127 and HD 216536. All three stars are exhausting the nuclear fuel in their cores and are gradually expanding in size. Their planetary-mass companions were detected with the radial velocity method. It involves a high resolution spectrograph which precisely measures the amount of redshift and blueshift in the star’s spectral lines as the star wobbles back and forth due to gravitational tugging from an orbiting planet.

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

BD+49 828 has 30 times the Sun’s luminosity, 1.5 times the Sun’s mass and 7.6 times the Sun’s radius. Its planetary-mass companion, BD+49 828 b, with the added suffix “b” to indicate its planetary nature, is 1.6 times Jupiter’s mass. The planet orbits its host star in a 2590-day orbit at a mean distance of 4.2 AU with eccentricity 0.35.

HD 95127 has 190 times the Sun’s luminosity, 1.2 times the Sun’s mass and 20 times the Sun’s radius. Its planetary-mass companion, HD 95127 b, is 5.0 times Jupiter’s mass and orbits its host star in a 482-day orbit at a mean distance of 1.3 AU with eccentricity 0.11.

HD 216536 has 63 times the Sun’s luminosity, 1.4 times the Sun’s mass and 13 times the Sun’s radius. Its planetary-mass companion, HD 216536 b, is 1.5 times Jupiter’s mass and orbits its host star in a 149-day orbit at a mean distance of 0.61 AU with eccentricity 0.38.

HD 95127 b and HD 216536 b are both orbiting close enough to their host stars that they will eventually be engulfed as their host stars continue to evolve and expand in size. For BD+49 828 b, its orbit is sufficiently large that it will not be affected by the evolution of its host star. BD+49 828 b is one of the longest period planets found using the radial velocity method. Interestingly, BD+49 828 b may also be located in the habitable zone (HZ) as inferred from the estimated luminosity of its host star. However, this is only transient because as the host star continues to evolve and grow in luminosity, the HZ will move beyond the orbit of BD+49 828 b. In fact, it may already have occurred.

Figure 2: Best-fit radial velocity curve indicating the presence of BD+49 828 b. Niedzielski et al. (2015).

 Figure 3: Best-fit radial velocity curve indicating the presence of HD 95127 b. Niedzielski et al. (2015).

Figure 4: Best-fit radial velocity curve indicating the presence of HD 216536 b. Niedzielski et al. (2015).

Reference:

Niedzielski et al. (2015), “Three red giants with substellar-mass companions”, arXiv:1501.07076 [astro-ph.SR]

A Star Came Close to the Sun 70,000 Years Ago

The Oort Cloud is a spherical cloud of icy planetesimals that surrounds the Sun out to a distance of about 50,000 AU. For comparison, Pluto’s orbit never brings it further than 49 AU from the Sun. Passing stars can perturb these icy planetesimals in the Oort Cloud and send them on trajectories towards the Sun, resulting in the flux of long-period comets that visit the planetary region of the Solar System. Some of these comets might have even impacted Earth and produced extinction events. A study by Mamajek et al. (2015) reports on the identification of the closest known flyby of a star to the Solar System. The star belongs to a nearby, low-mass binary system known as WISE J0720.

A star’s velocity through space consists of two components - radial and tangential. Radial motion is how fast the star appears to be approaching or receding along the line of sight, while tangential motion is the motion of the star across the plane of the sky, perpendicular to the line of sight. WISE J0720 seems peculiar with its low tangential velocity of only ~3 km/s because this suggests that most of its velocity could be in the radial component. Indeed, WISE J0720 was found to have a large radial velocity of 83 km/s, implying it once might have made a close pass to the Sun. By tracing its motion through space, WISE J0720 was determined to have passed within only ~50,000 AU of the Sun ~70,000 years ago.

Currently, WISE J0720 is about 20 light years away. This binary system weighs in at only ~0.15 times the Sun’s mass. It consists of an M9 red dwarf star and a T5 brown dwarf companion. Nevertheless, the low mass of WISE J0720 and the relatively high velocity of its close passage meant that the encounter probably has a negligible statistical impact on the flux of long-period comets into the planetary region of the Solar System. The discovery of WISE J0720 suggests more of such objects may be lurking in the Sun’s stellar neighbourhood.

WISE J0720 is extremely faint. Even at closest approach, it would have been invisible to the naked eyes. Nevertheless, the stellar component of WISE J0720 is an active red dwarf star that is perhaps capable of generating powerful flares. Such flares can cause the star to brighten by orders of magnitude. Although WISE J0720 is invisible in its quiescent state, an exceptionally powerful flare event might have allowed our Pleistocene ancestors to observe a short-lived transient in the night sky. From the vantage point of WISE J0720, the Sun would be by far the brightest star in the sky.

Reference:

Mamajek et al. (2015), “The Closest Known Flyby of a Star to the Solar System”, arXiv:1502.04655 [astro-ph.SR]

On Worlds Where Day Dominates Night

For most planets including Earth, it is reasonably accurate to assume that the nightside begins at a zenith angle of about 90°, which means approximately half the planet’s surface is in daylight while the other half experiences night. A zenith angle of 0° corresponds to the spot on the planet where its host star appears directly overhead (i.e. the sub-stellar point). However, for planets that orbit very close to their host stars, the large apparent size of the stellar disk means that an irradiance distribution which assumes the nightside begins at a zenith angle of 90° becomes inaccurate. On these planets, the nightside begins at a zenith angle significantly larger than 90°. This means the dayside covers a much greater extent than the nightside.

Planets that orbit close to their host stars are quite likely tidally-locked, where the same side of the planet always faces its host star, resulting in a permanent dayside and nightside. Extreme examples include COROT-7b, Kepler-10b and Kepler-78b. These planets orbit so close to their host stars that they each complete a year in a matter of hours. Temperatures at the sub-stellar point are expected to reach 2,500 K or more. Such temperatures are high enough to keep rock material molten, making it possible for lava oceans to exist on the hellish daysides of these planets.

On these intensely hot worlds, the host star looms so large in the sky that the nightside only begins at a zenith angle much larger than 90°, so that well over half the planet is always in daylight. In the case of Kepler-78b, more than two-thirds of the planet is always in daylight. These planets are too hot to hold onto any appreciable atmosphere than can effectively transport heat from the dayside to the nightside. As a result, even though the dayside can blaze as hot as the tungsten filament of an incandescent bulb, temperatures on the nightside can be as low as just a few tens of degrees above absolute zero.

COROT-7b, a rocky planet with 1.58 times Earth’s diameter, is in a 20.5-hour orbit at a distance of 0.0172 AU around a star measuring 0.82 times the Sun’s diameter. On COROT-7b, the nightside begins at a zenith angle of 102.6°, which means 61.2 percent of the planet’s surface is always in daylight. Kepler-10b is another rocky planet with 1.07 times Earth’s diameter. It is in a 20.1-hour orbit at a distance of 0.0168 AU around a Sun-like star measuring 1.07 times the Sun’s diameter. On Kepler-10b, the nightside begins at a zenith angle of 106.9°, which means 65.2 percent of the planet’s surface is always in daylight.

The most striking example is Kepler-78b, a rocky world with a girth that is 1.16 times the diameter of Earth. This planet is even crazier than COROT-7b and Kepler-10b when it comes to how close-in a planet can orbit its host star. Kepler-78b whizzes around its host star once every 8.5 hours. At a distance of 0.0089 AU from a star measuring 0.737 times the Sun’s diameter, Kepler-78b is only 1.6 stellar radii from the fiery surface of its host star. On Kepler-78b, the nightside begins at a zenith angle of 112.3°, which means a remarkable 70.5 percent of the planet’s surface is always in scorching daylight.

Low-Density Planet with a Non-Periodic Orbit

Kepler-289 c is a newly discovered low-mass, low-density planet in orbit around a Sun-like star. Its orbit lies between two other known planets: Kepler-289 b (inner planet) and Kepler-289 d (outer planet). Perturbations from its inner and outer planetary siblings cause the 66 day orbit of Kepler-289 c to vary by as much as ~5 hours. For comparison, the orbital period of Earth varies by only one second or so.

The non-periodic orbit of Kepler-289 c allows it to elude automated computer algorithms that search through the light curve data collected by NASA’s planet-hunting Kepler space telescope. In fact, the discovery of Kepler-289 c was made possible by enlisting the help of citizen scientists who visually scan through the light curve data collected by Kepler as part of the Planet Hunters program. Computers can’t find the unanticipated, but humans can.

Kepler-289 b, the inner planet, has an orbital period of 35 days, 7.3 times Earth’s mass, 2.15 times Earth’s radius and receives about 25 times more insolation than Earth gets from the Sun. Kepler-289 d, the outer planet, has an orbital period of 126 days, 132 times Earth’s mass, 11.6 times Earth’s radius and receives about 4.4 times more insolation than Earth gets from the Sun.

The middle planet, Kepler-289 c, has 4 times Earth’s mass and 2.7 times Earth’s radius, giving it a density of 1.2 g/cm³. Such a density is remarkably low for a planet of its mass. This requires about half the planet’s mass to be in the form of a substantial gaseous envelope of hydrogen and helium. Kepler-289 c joins a growing list of low-mass, low-density planets.

Kepler-289 b, c and d, are all too hot to be habitable. An interesting characteristic of the planetary system is that the three planets could be residing in a 1:2:4 Laplace resonance since the outer planet (Kepler-289 d) has an orbital period about twice as long as that of the middle planet (Kepler-289 c) and the middle planet’s orbital period is about twice as long as that of the inner planet (Kepler-289 b).

Reference:

Schmitt et al. (2014), “Planet Hunters VII. Discovery of a New Low-Mass, Low-Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)”, arXiv:1410.8114 [astro-ph.EP]

Kepler-407b: A Superheated Earth-Size Planet

Kepler-407b is a transiting Earth-size planet detected by NASA’s Kepler space telescope. Measuring 1.07 ± 0.02 times the radius of Earth, Kepler-407b also whizzes around its host star, a Sun-like star, in an extremely close-in orbit with a period of only 16 hours. At such close proximity to its host star, temperatures on the planet’s day side are expected to reach well over 2000 K.

Kepler-407b is most likely tidally-locked with the same side of the planet always pointed towards its host star. A lave ocean could exist on the planet’s intensely hot day side where temperatures are too high for rock material to remain solid. Kepler-407b joins a growing list of planets with orbital periods less than 24 hours. Other similar planets include Kepler-10b and Kepler-78b.

Follow-up radial velocity (RV) measurements to determine the mass of Kepler-407b could only provide a one sigma upper limit of 1.7 times the mass of Earth. These RV measurements also detected the partial orbit of an outer, non-transiting planet, hereafter identified as Kepler-407c. Given that only a quarter of a potentially ~10 year long orbit was measured by RV, the planet’s orbit and mass could only be weakly constrained.

Kepler-407c is estimated to have an orbital period of 3000 ± 500 days and somewhere between 5 to 20 times the mass of Jupiter. However, depending on the inclination of its orbit, Kepler-407c could also be a more massive object such as a brown dwarf or even a red dwarf star. Future RV measurements will be needed to better constrain the masses of both Kepler-407b and c.

Reference:

Marcy et al. (2014), “Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets”, arXiv:1401.4195 [astro-ph.EP]

Transforming Mini-Neptunes into Habitable Planets

After formation, a planet does not always stay in the same orbit around its host star. Various interactions can cause a planet to migrate nearer to or further from its host star. As a result, a planet formed outside the snow line (i.e. the region of the protoplanetary disk beyond which it is cold enough for water and other volatiles to condense into ice) can potentially be brought nearer to its host star, to within the habitable zone (HZ). Due to the large abundance of ices beyond the snow line, a planet coming into the HZ from there should contain a lot of water - an important criterion for habitability. Nonetheless, such a planet may also have acquired a thick gaseous envelope of hydrogen and helium, making it uninhabitable. A planet like this can be termed a “mini-Neptune”.

Mini-Neptunes are basically icy/rocky cores surrounded by massive gaseous envelopes. In a way, they are miniature versions of Neptune. A study by Luger et al. (2015) show mini-Neptunes that migrate into the HZ of M-dwarf stars can naturally shed their thick gaseous envelopes of hydrogen and helium, effectively transforming into potentially habitable, volatile-rich Earth-mass and super-Earth-mass planets. The pre-main sequence (PMS) phase of a star is the period during which the star has yet to fully contract. Sun-like stars spend less than 50 million years in the PMS phase, while M-dwarf stars can spend hundreds of millions of years in the PMS phase where they remain super-luminous. This can lead to greater atmospheric mass loss for planets around M-dwarf stars.

M-dwarf stars remain active for many hundreds of millions of years after formation. During this period they emit high levels of X-ray/extreme ultraviolet (XUV) radiation. Exposure to high levels of XUV radiation for such a long period of time can cause a mini-Neptune that has migrated into the HZ to lose its thick gaseous envelope of hydrogen and helium to space. After losing their massive gaseous envelopes, these objects are termed “habitable evaporated cores” (HECs). Such a process is most likely to occur for mini-Neptunes with solid cores ~1 Earth-mass or so and up to 50 percent hydrogen/helium by mass.

Mini-Neptunes with core masses grater than roughly twice the mass of Earth are not expected to form HECs because they are sufficiently massive to retain their thick gaseous envelopes. HECs are also unlikely to form around K-dwarf and G-dwarf stars because they have shorter super-luminous PMS phases and shorter XUV timescales. With abundant water content, HECs are expected to be water worlds with very deep oceans and are therefore not true terrestrial planets. On Earth, geochemical cycles are crucial for life. However, the oceans on HECs can be deep enough for high-pressure ice to form at the bottom, separating the ocean from the underlying mantle, possibly inhibiting geochemical cycling. Nevertheless, geochemical cycling can still go on via solid state ice convection.

Reference:

Luger et al., “Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs”, Astrobiology, Volume: 15, Issue 1, January 15, 2015

Thermally Bloated Low-Mass White Dwarfs

White dwarfs typically pack as much mass as the Sun into a volume the size of Earth. Given that Earth’s diameter is only one percent the Sun’s, white dwarfs are very compact objects. However, white dwarfs are not always as dense and compact as they typically are. A study by Rappaport et al. (2015) reports on the discovery of two thermally bloated low-mass white dwarfs in the binary star systems KIC 9164561 and KIC 10727668. Both white dwarfs happen to transit their primary host stars and were first detected using data from NASA’s Kepler space telescope.

Figure 1: Artist’s impression of a white dwarf.

KIC 9164561 consists of a white dwarf with 0.197 ± 0.005 times the Sun’s mass and 0.277 ± 0.005 times the Sun’s diameter in a 1.267 day orbit around a 2.02 ± 0.06 solar-mass A-type star. The other binary star system, KIC 10727668, consists of a white dwarf with 0.266 ± 0.035 times the Sun’s mass and 0.151 ± 0.004 times the Sun’s diameter in a 2.306 day orbit around a 2.22 ± 0.10 solar-mass A-type star. Both white dwarfs in KIC 9164561 and KIC 10727668 are hot, low-mass helium white dwarfs with estimated temperatures of 10,410 ± 200 K and 14,110 ± 440 K, respectively.

The addition of KIC 9164561 and KIC 10727668 brings the totally number of binary systems consisting of low-mass white dwarfs in tight orbits around A-type stars in the Kepler sample to six. The other four similar binary systems are KOI 74, KOI 81, KOI 1224 and KOI 1375. Of the six binary systems, KIC 9164561 has the shortest orbital period and the largest, most thermally bloated white dwarf. Nearly a quarter the Sun’s diameter but with only 1/5th the mass, the white dwarf in KIC 9164561 has a mean density of only 12.8 ± 1.2 g/cm³, making it less dense than gold. For comparison, white dwarfs typically have mean densities at ~1 ton per cubic centimetre.

Binary systems like KIC 9164561 and KIC 10727668 can form via two possible mechanisms. The first mechanism is known as common envelope (CE) evolution where the more massive star in the binary evolves into a giant star and undergoes unstable mass transfer to its companion star. The rate of mass transfer is sufficiently rapid to cause the companion star to spiral towards the core of the giant star. Such an in-spiralling process drives the transfer of angular momentum from the companion star to the envelope of the giant star. This causes the envelope to be expelled into space, leaving behind the helium core as a low-mass white dwarf in a close-in orbit around the companion star.

The second mechanism involves stable mass transfer where the more massive star (i.e. donor star) in the binary evolves quicker and grows in size before it starts to transfer mass to its companion star. During the mass transfer process, the donor star continues to evolve as a red giant and forms a helium core. Eventually, so much mass is transferred that only the helium core of the donor star remains in the form of a low-mass white dwarf in a tight binary.

Figure 2: Transit light curve for KIC 9164561. The smaller dip corresponds to the transit of the white dwarf in front of the primary A-type star while the larger dip corresponds to the occultation of the white dwarf as it passes behind the primary. The bottom panel shows the residuals of the data minus the fitted model. Rappaport et al. (2015).

Figure 3: As in Figure 2, but for KIC 10727668. Rappaport et al. (2015).

Reference:
Rappaport et al. (2015), “Thermally Bloated Low-Mass White Dwarfs”, arXiv:1502.02303 [astro-ph.SR]

Forming Icy Super-Earths in the Cold

Sedna, 2012 VP113 and several other objects belong to an intriguing population of small, icy worlds that orbit the Sun far beyond Pluto. These objects never come closer to the Sun than Neptune and have semimajor axis (i.e. the “average” distance from the Sun) greater than 150 AU. They are believed to be just a tiny fraction of a vast population of similar objects lurking in the dark far from the Sun. Observations of the orbits of these objects reveal a clustering of their arguments of perihelion around 0°. Such a distribution is statistically unlikely and suggests the presence of one or more super-Earths at 200 to 300 AU from the Sun “shepherding” the orbits of these objects.

Figure 1: Artist’s impression of an object orbiting far from the Sun.

However, the presence of one or more super-Earths at large distances from the Sun is not supported by the current understanding of planet formation which cannot account for the in-situ formation of such massive objects so far from a Sun-like star. Nonetheless, a recent study suggests it might be possible for one or more super-Earths to form in-situ at distances of around 125 to 250 AU from a Sun-like star. Unlike a typical planet which may take several million years to coalescence from a disk of protoplanetary material, it takes one to several billion years for icy material far from a Sun-like star to form super-Earths.

A disk of material with ~15 Earth-masses at 125 to 250 AU from a Sun-like star can potentially form super-Earths in-situ. When the disk material consists of planetesimals with initial radii of 1 cm to 1 m, super-Earths can form in 1 to 3 billion years at 125 AU and in 2 to 5 billion years at 250 AU. Larger planetesimals with initial radii of 10 m to 10 km take longer to form super-Earths; ~4 billion years or less at 125 AU and ~10 billion years or more at 250 AU (i.e. longer than the age of the Solar System). Discovering a super-Earth at ~200 AU from the Sun would imply the presence of a very large reservoir of material far from the Sun and would also change the current understanding of the layout of the Solar System.

Figure 2: Growth of the largest object at 125 AU for planetesimals with initial radii of 10 cm to 10 km. Kenyon & Bromley (2015).

Figure 3: Growth of the largest object at 250 AU for planetesimals with initial radii of 1 cm to 10 m. Planetesimals 10 m or more cannot grow into super-Earths over the age of the Solar System. Kenyon & Bromley (2015).

References:

- S. J. Kenyon, B. C. Bromley (2015), “Formation of Super-Earth Mass Planets at 125-250 AU from a Solar-type Star”, arXiv:1501.05659 [astro-ph.EP]

- C. de la Fuente Marcos, R. de la Fuente Marcos (2014), “Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets”, arXiv:1406.0715 [astro-ph.EP]

Adding More Earth-Like Planetary Candidates

Using 3 years (Q1 to Q12) of data from NASA’s Kepler space telescope, Rowe et al. (2015) report the discovery of 855 additional planetary candidates, bringing the current total to 3697. Of these planetary candidates, about 130 receive less than twice the flux Earth gets from the Sun (Sᴇ) and about 1100 are less than 1.5 times the Earth’s radius (Rᴇ). Potentially habitable planets are those that meet both criteria (< 2 Sᴇ and < 1.5 Rᴇ). This latest addition brings the number of candidate potentially habitable planets to over a dozen. The reason they are termed planetary candidates is because additional observations are required to confirm if they are bona fide planets. Nevertheless, most of the planetary candidates are expected to be true planets.

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

In this latest addition of planetary candidates, all of the candidate potentially habitable planets orbit cool K or M-dwarf stars. KOI-3138.01 is a Mars-sized (Rᴘ = 0.57 Rᴇ) planetary candidate that receives a Mars-like amount of flux (Sᴘ = 0.47 Sᴇ). It has an 8.7 day orbit around a very cool M-dwarf star which has an estimated temperature of 2703 K. KOI-3284.01, confirmed as a bona fide planet and designated Kepler-438b, is an Earth-sized (Rᴘ = 0.98 Rᴇ) planet which receives 31 percent more flux than Earth gets from the Sun (Sᴘ = 1.31 Sᴇ). It orbits a cool M-dwarf star every 35.2 days.

KOI-2418.01 (Rᴘ = 1.12 Rᴇ) and KOI-2626.01 (Rᴘ = 1.12 Rᴇ) are two Earth-sized planetary candidates that respectively receive 35 percent and 65 percent the flux Earth receives. Both planetary candidates orbit cool M-dwarf stars with orbital periods of 86 days (KOI-2418.01) and 38 days (KOI-2626.01). At the hotter inner edge of the habitable zone, KOI-2124.01 (Rᴘ = 1.00 Rᴇ) and KOI-3255.01 (Rᴘ = 1.37 Rᴇ) are Earth-sized planetary candidates that receive ~80 percent for flux than Earth. They are both more likely to resemble Venus than Earth. With more data from Kepler yet to be analysed and with improving data analysis methods, a large number of interesting planetary candidates still await discovery.

Figure 2: A plot of planet radius versus incident flux for all planet candidates known in the Q1-Q12 catalogue. (Note that some planet candidates lie outside the chosen axis limits for the plot, and thus are not shown.) The temperature of the host star is indicated via the colour of each point, and the signal-to-noise of the detection is indicated via the size of each point. Planet candidates that were newly designated in Q1-Q12 are indicated with black circles around the point. The two vertical dashed lines indicate the incident flux received by Mars and Venus, as a broad guide to the potential habitable zone. The horizontal dotted line is set at 1.5 Rᴇ as a suggested upper size limit to terrestrial-type planets (i.e. Earth-like planets). Rowe et al. (2015).

Reference:

Rowe et al. (2015), “Planetary Candidates Observed by Kepler V: Planet Sample from Q1-Q12 (36 Months)”, arXiv:1501.07286 [astro-ph.EP]