Lone Planet in Interstellar Space

There appears to be increasing evidence for the existence of isolated planetary-mass objects. These objects are either unbound (i.e. drifting alone in interstellar space) or orbit host stars in widely separated, distant orbits. Recently, Luhman (2014) reported the detection of a nearby brown dwarf with 3 to 10 times the mass of Jupiter and an estimated temperature of roughly 250K (-23°C). Such an object could have formed in-situ as stars do or it could be a planet that got ejected from its natal planetary system.

Using a novel technique, Freeman et al. (2014) reported the detection of an isolated planetary-mass object after re-analysis of observational data from the gravitational microlensing event MOA-2011-BLG-274. Gravitational microlensing occurs when the gravitational field of a foreground object (lens) bends and magnifies light from a background object (source). For this to happen, the observer, foreground object and background object must be in near perfect alignment.

MOA-2011-BLG-274 was previously reported by Choi et al. (2012). One indication the lens could be a planetary-mass object is the short duration of the gravitational microlensing event. At any given time, the observed amount of magnification and the time of peak magnification vary slightly from point to point on the Earth’s surface. This is known as the terrestrial parallax effect and it is particular pronounced for short duration gravitational microlensing events.

Because MOA-2011-BLG-274 was monitored from a number of separate locations on Earth, the terrestrial parallax effect could be measured by Freeman et al. (2014), making it possible to pin down the distance and hence the mass of the lens, identified in this case as MOA-2011-BLG-274L with the suffix “L”. The best fit to the data shows MOA-2011-BLG-274L is 0.8 ± 0.3 times the mass of Jupiter and located 2.6 ± 0.8 thousand light years away.

Also, the data excludes the presence of a host star out to ~40 AU (i.e. 40 times the distance of Earth from the Sun). MOA-2011-BLG-274L is either an isolated planetary-mass object drifting in the depths of interstellar space or a planet in a far-flung orbit around a host star, sufficiently far out that the host star has no effect on the gravitational microlensing light curve.

References:
- Luhman (2014), “Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun”, ApJ 786 L18
- Freeman et al. (2014), “Can the masses of isolated planetary-mass gravitational lenses be measured by terrestrial parallax?”, arXiv:1412.1546 [astro-ph.EP]
- Choi et al. (2012), “Characterizing lenses and lensed stars of high-magnification single-lens gravitational microlensing events with lenses passes over source stars”, arXiv:1111.4032 [astro-ph.SR]

A Hot Giant Planet as Black as Charcoal

Kepler-423b is a hot-Jupiter which zips around its host star every 2.7 days in a close-in orbit. On each orbit, Kepler-423b crosses in front of its host star, causing an observable dip in the star’s brightness, thereby allowing the planet’s size to be measured. Kepler-423b is estimated to have 60 percent the mass of Jupiter and 1.2 times its diameter. The host star of Kepler-423b is a very old Sun-like star with an estimated age of 11 ± 2 billion years.

The orbit of Kepler-423b also periodically brings it behind its host star in what is known as a secondary eclipse. During secondary eclipse, the star occults any emission from the planet, leading to a slight decrease in the combined star-planet flux. For Kepler-423b, the drop in the combined star-planet flux is remarkably small. This indicates that Kepler-423b is a very dark object (i.e. low reflectivity). Kepler-423b reflects less than 4 percent of the light it receives from its host star, making the planet as reflective as charcoal. If Kepler-423b were more reflective, the secondary eclipse signature would be stronger since the planet would contribute more to the combined star-planet flux.

Even though Kepler-423b is as black as charcoal, the planet’s dayside would still appear utterly glaring. The extreme closeness to its host star means that the insolation the planet receives is a few hundred times more intense than what Earth receives from the Sun. Reflecting just a few percent of that intense insolation would still create nothing short of a blazing glare. Temperatures on the planet’s dayside can get as high as ~2000K.

Reference:
Gandolfi et al. (2015), “Kepler-423b: a half-Jupiter mass planet transiting a very old solar-like star”, arXiv:1409.8245 [astro-ph.EP]

Strange Quark Matter (SQM) Planets

At extreme densities, normal matter may exist in the form of strange quark matter (SQM). A consequence of this hypothesis is the existence of SQM stars which are hard to distinguish from neutron stars. SQM stars and neutron stars are both ultra-compact stars that measure only several kilometres across but can contain 1 to 2 times the Sun’s mass. Unlike neutron stars which have a minimum mass limit, SQM stars can have arbitrarily small masses. Since SQM is stable in bulk, planetary-mass clumps of SQM can exist (i.e. SQM planets). The detection of SQM planets would be very useful for testing the SQM hypothesis.

Gravitational waves are ripples in the curvature of spacetime that propagate outward from their source. Sources of detectable gravitational waves include binary systems composed of compact objects such as white dwarfs, neutron stars or black holes. A more tightly bound binary system emits stronger gravitational waves. For a normal matter planet orbiting an ultra-compact star, the emitted gravitational wave power is negligible (i.e. non-detectable) since the planet cannot come close enough to the star without being tidally disrupted.

However, things become very different for a SQM planet orbiting a SQM star. Due to its extreme compactness, a SQM planet can spiral very close to its host SQM star without being tidally disrupted. Such a compact system then becomes very efficient in producing strong gravitational waves. Upcoming gravitational wave detectors such as Advanced LIGO and the Einstein Telescope can detect gravitational waves arising from the in-spiral of a SQM planet into its host SQM star.

There are a number of possible mechanisms that can result in the formation of a SQM planet. One mechanism involves newly-born SQM stars that are very hot and exceedingly turbulent. The strong turbulence can eject planetary-mass clumps of SQM. If these clumps remain gravitationally bound, SQM planets are produced. A SQM planet with 1/10th the mass of Jupiter (i.e. 32 Earth masses) would measure only ~1000m in diameter.

Reference:
J. J. Geng, Y. F. Huang, T. Lu (2015), “Coalescence of Strange-Quark Planets with Strange Stars: a New Kind of Sources for Gravitational Wave Bursts”, arXiv:1501.02122 [astro-ph.HE]

Trio of Super-Earths Circling a Nearby Star

Following the failure of two of its four reaction wheels in May 2013, NASA’s planet-hunting Kepler space telescope was ingeniously repurposed for a new mission plan named K2. On 18 December 2014, it was announced that the K2 mission had detected its first confirmed exoplanet, a super-Earth or mini-Neptune designated HIP 116454 b (Vanderburg et al. 2014). The detection of HIP 116454 b was based on data collected during the testing run to prepare the space telescope for the nominal K2 mission.

Using data from the K2 mission covering 30 May to 21 August 2014, Crosseld et al. (2015) report the discovery of three super-Earths orbiting a nearby M dwarf star slightly larger than half the size of the Sun. This M dwarf star is designated EPIC 201367065 and it lies at a relatively nearby distance of about 150 light years. The three planets are 2.1, 1.7 and 1.5 times the size of Earth, and take 10.1, 24.6 and 44.6 days to circle the host star, respectively. From their sizes, the planets appear to span the range between rock-dominated “Earths/super-Earths” and lower-density “mini-Neptunes” with substantial volatile content.

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

Figure 2: Transit light curves of the three planets around EPIC 201367065. Top: Vertical ticks indicate the location of each planet’s transit. Bottom: Phase-folded photometry and best-fit light curves for each planet. Crosseld et al. (2015)

The outermost of the three planets, designated as planet “d”, is 1.5 times the size of Earth and receives somewhat more insolation from its host star than Earth receives from the Sun. This places the planet at the inner edge of the habitable zone where temperatures might be cool enough for the planet to support Earth-like conditions, and possibly life. If temperatures turn out to be too hot, then the planet is more likely a super-Venus with conditions too inhospitable for life.

All three planets around EPIC 201367065 are probably tidally-locked. This means the same side of each planet always faces the host star (i.e. permanent day and night sides). Since M dwarf stars are much cooler and fainter than the Sun, a planet around an M dwarf star needs to be much closer-in to get a similar amount of insolation as Earth gets from the Sun. As a result, the planet experiences stronger tidal forces from the host star and is expected to be tidally-locked.

A study by Yang et al. (2013) suggests that a tidally-locked planet can support thick water clouds on its dayside as the high amount of insolation drives strong dayside convection. The water clouds reflect away incoming radiation from the host star, leading to lower surface temperatures. If such a stabilizing cloud feedback mechanism works on planet “d”, then its surface temperatures can be lower than would otherwise be, allowing the planet to support cool and clement conditions even though its receives somewhat more insolation than Earth.

The trio of planets around EPIC 201367065 can be conveniently studied in further detail as the host star is relatively bright and nearby. Both the Hubble Space Telescope (HST) and the upcoming James Webb Space Telescope (JWST) have the capabilities to reveal more about this planetary system. Such a discovery so early in the mission shows the ubiquity of planetary systems and that the K2 mission will extend the legacy of Kepler for years to come.

References:
- Vanderburg et al. (2014), “Characterizing K2 Planet Discoveries: A super-Earth transiting the bright K-dwarf HIP 116454”, arXiv:1412.5674 [astro-ph.EP]
- Crosseld et al. (2015), “A nearby M star with three transiting super-Earths discovered by K2”, arXiv:1501.03798 [astro-ph.EP]
- Yang et al. (2013), “Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets”, arXiv:1307.0515 [astro-ph.EP]