Tuesday, June 4, 2013

Jet Streams on Uranus and Neptune

Eight from the Sun to see in orbit.
But no one mentions Neptune for that odyssey.
What is it about it.
So vast it is...
Sixty Earths could sit within its grasp.
- Lawrence S. Pertillar, “Neptune”, stanza 3 line 1-5.

Figure 1: An illustration of Neptune’s interior.

In the Solar System, the known planets can be classified into three categories - terrestrial planets (Mercury, Venus, Earth and Mars), gas giant planets (Jupiter and Saturn) and ice giant planets (Uranus and Neptune). Compared to the other planets in the Solar System, very little is known about Uranus and Neptune because the only spacecraft to have ever visited them was NASA’s Voyager 2 which flew by Uranus in 1986 and Neptune in 1989. The bulk composition of an ice giant planet is very different from a gas giant planet such as Jupiter or Saturn. An ice giant planet consists of a rocky core, an icy mantle and an outer gaseous hydrogen-helium envelop. The icy mantle comprises the bulk of the planet’s mass and is what defines an ice giant planet. In contrast, a gas giant planet is almost entirely made of hydrogen and helium.

Figure 2: Zonal winds on Uranus and Neptune. The data points correspond to atmospheric wind speed measurements of Uranus and Neptune by the Voyager 2 spacecraft (circles) and the Hubble Space Telescope (squares). The solid lines for Uranus and Neptune are empirical fits to the data, constrained to zero at the poles. Credit: Yohai Kaspi et al. (2013).

Both Uranus and Neptune have fast atmospheric jets that flow westward near the Equator and flow eastward at higher latitudes. Neptune in particular, has the fastest planetary winds anywhere in the Solar System. A paper by Yohai Kaspi et al. (2013) show that these atmospheric jets extend to depths of not more than ~1100 km for both planets. On Uranus, this depth corresponds to a pressure of ~2000 bar or the outermost 0.15 percent of the planet’s mass. For Neptune, this depth corresponds to a pressure of ~4000 bar or the outermost 0.20 percent of the planet’s mass. When one considers that Uranus and Neptune have mean diameters of 50,700 km and 49,200 km respectively, a depth of not more than ~1100 km implies that the atmospheric dynamics on Uranus and Neptune do not extend deeply into the planetary interiors. As a result, the dynamics of these atmospheric jets are probably driven by shallow processes.

Figure 3: Artist’s impression of a Uranus/Neptune-like ice giant planet.

In the upper atmosphere of Uranus and Neptune, the temperature is cool enough for methane to condense to form clouds. Deeper down, at pressures of up to a few bars, are clouds of ammonia and hydrogen sulphide. Yet deeper, at tens of bars or more, water condenses into clouds. A plausible mechanism that can drive the atmospheric jets on Uranus and Neptune is by latent heat release from moist convection. Deep in the atmosphere at pressures of ~300 bars, the condensation of water to form clouds can release sufficient latent heating to drive the atmospheric jets. Moist convection on Uranus and Neptune cannot be powered by energy from sunlight because sunlight only penetrates to pressures of a few bars. Instead, moist convection on Uranus and Neptune can only be driven by internal heat radiating out from the planetary interior. The internal flux to solar flux ratios of Uranus and Neptune are 0.06 and 1.6 respectively. For Neptune, such a high ratio means that the internal heat radiating out from the planet’s interior is greater than the energy the planet receives from the Sun. As a consequence, Neptune’s high internal heat flux drives a more vigorous moist convection and probably explains why it has the fastest winds in the Solar System.

Yohai Kaspi et al., “Atmospheric confinement of jet streams on Uranus and Neptune”, Nature 497, 344-347 (16 May 2013)

Monday, June 3, 2013

Radiation-Blasted Exoplanets

Figure 1: Artist’s impression of CoRoT-2b - a hot-Jupiter being blasted by intense radiation from its parent star. Credit: NASA/CXC/M.Weiss.

Hot-Jupiters are a class of Jupiter-sized exoplanets that orbit very close to their parent stars and receive intense amounts of stellar radiation. A typical hot-Jupiter orbits its parent star at ~1/20th the Sun-Earth distance and receives ~10,000 times more radiation that Jupiter does from the Sun. Being so close to its parent star, a hot-Jupiter is expected to be tidally-locked whereby one side continuously faces it parent star while the other side points away into the darkness of space. A study done by Daniel Fabrycky (2008) shows that reradiated thermal radiation from a hot-Jupiter carries away momentum and this can gradually change the star-planet separation distance by ~1 percent over the planet’s lifetime. For more extreme cases, this change can be as much as a few percent.

Radiation is comprised of photons which carry momentum. As a result, a force is exerted on a planet when radiation is reflected, absorbed or reradiated by the planet. Radiation that is reflected or absorbed by a hot-Jupiter simply pushes it away from its parent star. However, the direction of force exerted on a hot-Jupiter by reradiated thermal radiation is more complicated. Three-dimensional models suggest that the hottest region on a hot-Jupiter is unlikely to lie at the planet’s substellar point. Instead, the hottest region is displaced eastward by a superrotating wind. This causes reradiated thermal radiation from a hot-Jupiter to be emitted in a preferential direction and thus acts as a radiative thruster which adds angular momentum to the planet, pushing it into a wider orbit around its parent star.

Figure 2: The force vectors due to radiation asymmetry as a function of orbital position for the HAT-P-2b - a hot-Jupiter in an eccentric 5.63-day orbit around a star that is slightly bigger and hotter than the Sun. Credit: Daniel Fabrycky (2008).

The radiative thruster effect is amplified for hot-Jupiters that orbit closer than typical to their parent stars or for hot-Jupiters with inflated cross-sectional areas. For example, OGLE-TR-56b is a hot-Jupiter which orbits in a very tight 1.21-day orbit around its parent star. As a consequence, the stupendous amount of stellar radiation received by OGLE-TR-56b amplifies the radiative thruster effect. The result is an estimated increase in the star-planet separation distance of OGLE-TR-56b by ~5 percent over the planet’s lifetime.

This study shows that the intense radiation received and reradiated by a close-in exoplanet can change the planet’s orbit over its lifetime. Although the study done by Daniel Fabrycky (2008) considered only hot-Jupiters, the radiative thruster effect is expected to be more pronounced for Earth-sized rocky planets in close-in orbits around their parent stars. A number of such exoplanets have already been discovered, and they include planets such as Kepler-10b and KIC 8435766b. These terrestrial-sized planets are likely to have a smaller cross-section per unit mass than hot-Jupiters and are likely to be more influenced by the radiative thruster effect.

Daniel Fabrycky (2008), “Radiative Thrusters on Close-in Extrasolar Planets”, arXiv:0803.1839 [astro-ph]

Sunday, June 2, 2013

Jet-Black Exoplanet

TrES-2b is a Jupiter-sized exoplanet discovered in 2006 by the Trans-Atlantic Exoplanet Survey (TrES). It happens to be in the field-of-view of NASA’s Kepler space telescope and observations by Kepler found that it reflects less than one percent of the sunlight falling on it. This makes TrES-2b one of the darkest exoplanets currently known. TrES-2b orbits just five million kilometres from its parent star and has an orbital period of only 2.47 days. It is in a category of planets known as hot-Jupiters. Being so close to its parent star, TrES-2b is superheated to 1000 degrees Centigrade.

Figure 1: Artist’s conception of TrES-2b with a hypothetical moon in the foreground.

Figure 2: Orbital photometric phase variations of TrES-2b showing a contrast of 6.5 parts per million between the planet’s day-side and night-side. Credit: David M. Kipping & David S. Spiegel (2011).

Since Kepler is a planet-hunting telescope designed to measure the tiny dip in brightness when an Earth-sized planet crosses in front of its parent star, its exquisite photometric precision allows the reflectivity of TrES-2b to be measured. This is done by measuring the combined brightness of the star-planet system as the planet’s day-side rotates in and out of view. Measurements by Kepler showed that the contrast between the day-side and night-side photon flux of TrES-2b is only 6.5 parts per million. Such a tiny day-night contrast indicates that TrES-2b is exceptionally dark because a more reflective planet would have shown larger brightness variations as its day-side rotates in and out of view. TrES-2b appears to have an incredibly low reflectivity of less than one percent. In fact, best fit models show that its reflectivity is a mere 0.04 percent.

Jupiter reflects more than one-third of the sunlight that reaches it due to the presence of bright reflective clouds in its atmosphere. Unlike Jupiter, TrES-2b lacks reflective clouds. However, that alone is far from sufficient to explain the planet’s extremely low reflectivity. The presence of light-absorbing chemicals such as gaseous sodium and potassium in the hot atmosphere of TrES-2b may help explain the planet’s jet-black appearance. Nevertheless, the cause for the extremely low reflectivity of TrES-2b still remains unknown, although it may be explained by the presence of yet unknown atmospheric constituents.

“It’s darker than the blackest lump of coal, than dark acrylic paint you might paint with. It’s bizarre how this huge planet became so absorbent of all the light that hits it”, says astronomer David Spiegel of Princeton University. “However, it’s not completely pitch black. It’s so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove.”

David M. Kipping & David S. Spiegel (2011), “Detection of Visible Light from the Darkest World”, arXiv:1108.2297 [astro-ph.EP]

Saturday, June 1, 2013

Trojan Earths

In a protoplanetary disk around a young star, it is believed that regions with higher gas densities tend to concentrate solid material. If these high gas density regions are sufficiently long-lived, the aggregates of solid material within them can gravitationally collapse to form planets. The presence of a gas giant planet like Jupiter can create such long-lived, high gas density regions; especially around the gas giant planet’s leading (L4) and trailing (L5) Lagrangian points. The high gas density regions around L4 and L5 tend to concentrate solid material in a process known as Lagrangian trapping. A paper by Lyra et al. (2008) show that Lagrangian trapping can lead to gravitational collapse of aggregated solid material and form objects with masses in the regime of terrestrial planets (~ 0.1 to 10 Earth masses).

Figure 1: A contour plot showing the 5 Lagrangian points of the Sun-Earth system. The same layout of the 5 Lagrangian points also applies for the Sun-Jupiter system. The regions around the leading (L4) and trailing (L5) Lagrangian points are “islands of stability”.

As a gas giant planet orbits around its infant star, it clears out a wide gap in the protoplanetary disk. The environment within the gap is depleted of gas and other materials. Nevertheless, the regions around L4 and L5 serve as “islands of stability” where high gas densities are retained. The high gas densities create drag and damp the motion of solid particles. This allows Lagrangian trapping to occur where solid particles become trapped in the high gas density regions around L4 and L5. Simulations were conducted by Lyra et al. (2008) to determine the effectiveness of Lagrangian trapping in forming terrestrial planets with masses ranging from sub-Earth-mass to super-Earth-mass. A planet formed at L4 or L5 is known as a Trojan planet or Trojan Earth (if it has ~ 1 Earth mass). The word “Trojan” comes from the Trojan asteroids that exist around Jupiter’s L4 and L5 Lagrangian points.

Lyra et al. (2008) investigated the formation of Trojan planets by running simulations using a gas giant planet with the mass of Jupiter and solid particles with diameters of 1 cm, 10 cm, 30 cm and 1 m. The 10 cm and 30 cm solid particles readily underwent gravitational collapse at the Lagrangian points. For the 1 cm solid particles, they were so well coupled to the gas that gravity is not the main factor driving their collapse. Instead, the 1 cm solid particles collapse as the high gas density regions around L4 and L5 gradually shrunk. For the large 1 m solid particles, they remained unbound because they were too heavy to be captured by gas drag into the high gas density regions.

Each simulation by Lyra et al. (2008) was run using only one particle size. Gravitational collapse was more efficient for 10 cm solid particles than for 30 cm ones. The 10 cm solid particles collapsed to form a 0.6 Earth-mass planet at L4 and a 2.6 Earth-mass planet at L5. For the case involving 30 cm solid particles, a 0.1 Earth-mass planet was formed at L4, but the solid particles remained unbound at L5. These planet masses are only applicable for the particular set of assumptions used in the simulations by Lyra et al. (2008).

Figure 2: Artist’s conception of an Earth-size planet with vegetation covering most of the planet’s surface from pole-to-pole. Credit: Goran Licanin.

Figure 3: Artist’s conception of a habitable Earth-size planet with a large ice sheet on one of the planet’s poles.

Lagrangian trapping can produce a wide range of planetary masses depending on many conditions such as mass of gas giant planet, density of protoplanetary disk, size distribution of solid particles, etc. As a result, a huge diversity of Trojan planets is expected. Trojan planets formed at the Lagrangian points of a gas giant planet orbiting within the habitable zone of its parent star can potentially be Earth-like and habitable. Such “Trojan Earths” can appear very similar to the Earth. For a gas giant planet orbiting further from its parent star, Trojan planets formed at its Lagrangian points can acquire a significant fraction of icy material if temperatures are cool enough for ices to exist in the protoplanetary disk. Such a Trojan planet is expected to have a surface ice shell overlying a vast global ocean of liquid water.

If Trojan objects with masses in the regime of terrestrial planets can form, it is worth considering why Trojan planets are absent in the Solar System. One reason could be that the initial Trojan objects that formed at Jupiter’s L4 and L5 Lagrangian points were de-stabilized when Jupiter and Saturn crossed the 2:1 mean motion resonance. Following that, Jupiter acquired a new population of Trojan objects. However, without gas drag, the new population of Trojan objects could not assembly into planets and hence left behind what is now observed as the Trojan asteroids at Jupiter’s Lagrangian points. As a result, Trojan planets are probably more common in planetary systems with only one gas giant planet or in planetary systems where the gas giant planets did not undergo resonance crossing.

Lyra et al. (2008), “Standing on the shoulders of giants: Trojan Earths and vortex trapping in low mass self-gravitating protoplanetary disks of gas and solids”, arXiv:0810.3192 [astro-ph]