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Astronomers have confirmed that the planet Gliese 436b seems to be trailing a gigantic, comet-like cloud of hydrogen.
The discovery of the first exoplanets nearly 20 years ago ushered in a new domain of astronomy. For the first time, we knew that our solar system was not alone in the galaxy. In the past two decades, astronomers have revealed a complex and diverse menagerie of exoplanets spanning a wide range of orbital and physical properties, from giant worlds several times Jupiter’s size to a seven-planet system that would fit within Earth’s orbit.
Among the members of the exoplanet zoo is Gliese 436b. This planet is nearby, with a distance just over 30 light-years, and about the mass and size of Neptune. However, unlike Neptune, Gliese 436b orbits its red dwarf star in just 2.6 days, making it much warmer than our solar system’s ice giant.
In 2014, Jennifer Kulow (University of Colorado, Boulder) and colleagues looked at Gliese 436b in ultraviolet light using the Hubble Space Telescope. They watched the planet pass once in front of its star from our perspective and found that, at ultraviolet wavelengths, the star stayed dim long after the planet had ostensibly finished passing. The team suggested that the planet might be trailing a comet-like tail behind itself that continued to block starlight after the main transit.
This led an international team of astronomers, led by David Ehrenreich (Geneva Observatory, Switzerland), to take a closer look at Gliese 436b with the Hubble Space Telescope and the Chandra X-ray Observatory. What they discovered has shed new light on the formation and evolution of exoplanets.
Gliese 436b blocks less than 1% of its host star’s optical light during each transit. But when viewed in ultraviolet (UV) light, an entirely different view emerges. The star Gliese 436 emits UV light (the high-energy cause of sunburns here on Earth during these summer months), including a specific wavelength called Lyman-alpha, which is unique to hydrogen. Ehrenreich's team studied the planet's transit at this wavelength during three transits and confirmed that the transits dimmed significantly more UV starlight than visible starlight, blocking over 50% of the host star's UV light!
Because the Lyman-alpha signal would only be reduced this way if there’s a lot of hydrogen between us and the star, the team argues in the June 24th Nature that this extended UV eclipse means that Gliese 436b is losing hydrogen as its atmosphere evaporates, producing a cloud of hydrogen much larger than the planet — even larger than the star. Astronomers have seen this effect in other exoplanets, but never in a planet this small.
Because Earth's atmosphere blocks most ultraviolet light from reaching us here on the surface, astronomers needed a space telescope with Hubble's ultraviolet capability and exquisite precision to view the cloud. "You wouldn't be able to see it at visible wavelengths," says Ehrenreich. "But when you turn the ultraviolet eye of Hubble onto the system, it's really quite a transformation — the planet turns into a monstrous thing."
Using the observations, Ehrenreich’s team was able to estimate how much hydrogen the planet has lost and its atmosphere’s evaporation rate. The numbers are astounding: 100 million to 1 billion grams (100 to 1,000 metric tons) of hydrogen evaporating from the planet every second. Assuming that hydrogen makes up 10% of Gliese 436b’s mass, that would mean the planet loses about one-tenth of a percent of its mass every billion years. But the star is about 6 billion years old now, and red dwarfs are notoriously more active in their early years. At first, the star could have blasted the planet with up to 100 times as much UV and X-ray radiation, whittling away about 10% of the planet’s atmosphere within its first billion years. This mass loss rate is larger than some comets in our solar system, but not large enough to totally destroy the planet's atmosphere over its lifetime.
Atmospheric evaporation such as this may also have happened in the earlier history of the solar system, when Earth had a hydrogen-rich atmosphere that dissipated. It may happen again to Earth's atmosphere at the end of our planet's life, when the Sun swells up to become a red giant and boils off our remaining atmosphere, before potentially engulfing our planet completely.
With upcoming large-scale programs dedicated to exoplanet studies, such as the Transiting Exoplanet Survey Satellite (TESS), astronomers expect to find other planets like Gliese 436b in the future. Gliese 436b will likely be the first in a class of small, evaporating planets that may shed some light on the importance of atmosphere evaporation in planetary evolution.
Below, watch a video with different animations of the Gliese 436 system. That's one gigantic cloud of hydrogen!
Reference: D. Ehrenreich et al. "A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b." Nature, June 25, 2015.
Want to introduce the kids to stargazing this summer? Start with the delightful bedtime book There Once Was a Sky Full of Stars.
X-ray echoes from binary star system Circinus X-1 are helping astronomers measure its distance from Earth.
Imagine ripples spreading out from a drop of water falling on a tepid lake. The concentric circles that radiate out from the center all have a common center, a geometry common in wave interactions, such as sound waves radiating from a speaker or light echoing in space.
Light echoes occur when a flash of radiation from an astronomical source collides with intervening matter, giving astronomers a new perspective on celestial happenings. In the case of the X-ray-emitting binary system named Circinus X-1, light echoes provide an unexpected opportunity to measure its distance directly, putting an end to years of debate. On June 20th Sebastian Heinz (University of Wisconsin-Madison) and colleagues reported on X-ray light echoes around this system in the Astrophysical Journal.
In late 2013 the neutron star at the center of Circinus X-1 flared, creating four concentric rings that astronomers spotted a few months later. The brief flare bounced off intervening dust clouds to form the concentric circles, which look like they circle the neutron star — but it turns out this is an optical illusionHow did the Rings Form?
When the neutron source flared, it emitted a brief flash of X-rays in every direction. Some of these X-rays traveled straight to Earth, but some of them scattered off intervening dust. The scattered X-rays take longer to arrive, and the lag in their arrival time gives a precise geometric measurement of the distance to the source.
The figure on the right outlines light echoes’ optical illusion. An X-ray travels outward from the neutron star at an angle “α,” but bounces off a screen of dust between the neutron star and the observer. Because of its detour, the observer sees the photon arrive at an angle “θ,” as if it came not from the neutron star but from above it. In a given moment, the observer will see all the X-rays scattering at a certain angle. Just like a protractor, all of these scattered photons come in at the same angle and form an illusory ring of light. If astronomers watched for long enough, they would see this circle ripple outward, as X-rays come in from ever larger angles.
In the case of Circinus X-1, there’s not just one but four dust clouds between the neutron star and us. So rather than watch a single ring ripple outward over time, the astronomers spotted four concentric rings.
The team collected the X-ray data via the Chandra X-Ray Observatory. They also studied carbon monoxide maps of the clouds themselves, compiled by the Mopra radio telescope in Australia, which told them how far away the clouds were from Earth. Together, these data sets allowed them to calculate Circinus X-1’s distance from Earth using simple geometry: 30,700 light-years, more than twice a previously published distance.
Knowing an object’s distance from Earth can tell us a lot about it, such as its intrinsic brightness. Just as a lamp right next to you seems to shine brighter than one across the street, knowing the distance to Circinus X-1 puts its apparent brightness into perspective. Now astronomers can begin to understand how the neutron star’s flares fit into its total energy output.
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The cover story of the October 2015 issue features the conundrum of massive star formation: how do these stars can form at all in the hostile environment they themselves create? Here, you'll find accompanying videos.
The nearest massive star formation region is Orion, about 1,500 light-years away. This WorldWide Telescope tour guides you through the star-forming regions in this constellation, showing protostars still enshrouded in heavy mantles of gas and dust, proplyds (protostars surrounded by protoplanetary disks), and massive just-born stars that wreak havoc on their surroundings. Watch it here, or go to WorldWide Telescope to watch an interactive version.
About five times further away, the Carina Nebula shows additional evidence for the beautiful destruction of massive stars. This movie is also available as a WorldWide Telescope interactive tour.
Despite the destruction their own intense radiation and powerful winds can cause, massive stars still appear to form much as low-mass stars do. A growing pile of evidence shows that massive stars, like their low-mass brethren, form out of the monolithic collapse of clumps of gas. Among this pile is a two-year compilation of observations of a future B-type star known as Orion Source I (see video below). The video shows a gas clumps running away from the protostar, a clear signature of an accretion disk feeding the protostar and the wind that flows off of it.
The protostar is unseen, but marked by a red circle. The accretion disk is also too opaque to show up in this movie, but it can be seen by its absence, a black diagonal bar at the image center.
Each dot in the "X" of colored lights is a clump of silicon monoxide gas speeding away from the disk and the central protostar. The clumps are color-coded for direction — red-colored spots move away from us, while blue-colored spots move toward us. (Some of the gas, colored green and yellow, rotates in a bridge connecting the two bottom arms of the X — a peek at the disk itself.)
These movies tell only part of the complex story of massive star formation. For more, check out our October 2015 issue.