Why Is Earth So Dry?

A Tale of Two Disk Models
Credit: NASA, ESA, and A. Feild (STScI)

With large swaths of oceans, rivers that snake for hundreds of miles, and behemoth glaciers near the north and south poles, Earth doesn't seem to have a water shortage. And yet, less than one percent of our planet's mass is locked up in water, and even that may have been delivered by comets and asteroids after Earth's initial formation.

Astronomers have been puzzled by Earth's water deficiency. The standard model explaining how the solar system formed from a protoplanetary disk, a swirling disk of gas and dust surrounding our Sun, billions of years ago suggests that our planet should be a water world. Earth should have formed from icy material in a zone around the Sun where temperatures were cold enough for ices to condense out of the disk. Therefore, Earth should have formed from material rich in water. So why is our planet comparatively dry?

A new analysis of the common accretion-disk model explaining how planets form in a debris disk around our Sun uncovered a possible reason for Earth's comparative dryness. Led by Rebecca Martin and Mario Livio of the Space Telescope Science Institute in Baltimore, Md., the study found that our planet formed from rocky debris in a dry, hotter region, inside of the so-called "snow line." The snow line in our solar system currently lies in the middle of the asteroid belt, a reservoir of rubble between Mars and Jupiter; beyond this point, the Sun's light is too weak to melt the icy debris left over from the protoplanetary disk. Previous accretion-disk models suggested that the snow line was much closer to the Sun 4.5 billion years ago, when Earth formed.

"Unlike the standard accretion-disk model, the snow line in our analysis never migrates inside Earth's orbit," Livio said. "Instead, it remains farther from the Sun than the orbit of Earth, which explains why our Earth is a dry planet. In fact, our model predicts that the other innermost planets, Mercury, Venus, and Mars, are also relatively dry. "

The results have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

In the conventional model, the protoplanetary disk around our Sun is fully ionized (a process where electrons are stripped off of atoms) and is funneling material onto our star, which heats up the disk. The snow line is initially far away from the star, perhaps at least one billion miles. Over time, the disk runs out of material, cools, and draws the snow line inward, past Earth's orbit, before there is sufficient time for Earth to form.

"If the snow line was inside Earth's orbit when our planet formed, then it should have been an icy body," Martin explained. "Planets such as Uranus and Neptune that formed beyond the snow line are composed of tens of percents of water. But Earth doesn't have much water, and that has always been a puzzle."

Martin and Livio's study found a problem with the standard accretion-disk model for the evolution of the snow line. "We said, wait a second, disks around young stars are not fully ionized," Livio said. "They're not standard disks because there just isn't enough heat and radiation to ionize the disk."

"Very hot objects such as white dwarfs and X-ray sources release enough energy to ionize their accretion disks," Martin added. "But young stars don't have enough radiation or enough infalling material to provide the necessary energetic punch to ionize the disks."

So, if the disks aren't ionized, mechanisms that would allow material to flow through the region and fall onto the star are absent. Instead, gas and dust orbit around the star without moving inward, creating a so-called "dead zone" in the disk. The dead zone typically extends from about 0.1 astronomical unit to a few astronomical units beyond the star. (An astronomical unit is the distance between Earth and the Sun, which is roughly 93 million miles.) This zone acts like a plug, preventing matter from migrating towards the star. Material, however, piles up in the dead zone and increases its density, much like people crowding around the entrance to a concert, waiting for the gates to open.

The dense matter begins to heat up by gravitational compression. This process, in turn, heats the area outside the plug, vaporizing the icy material and turning it into dry matter. Earth forms in this hotter region, which extends to around a few astronomical units beyond the Sun, from the dry material. Martin and Livio's altered version of the standard model explains why Earth didn't wind up with an abundance of water.

Martin cautioned that the revised model is not a blueprint for how all disks around young stars behave. "Conditions within the disk will vary from star to star," Livio said, "and chance, as much as anything else, determined the precise end results for our Earth."

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Mario Livio
Space Telescope Science Institute, Baltimore, Md.
410-338-4439
mlivio@stsci.edu

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The protoplanetary disc around Beta Pictoris

Beta Pictoris b

In the image the dashed line indicates the true disc plane
Credit: Amateur astronomer Rolf Wahl Olsen

This image shows the famous protoplanetary disc of debris and dust orbiting the star Beta Pictoris 63.4 light years away. This is a very young system thought to be only around 12 million years old and is essentially similar to how our own Solar System must have formed some 4.5 billion years ago. The disc is seen edge-on from our perspective and appears in professional images as thin wedges or lines protruding radially from the central star in opposite directions.

For the last couple of years I have been wondering if it was possible for amateurs to capture this special target but have never come across any such images. The main difficulty is the overwhelming glare from Beta Pictoris itself which completely drowns out the dust disc that is circling very close to the star. Images of the disc taken by the Hubble Space Telescope, and from big observatories, are usually made by physically blocking out the glare of Beta Pictoris itself within the optical path.

But recently I then found this 1993 paper 'Observation of the central part of the beta Pictoris disk with an anti-blooming CCD' (Lecavelier des etangs, A., Perrin, G., Ferlet, R., Vidal-Madjar, A., Colas, F., et al., 1993, A&A, 274, 877) .

Full article available here: http://adsabs.harvard.edu/abs/1993A%26A...274..877L

I then realised that it might not be entirely impossible to also record this object with my own equipment. I followed the technique described in the paper above, which basically consists of imaging Beta and then taking another image of a similar reference star under the same conditions. The two images are subtracted from each other to eliminate the stellar glare, and the dust disc should then hopefully reveal itself.

First I collected 55 images of Beta Pictoris at 30 seconds each. The dust disc is most prominent in IR so ideally a better result would be expected with the use of an IR pass filter. Since I only have a traditional IR/UV block filter I just imaged without any filter, to at least get as much IR light through as possible.

Next step was to capture a similar image of a reference star under the same conditions. For this purpose I used Alpha Pictoris as the paper suggested. This star is of nearly the same spectral type (A7IV compared to Beta's A6V) and is also close enough to Beta in the sky so that the slight change in telescope orientation should not affect the diffaction pattern. However, since the two stars have different magnitudes I needed to calculate how long to expose Alpha for in order to get a similar image which I could subtract from the Beta image.

The magnitude difference between the stars is 3.86(Beta) - 3.30(Alpha) = 0.56
Due to the logarithmic nature of the magnitude scale we know that a difference of 1 magnitude equals a brightness ratio of 2.512. Therefore 2.512 to the power of the numerical magnitude difference then equals the variation in brightness.
2.512^0.56 = 1.67, so it appears Alpha is 1.67 times brighter than Beta. This means that exposure for Alpha should be 1/1.67 = 0.597x that of Beta. I took the liberty of using 0.6x for simplicity's sake...
So I collected 55 images of 18 seconds (30 x 0.6) for Alpha.

Both sets of images were stacked separately in Registax and I then imported these into Photoshop, layered Alpha in 'Difference' mode on top of Beta and flattened the result. This produces a very dark image (which it should!) apart from the different background stars. But after some curves adjustment I was able to see clear signs of the actual dust disc protruding on both sides from the glare of the star. I was very happy to conclude that the position angle with regards to the background stars matched the official images exactly.
This raw Difference image looked rather horrible though, so to produce a more natural looking result I took the original stacked Beta image and then blended in the central parts from the Difference image that showed the dust disc. I decided to also keep the black spot of the central glare from the Difference image since the contrast with the protruding disc seems better this way.

And the result is, I believe, the first amateur image of another solar system: The protoplanetary disc around Beta Pictoris. I must say it feels really special to have actually captured this.

Copyright Rolf Wahl Olsen 2010

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Herschel detects abundant water in planet-forming disc

This image shows an artist's impression of the icy protoplanetary disc around the young star TW Hydrae (upper panel) and the spectrum of the disc as obtained using the HIFI spectrometer on ESA's Herschel Space Observatory (lower panel).

By analysing the spectrum, astronomers have detected the emission from cold water vapour in the planet-forming disc. The vapour arises when highly energetic radiation from the central star interacts with icy grains in the disc. The detection thus hints at a copious and otherwise undetectable supply of water ice hidden in the disc's deeper and colder layers.

The graph in the lower panel shows the spectral signature of water vapour in the disc. Water molecules come in two "spin" forms, called ortho and para, in which the two spins of the hydrogen nuclei have different orientations. By comparing the relative amounts of ortho and para water, astronomers can determine the temperatures under which the water formed. Lower ratios indicate cooler temperatures, though in practice the analysis is much more complicated. The ratio of ortho to para water observed in TW Hydrae's protoplanetary disc is low enough to point to the presence of cold water vapour.Credits: ESA/NASA/JPL-Caltech/M. Hogerheijde (Leiden Observatory) HI-RES JPEG (Size: 1196 kb)

This artist's impression illustrates an icy protoplanetary disc around the young star TW Hydrae, located about 175 light-years away in the Hydra, or Sea Serpent, constellation.

Astronomers using the HIFI spectrometer on ESA's Herschel Space Observatory detected copious amounts of cold water vapour, illustrated in blue, emanating from the star's planet-forming disc of dust and gas. The water vapour, corresponding to temperatures lower than 100 K, is distributed across the entire extent of the disc and is likely confined to a thin layer at an intermediate depth in the disc. The vapour arises when highly energetic radiation from the central star interacts with icy grains in the disc, the very same grains that should ultimately coalesce into icy planetesimals, such as comets. The detection thus hints at a copious and otherwise undetectable supply of water ice hidden in the disc's deeper and colder layers.

In our own Solar System, comets are thought to have carried water to Earth, creating our oceans. A similar process might be taking place around TW Hydrae, where comets could, over the next several millions of years, transport water to young worlds. The Herschel results demonstrate that vast reservoirs of water are available around stars for creating these hypothetical water worlds. Credits: ESA/NASA/JPL-Caltech. HI-RES JPEG (Size: 1934 kb)

ESA’s Herschel space observatory has found evidence of water vapour emanating from ice on dust grains in the disc around a young star, revealing a hidden ice reservoir the size of thousands of oceans.

TW Hydrae, a star between 5-10 million years old, and only 176 light-years away, is in the final stage of formation, and is surrounded by a disc of dust and gas that may condense to form a complete set of planets.

It is believed that a large proportion of Earth’s water may have come from ice-laden comets that bombarded our world during and after its formation. Recent studies of comet 103P/Hartley 2 with Herschel shed new light on how water may have come to Earth, with its findings of the first Earth-like water in a comet. Until now, however, almost nothing was known about reservoirs in planet-forming discs around other stars.

This new detection is the first of its kind and has been made possible by Herschel’s HIFI instrument.

The tell-tale water vapour signature, believed to be produced when the ice coated dust grains are warmed by interstellar UV radiation, has been detected throughout the disc around TW Hydrae, and, though weaker than expected, it hints at a substantial reservoir of ice. This could be a rich source of water for any planets that form around this young star.

"The detection of water sticking to dust grains throughout the disc would be similar to events in our own Solar System's evolution, where over millions of years, similar dust grains then coalesced to form comets," says Michiel Hogerheijde of Leiden University in the Netherlands, who led the study.

"These comets we believe became a contributing source of water for the planets."

The scientists ran detailed simulations, combining the new data with previous ground-based observations and some from NASA’s Spitzer telescope. From this they calculated the size of the ice reservoirs in the planet-forming regions.

Their results show that the total amount of water in the disc around TW Hydrae would fill several thousand Earth oceans.

"We already have approved time on Herschel to study more planet-forming regions around three other stars," says Dr Hogerheijde.

"We believe that will show similar results in terms of the water detections, but as our next observations will be of objects up to three times further in distance away, we'll need many more hours of observation time."

This research breaks new ground in understanding water’s role in planet-forming discs and gives scientists a new testing ground for looking at how water came to our own planet.

"With Herschel we can follow the trail of water through all the steps of star and planet formation," comments Göran Pilbratt, Herschel Project Scientist at ESA.

"Here we are studying the 'raw material' for planet formation, which is fundamental to an understanding of how planetary systems such as our own Solar System once formed."

Notes to editors

HIFI is the Heterodyne Instrument for the Far-Infrared spectrometer on the Herschel Space Observatory. It was designed to observe water in a wide variety of objects, and aims to study not only planet-forming discs and star formation, but also galactic evolution. Its capability for highly detailed chemical identification of individual atoms and molecules makes it the instrument of choice for studying chemistry throughout space, particularly around embryonic and dying stars.

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Email: markus.bauer@esa.int
Tel: +31 71 565 6799
Mob: +31 61 594 3 954

Michiel Hogerheijde
Leiden Observatory
Tel: +31 71 527 5590
Email: michiel@strw.leidenuniv.nl

Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@rssd.esa.int

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Duo of Big Telescopes Probes the Depths of Binary Star Formation

Figure 1: A composite image toward Taurus FS A binary system. The green color shows the intensity of visible light, based on optical data from the Hubble Space Telescope. The red color displays near-infrared data from the Subaru Telescope. The ellipse marks an artifact from data processing of the central bright star. The dashed lines denote the directions of the support of the secondary mirror of the telescope. The field of view is 16.5" x 17.5". North is up, and east is to the left.

Figure 2: The polarization distribution in visible light, overlaid on the visible image (top) and near-infrared image (bottom). The color-coding refers to the offset from the expected centro-symmetric pattern around FS A star and FS B star. The blue and red colors show the circular pattern around FS A while the green encircles FS B. The area toward the southeast of FS A is bright in the near infrared, and the offset, denoted by the yellow color, is larger. The fields of view are 19" x 31" (top) and 14" x 17" (bottom), respectively. North is up and east is to the left.

A team of researchers from four Japanese universities (Kobe, Saitama, Osaka, and Tokyo) has been able to delineate the intricate structure of the circumbinary disk that surrounds a young binary star system from the observation with the Subaru Telescope and the Hubble Space Telescope. By using different wavelengths to examine the system's internal structure, they succeeded in demonstrating a distinct color difference between its northern and southern portions (figure 1). The researchers are now prepared to apply their approach of combining optical and near-infrared observations to other regions of binary formation.

Previous observations have demonstrated that protoplanetary disks, composed of a ring of dense gas surrounding a star like our Sun, not only accompany many infant stars but also are sites that generate planetary systems such as the one to which our Earth belongs. Therefore, these disks provide important information about the formation of stars and planets.

Past observations have focused on the protoplanetary disks of single stars. However, stellar research reveals that the majority of stars are members of binary or multiple star systems rather than ones composed of a single star. The research team addressed the issue of limited research on binary systems by pointing the Subaru Telescope toward the FS star system in the constellation Taurus. The separation between the primary (A) and companion (B) stars is 20 arc seconds (2800 AU; astronomical unit, the distance between the Sun and the Earth) and the FS A star itself is also a binary system with only 0.2 arc seconds (30 AU) of separation between its stars. The research team succeeded in detecting a circumbinary disk by using the Subaru Telescope's near-infrared camera CIAO (Coronagraphic Imager with Adaptive Optics), which blocks out the bright light of the central star. The disk's size of 630 AU is equivalent to the aphelion (the furthest point from the Sun in its orbit) of Sedona, one of the trans-Neptunian objects.

The team then compared its near-infrared image with the optical image taken by the Advanced Camera System (ACS) aboard the Hubble Space Telescope (HST). The area north of the FS A binary is brighter in the visible light (optical), while that south of the binary stands out in the near-infrared. In other words, the north is blue, and the south is red. The protoplanetary disk reflects the visible or the infrared light from the central star, but it does not emit light by itself. The highly distinct color contrast between the northern and southern portions of FS A's protoplanetary disk is a very unique characteristic of the system.

The question becomes why the color is different in these regions around the binary. Part of the answer relates to the degree of the polarization dispersed from the surface of the disk. Regardless of whether the light is visible or near-infrared, it displays the properties of a wave as well as a particle, and its reflection shows polarization. The degree and the direction of the polarization provide information about the object that reflects the light. This is why measurement of the polarization is important for understanding the structure of the protoplanetary disk. The observation with the Hubble Space Telescope included information on polarization, and figure 2 shows the distribution of the polarized light. The majority of the disk shows a typical concentric pattern around the central star. Other protoplanetary disks show similar patterns.

In addition to its circular pattern, the outer region to the north reflects the light from the FS B star, which is much further away from the FS A system. The research team interpreted this feature as an effect of abundant interstellar material in front of the FS A circumbinary disk and the influence of the FS B. However, the polarization data show that the inner region to the north is part of the protobinary disk surrounding the FS A binary. The mystery of color difference remains.

In sum, the research team established that there is a distinct color difference between the areas to the north and south of the circumbinary disk of the FS A star. They want to continue observations of protoplanetary disks so that they can identify their common characteristics and chronicle their evolution. Their ultimate goal is to understand the planetary formation process in circumstellar/circumbinary disks.

Reference:
"High-Resolution Optical and Near-Infrared Images of the FS Tauri Circumbinary Disk", Tomonori Hioki, Yoichi Itoh, Yumiko Oasa, Misato Fukagawa, Masahiko Hayashi. June 2011 issue of the Publications of the Astronomical Society of Japan.

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