Universe Today

Artist illustratino of a planetary zone filled with pebbles. Image credit: CfA. Click to enlarge.
Interstellar travelers might want to detour around the star system TW Hydrae to avoid a messy planetary construction site. Astronomer David Wilner of the Harvard-Smithsonian Center for Astrophysics (CfA) and his colleagues have discovered that the gaseous protoplanetary disk surrounding TW Hydrae holds vast swaths of pebbles extending outward for at least 1 billion miles. These rocky chunks should continue to grow in size as they collide and stick together until they eventually form planets.

“We’re seeing planet building happening right before our eyes,” said Wilner. “The foundation has been laid and now the building materials are coming together to make a new solar system.”

Wilner used the National Science Foundation’s Very Large Array to measure radio emissions from TW Hydrae. He detected radiation from a cold, extended dust disk suffused with centimeter-sized pebbles. Such pebbles are a prerequisite for planet formation, created as dust collects together into larger and larger clumps. Over millions of years, those clumps grow into planets.

“We’re seeing an important step on the path from interstellar dust particles to planets,” said Mark Claussen (NRAO), a co-author on the paper announcing the discovery. “No one has seen this before.”

A dusty disk like that in TW Hydrae tends to emit radio waves with wavelengths similar to the size of the particles in the disk. Other effects can mask this, however. In TW Hydrae, the astronomers explained, both the relatively close distance of the system and the stage of the young star’s evolution are just right to allow the relationship of particle size and wavelength to prevail. The scientists observed the young star’s disk with the VLA at several centimeter-range wavelengths. “The strong emission at wavelengths of a few centimeters is convincing evidence that particles of about the same size are present,” Claussen said.

Not only does TW Hydrae show evidence of ongoing planet formation, it also shows signs that at least one giant planet may have formed already. Wilner’s colleague, Nuria Calvet (CfA), has created a computer simulation of the disk around TW Hydrae using previously published infrared observations. She showed that a gap extends from the star out to a distance of about 400 million miles – similar to the distance to the asteroid belt in our solar system. The gap likely formed when a giant planet sucked up all the nearby material, leaving a hole in the middle of the disk.

Located about 180 light-years away in the constellation Hydra the Water Snake, TW Hydrae consists of a 10 million-year-old star about four-fifths as massive as the Sun. The protoplanetary disk surrounding TW Hydrae contains about one-tenth as much material as the Sun – more than enough to form one or more Jupiter-sized worlds.

“TW Hydrae is unique,” said Wilner. “It’s nearby, and it’s just the right age to be forming planets. We’ll be studying it for decades to come.”

This research was published in the June 20, 2005, issue of The Astrophysical Journal Letters.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: Harvard CfA News Release

Simulation of interstellar grains of dust. Image credit: OSU. Click to enlarge.
Science fiction writer Harlan Ellison once said that the most common elements in the universe are hydrogen and stupidity.

While the verdict is still out on the volume of stupidity, scientists have long known that hydrogen is indeed by far the most abundant element in the universe. When they peer through their telescopes, they see hydrogen in the vast clouds of dust and gas between stars ?- especially in the denser regions that are collapsing to form new stars and planets.

But one mystery has remained: why is much of that hydrogen in molecular form ?- with two hydrogen atoms bonded together ?- rather than its single atomic form? Where did all that molecular hydrogen come from? Ohio State University researchers recently decided to try to figure it out.

They discovered that one seemingly tiny detail — whether the surfaces of interstellar dust grains are smooth or bumpy — could explain why there is so much molecular hydrogen in the universe. They reported their results at the 60th International Symposium on Molecular Spectroscopy, held at Ohio State University .

Hydrogen is the simplest atomic element known; it consists of just one proton and one electron. Scientists have always taken for granted the existence of molecular hydrogen when forming theories about where all the larger and more elaborate molecules in the universe came from. But nobody could explain how so many hydrogen atoms were able to form molecules — until now.
When it comes to making molecular hydrogen, the ideal microscopic host surface is ?less like the flatness of Ohio and more like a Manhattan skyline.?

For two hydrogen atoms to have enough energy to bond in the cold reaches of space, they first have to meet on a surface, explained Eric Herbst, Distinguished University Professor of physics at Ohio State.

Though scientists suspected that space dust provided the necessary surface for such chemical reactions, laboratory simulations of the process never worked. At least, they didn’t work well enough to explain the full abundance of molecular hydrogen that scientists see in space.

Herbst, professor of physics, chemistry, and astronomy, joined with Herma Cuppen, a postdoctoral researcher, and Qiang Chang, a doctoral student, both in physics, to simulate different dust surfaces on a computer. They then modeled the motion of two hydrogen atoms tumbling along the different surfaces until they found one another to form a molecule.

Given the amount of dust that scientists think is floating in space, the Ohio State researchers were able to simulate the creation of the right amount of hydrogen, but only on bumpy surfaces.

When it comes to making molecular hydrogen, the ideal microscopic host surface is ?less like the flatness of Ohio and more like a Manhattan skyline,? Herbst said.

The problem with past simulations, it seems, is that they always assumed a flat surface.

Cuppen understands why. ?When you want to test something, starting with a flat surface is just faster and easier,? she said

She should know. She’s an expert in surface science, yet it still took her months to assemble the bumpy dust model, and she’s still working to refine it. Eventually, other scientists will be able to use the model to simulate other chemical reactions in space.

In the meantime, the Ohio State scientists are collaborating with colleagues at other institutions who are producing and using actual bumpy surfaces that mimic the texture of space dust. Though real space dust particles are as small as grains of sand, these larger, dime-sized surfaces will enable scientists to test whether different textures help molecular hydrogen to form in the lab.

Original Source: OSU News Release

Sky map of the June 25th planetary alignment. Image credit: NASA. Click to enlarge.
Saturn, which has been prominent, in the constellation Gemini all winter is slowly exiting our skies. But the Ringed Planet has one last show to put on for us, and the stage has been set. On June 18th, Saturn was joined by Venus, followed by Mercury on the 19th. On these dates the trio formed a long string stretching from the stars Castor and Pollux to just above the horizon. As the week progressed, the two faster planets slowly drew closer to Saturn. On the evenings of the 24th and 25th the trio will form a very close conjunction with Venus being just 1 degree from Saturn and less than 1 degree from Mercury.

For the next few nights all 3 planets should be visible in the wide field of view of a pair of binoculars or small telescope. By the 27th, Mercury and Venus will have drawn away from Saturn somewhat but will lie just 8 arc-minutes from one another, nearly indistinguishable to the unaided eye.

As June turns to July, Saturn will be lost in the glare of the setting sun. But Mercury and Venus will stay in close conjunction well into the month. On July 8th look for a very slim waxing crescent moon hovering just above the pair. Around July 15th the apparent separation of Mercury and Venus will have increased to 5 degrees. At this point Mercury will begin looping back toward the sun, while Venus continues to climb higher in our evening skies.

Contrary to popular belief, planetary conjunctions are fairly common. All the planets and the sun appear to travel along an imaginary line in the sky known as the ecliptic. Because our solar system is essentially a disk, the objects in our solar system appear to follow the same path year after year after year. Since we see these objects from Earth, which is itself moving, the planets occasionally appear to get close together in the sky. Conjunctions of 2 or 3 planets happen quite often particularly when one of them is Venus. The faster planets seem to ?catch up with? and ?pass? the slower moving ones, as we see in June.

Throughout recorded history humans have observed planetary conjunctions. In ancient times they were thought to be signs or omens. Not until recent centuries have we been able to model and therefore marvel at the workings of our solar system. Even though the conjunction of Mercury, Venus and Saturn doesn?t portend events, it is nonetheless a spectacular sight to behold.

Written by Rod Kennedy