TW Hydrae is a bright, young red dwarf star dwelling a mere 190 light-years from our planet in the southern constellation Hydra the Water Snake. TW Hydrae shines with a light that is orange, and it weighs in at about 80 percent that of our own Star, the Sun. It is probably less than 10 million years old–a mere stellar baby–and is still in the process of accreting gas from a surrounding disk of nourishing material. Two studies released in June 2013 reveal that this small red star has some very important things to tell curious astronomers–TW Hydrae can provide welcome clues to the way our Sun once was very long ago and, even more extraordinary, it also may be orbited by an extremely distant planet that is 7.5 billion light-years away from it! This new observation of a planet, dwelling so far away from its parent star, may well challenge current theories about how baby planets are born and evolve.
“By studying TW Hydrae, we can watch what happened to the Sun when it was a toddler,” commented Dr. Nancy Brickhouse of the Harvard-Smithsonian Center for Astrophysics (CfA), at a June 6, 2013 press conference held at the summer meeting of the American Astronomical Society (AAS) in Indianapolis, Indiana. Dr. Brickhouse presented new findings at the AAS meeting, indicating that her team’s study of гидра онион suggests that our own Sun when it was young was both highly active and “feisty” at the same age–growing in fits and starts while emitting little bursts of X-rays.
Dr. Brickhouse and her colleagues came to this conclusion by observing the young red dwarf. In order to grow, the little star “eats” gas from the surrounding accretion disk. However, the disk does not reach the star’s surface, so the star cannot feast off of it directly. Instead, infalling gas gets sipped up along magnetic field lines to the star’s poles.
This infalling buffet crashes into the small young star, forming a vicious shock wave that heats the accreting feast of gas to temperatures higher than 5 million degrees Fahrenheit. The gas shines with high-energy X-rays, but as it continues to fall inward, it cools and its glow shifts to optical wavelengths of light. Dr. Brickhouse and her colleagues combined their observations from NASA’s Chandra X-ray Observatory with those from ground-based optical ‘scopes.
“By gathering data in multiple wavelengths we followed the gas all the way down. We traced the whose accretion process for the first time,” Dr. Brickhouse continued to note at the AAS press conference.
The team found that accretion was both episodic and clumpy in the construction of the stellar baby. At one point the quantity of star-building material crashing into the little star altered by a factor of five over the course of only a few days.
Team member Dr. Andrea Dupree commented at the June 6, 2013 AAS press conference that “The accretion process changes from night to night. Things are happening all the time.”
Some of the infalling buffet is pushed away by the fierce winds emanating from the young star–very much like the solar wind that blasts through our own entire Solar System. Astronomers have long known that bouncy, youthful baby stars are more magnetically vigorous than our middle-aged Sun is at over 4.5 billion years of age–but now they can actually investigate the interplay between the star’s magnetic fields and the accretion disk surrounding it.
“The very process of accretion is driving magnetic activity on TW Hydrae,” Dr. Brickhouse explained at the AAS press conference.
A Stellar Feast
As a very dense pocket embedded within an immense, dark, and frigid molecular cloud collapses under its own gravity to give birth to a new baby star, it usually leaves in its wake a disk of tiny dust particles that are very sticky in nature, and therefore tend to meld themselves together to form increasingly larger and larger objects that eventually grow into full-fledged planets. Our own Solar System was born approximately 4.568 billion years ago with the gravitational collapse of a relatively minute segment of a giant, dark molecular cloud. Most of the collapsing mass congealed at the center, giving birth to our own Sun, while the rest flattened out into a pancake-like disk of gas and dust–the protoplanetary disk (accretion disk)–from which the eight major planets, their myriad of lovely moons, the asteroids, comets, and other small Solar System bodies emerged.
Protoplanetary disks have been spotted around a number of young stars inhabiting newborn star clusters in our Milky Way Galaxy, and they apparently form at about the same time that the baby star is born! The disk that nourishes the young, active, central protostar is believed to be very massive–and very hot. Such disks can linger around a young star for about 10 million years. By the time the vibrant baby star reaches what is termed the T Tauri stage, the disk has thinned out considerably and greatly cooled down. A T Tauri star is a youthful, active, variable star that is less than 10 million years old, and possesses a mass that is similar to or somewhat less than that of our own Sun. However, a T Tauri star can sport a diameter that is several times greater than our Sun’s, and is still in the act of shrinking. By the time the new, brilliant star has reached this stage, less volatile materials have started to condense near the center of the surrounding disk. The sticky dust particles collide and glue themselves together to form every larger objects up to several centimeters in size. Further aggregation eventually results in the formation of planetesimals (the building blocks of planets) measuring 1 kilometer across or larger. Eventually, the planetesimals themselves collide with each other, and then meld together to create large planet-size bodies.
The ultimate disintegration of the surrounding disk is triggered by many different mechanisms. The inner portion of the disk is either consumed by the hungry new star or is tossed into Space by the star’s vigorous bipolar jets. On the other hand, the outer part can evaporate away due to the young star’s intense ultraviolet radiation during its very active T Tauri stage, or else by way of cataclysmic encounters with closely neighboring stars that may be sisters of its own stellar parent.
The gas in the center of the disk can either be swallowed or ejected by the growing baby planets, while the exquisitely tiny dust grains are tossed out due to the radiation pressure of the central, hot, active star. Ultimately, one of three things will be left: a planetary system, a remnant disk that is devoid of planets and composed only of dust, or nothing at all if planetesimals failed to form.
The Distant Planet Around TW Hydrae
A team of astronomers using NASA’s venerable Hubble Space Telescope (HST) have spotted strong hints that a planet dwells about 7.5 billion miles away from its parent star, TW Hydrae. This great distance defies current theories about how planets are born around their parent stars!
Of the approximately 900 planets confirmed so far, this candidate planet is the very first to be observed lurking at such an immense distance from its star. HST spotted a weird gap in TW Hydrae’s surrounding disk of gas and dust swirling around it. The mysterious gap is 1.9 billion miles wide and the disk itself is 41 billion miles wide. The existence of the gap is most likely the result of an unseen, growing baby planet that is gravitationally scooping up material from the disk, thus sweeping out a clearing for itself.
The planet is believed to be a relatively small one, at 6 to 28 times the mass of our own Earth. TW Hydrae’s great distance away from its parent star suggests that it is traveling very slowly around it in its orbit. If the candidate planet was a member of our own Solar System, it would be situated twice as far from our Sun as the dwarf planet Pluto.
Planets are believed to form over a time span of tens of millions of years, and therefore the construction of a planet is slow–but relentless. The planet-to-be snatches up rocks, gas, and dust from the surrounding disk. A planet lurking 7.5 billion miles away from its parent star should take over 200 times longer to be born than Jupiter did (about 10 million years) at its distance of about 500 million miles from our Star, due to its much more lazy orbital speed and the scarcity of material in the disk at that distance.
Little orange TW Hydrae is only about 8 million years old. Therefore, it is quite weird that it hosts such a planet, according to theory. There has simply not been sufficient time for such a planet to grow through the gradual swooping up of smaller pieces of debris. Even more perplexing is the fact that TW Hydrae is only about 55% as massive as our Sun.
“It’s so intriguing to see a system like this. This is the lowest-mass star for which we’ve observed a gap so far out,” commented Dr. John Debes in a June 13, 2013 HubbleSite Press Release. Dr. Debes is of the Space Telescope Science Institute in Baltimore, Maryland.
However, there is an alternative planet-formation theory that indicates that a portion of the disk can become gravitationally unstable and then collapse in on itself. According to this theory, a planet would be able to form much more quickly–in only a few thousand years.
“If we can actually confirm that there’s a planet there, we can connect its characteristics to measurements of the gap properties. That might add to planet formation theories as to how you can actually form a planet very far out,” Dr. Debes continued to explain.
To complicate the matter, the disk circling TW Hydrae is devoid of large dust particles in its outer limits. Observations conducted with the Atacama Large Millimeter Array in northern Chile’s desert, reveal that dust particles about the size of a grain of sand are not present beyond approximately 5.5 billion miles from the little orange star, just short of the mysterious gap.
The team of astronomers used HST’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to see the star in infrared light. The scientists then compared the NICMOS images with archival HST data and optical and spectroscopic observations from HST’s Imaging Spectrograph (STIS). Dr. Debes further noted, in the June 13, 2013 Press Release, that the gap was seen at all wavelengths–indicating that it is truly a structural entity and not an illusion caused either by scattered light or the instruments being used. The team’s research was published online on June 14, 2013 in The Astrophysical Journal.
“Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet formation models,” Dr. Debes added.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.