Mystery Object Found Orbiting Brown Dwarf

This Hubble Space Telescope image of young brown dwarf 2M J044144 show it has a companion object at the 8 o'clock position that is estimated to be 5-10 times the mass of Jupiter.Credit: NASA, ESA, and K. Todorov and K. Luhman (Penn State University)

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Big planet or companion brown dwarf? Using the Hubble Space Telescope and the Gemini Observatory, astronomers have discovered an unusual object orbiting a brown dwarf, and its discovery could fuel additional debate about what exactly constitutes a planet. The object circles a nearby brown dwarf in the Taurus star-forming region with an orbit approximately 3.6 billion kilometers (2.25 billion miles) out, about the same as Saturn from our sun. The astronomers say it is the right size for a planet, but they believe the object formed in less than 1 million years — the approximate age of the brown dwarf — and much faster than the predicted time it takes to build planets according to conventional theories.

Kamen Todorov of Penn State University and his team conducted a survey of 32 young brown dwarfs in the Taurus region.

The object orbits the brown dwarf 2M J044144 and is about 5-10 times the mass of Jupiter. Brown dwarfs are objects that typically are tens of times the mass of Jupiter and are too small to sustain nuclear fusion to shine as stars do.

Artist's conception of the binary system 2M J044144. Science Credit: NASA, ESA, and K. Todorov and K. Luman (Penn State University) Artwork Credit: Gemini Observatory, courtesy of L. Cook

While there has been a lot of discussion in the context of the Pluto debate over how small an object can be and still be called a planet, this new observation addresses the question at the other end of the size spectrum: How small can an object be and still be a brown dwarf rather than a planet? This new companion is within the range of masses observed for planets around stars, but again, the astronomers aren’t sure if it is a planet or a companion brown dwarf star.

The answer is strongly connected to the mechanism by which the companion most likely formed.

The Hubble new release offers these three possible scenarios for how the object may have formed:

Dust in a circumstellar disk slowly agglomerates to form a rocky planet 10 times larger than Earth, which then accumulates a large gaseous envelope; a lump of gas in the disk quickly collapses to form an object the size of a gas giant planet; or, rather than forming in a disk, a companion forms directly from the collapse of the vast cloud of gas and dust in the same manner as a star (or brown dwarf).

If the last scenario is correct, then this discovery demonstrates that planetary-mass bodies can be made through the same mechanism that builds stars. This is the likely solution because the companion is too young to have formed by the first scenario, which is very slow. The second mechanism occurs rapidly, but the disk around the central brown dwarf probably did not contain enough material to make an object with a mass of 5-10 Jupiter masses.

“The most interesting implication of this result is that it shows that the process that makes binary stars extends all the way down to planetary masses. So it appears that nature is able to make planetary-mass companions through two very different mechanisms,” said team member Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Penn State University.

If the mystery companion formed through cloud collapse and fragmentation, as stellar binary systems do, then it is not a planet by definition because planets build up inside disks.

The mass of the companion is estimated by comparing its brightness to the luminosities predicted by theoretical evolutionary models for objects at various masses for an age of 1 million years.

Further supporting evidence comes from the presence of a very nearby binary system that contains a small red star and a brown dwarf. Luhman thinks that all four objects may have formed in the same cloud collapse, making this in actuality a quadruple system.

“The configuration closely resembles quadruple star systems, suggesting that all of its components formed like stars,” he said.

The team’s research is being published in an upcoming issue of The Astrophysical Journal.

The team’s paper: Discovery of a Planetary-Mass Companion to a Brown Dwarf in Taurus

Source: HubbleSite

Dwarf Star

A comparison of the Sun in its yellow dwarf phase and red giant phase

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A dwarf star is a star that is not a giant or supergiant … in other words, a dwarf star is a normal star! Of course, some dwarf stars are much smaller (less massive, have a smaller radius, etc) than normal (or main sequence, not really massive) stars … and these have names, like white dwarf, red dwarf, brown dwarf, and black dwarf. Our very own Sol (the Sun) is a dwarf star … a yellow dwarf.

Looking more closely at this rather confusing class of objects: a dwarf star has a mass of up to about 20 sols, and a luminosity (a.k.a. intrinsic brightness) of up to about 20,000 sols (‘sol’ is a neat unit; it can mean ‘the mass of the Sun’, or ‘the luminosity of the Sun’, or …!). So just about every star is a dwarf star! Why? Because most stars are on the main sequence (which means almost all have luminosities below 20,000 sols), and only a tiny handful of main sequence stars are more massive than 20 sols. In addition, once a star has burned through all its fuel, it becomes a white dwarf (and, one day, a black dwarf), all of which are dwarf stars by this definition.

The most interesting class of dwarf star is, perhaps, the black dwarf star; it’s hardly a star at all (it doesn’t burn any fuel, except, perhaps, deuterium, for a few million years or so).

So why do astronomers have this classification at all? Hitting the history books gives us a clue … back when spectroscopy was getting started, among astronomers – and well before there was any kind of astronomy except that in the optical (or visual) waveband; think the second half of the 19th century – a curious fact about stars was discovered: the spectra of stars with the same colors could still be very different (and when their distances were estimated, these spectral differences were found to track luminosity). So while dwarf stars overwhelmingly dominate, in terms of numbers, the giants (and sub-giants, and supergiants) pretty much rule in terms of what you can see with your unaided vision.

Neatly linking one kind of dwarf (the Sun, as a yellow dwarf) to another (white dwarf) is Universe Today’s The Sun as a White Dwarf. Other Universe Today articles on dwarf stars (not only white dwarfs!) include Astronomers Discover Youngest and Lowest Mass Dwarfs, Brown Dwarfs Form Like Stars, and Observing an Evaporating Extrasolar Planet.

Astronomy Cast’s episode Dwarf Stars has more on this topic.

Baby Brown Dwarfs Provide Clues to Solve Mystery

Why – and how — do brown dwarfs form? Since these cosmic misfits fall somewhere between planets and stars in terms of their temperature and mass, astronomers haven’t yet been able to determine how they form: are their beginnings like planets or stars? Now, the Spitzer Space Telescope has found what could be two of the youngest brown dwarfs. While astronomers are still looking to confirm the finding of these so-called “proto brown dwarfs” it has provided a preliminary answer of how these unusual stars form.

The baby brown dwarfs were found in Spitzer data collected in 2005. Astronomers had focused their search in the dark cloud Barnard 213, a region of the Taurus-Auriga complex well known to astronomers as a hunting ground for young objects.

“We decided to go several steps back in the process when (brown dwarfs) are really hidden,” said David Barrado of the Centro de Astrobiología in Madrid, Spain, lead author of the paper, published in the Astronomy & Astrophysics journal. “During this step they would have an (opaque) envelope, a cocoon, and they would be easier to identify due to their strong infrared excesses. We have used this property to identify them. This is where Spitzer plays an important role because Spitzer can have a look inside these clouds. Without it this wouldn’t have been possible.”

Barrado said the findings potentially solve the mystery about whether brown dwarfs form more like stars or planets. The team’s findings? Brown dwarfs form like low-mass stars.

Brown dwarfs are cooler and more lightweight than stars and more massive (and normally warmer) than planets. They are born of the same dense, dusty clouds that spawn stars and planets. But while they may share the same galactic nursery, brown dwarfs are often called “failed” stars because they lack the mass of their hotter, brighter stellar siblings. Without that mass, the gas at their core does not get hot enough to trigger the nuclear fusion that burns hydrogen — the main component of these molecular clouds — into helium. Unable to ignite as stars, brown dwarfs end up as cooler, less luminous objects that are more difficult to detect — a challenge that was overcome in this case by Spitzer’s heat-sensitive infrared vision.

This artist's rendering gives us a glimpse into a cosmic nursery as a star is born from the dark, swirling dust and gas of this cloud. Image credit: NASA/JPL-Caltech
This artist's rendering gives us a glimpse into a cosmic nursery as a star is born from the dark, swirling dust and gas of this cloud. Image credit: NASA/JPL-Caltech

Young brown dwarfs also evolve rapidly, making it difficult to catch them when they are first born. The first brown dwarf was discovered in 1995 and, while hundreds have been found since, astronomers had not been able to unambiguously find them in their earliest stages of formation until now.

Spitzer’s longer-wavelength infrared camera penetrated the dusty natal cloud to observe STB213 J041757. The data, confirmed with near-infrared imaging from Calar Alto Observatory in Spain, revealed not one but two of what would potentially prove to be the faintest and coolest brown dwarfs ever observed.

The twins were observed from around the globe, and their properties were measured and analyzed using a host of powerful astronomical tools. One of the astronomers’ stops was the Caltech Submillimeter Observatory in Hawaii, which captured the presence of the envelope around the young objects. That information, coupled with what they had from Spitzer, enabled the astronomers to build a spectral energy distribution — a diagram that shows the amount of energy that is emitted by the objects in each wavelength.

From Hawaii, the astronomers made additional stops at observatories in Spain (Calar Alto Observatory), Chile (Very Large Telescopes) and New Mexico (Very Large Array). They also pulled decade-old data from the Canadian Astronomy Data Centre archives that allowed them to comparatively measure how the two objects were moving in the sky. After more than a year of observations, they drew their conclusions.

“We were able to estimate that these two objects are the faintest and coolest discovered so far,” Barrado said. This theory is bolstered because the change in brightness of the objects at various wavelengths matches that of other very young, low-mass stars.

While further study will confirm whether these two celestial objects are in fact proto brown dwarfs, they are the best candidates so far, Barrado said. He said the journey to their discovery, while difficult, was fun. “It is a story that has been unfolding piece by piece. Sometimes nature takes its time to give up its secrets.”

Lead image caption: This image shows two young brown dwarfs, objects that fall somewhere between planets and stars in terms of their temperature and mass. Image credit: NASA/JPL-Caltech/Calar Alto Obsv./Caltech Sub. Obsv.

Source: JPL