What Strange Places Are Habitable?

What Strange Places are Habitable
What Strange Places are Habitable

Everywhere we look on Earth, we find life. Even in the strangest corners of planet. What other places in the Universe might be habitable?

There’s life here on Earth, but what other places could there be life? This could be life that we might recognize, and maybe even life as we don’t understand it.

People always accuse me of being closed minded towards the search for life. Why do I always want there to be an energy source and liquid water? Why am I so hydrocentric? Scientists understand how life works here on Earth. Wherever we find liquid water, we find life: under glaciers, in your armpits, hydrothermal vents, in acidic water, up your nose, etc.

Water acts as a solvent, a place where atoms can be moved around and built into new structures by life forms. It makes sense to search for liquid water as it always seems to have life here. So where could we go searching for liquid water in the rest of the Universe?

Under the surface of Europa, there are deep oceans. They’re warmed by the gravitational interactions of Jupiter tidally flexing the surface of the moon. There could be life huddled around volcanic vents within its ocean. There’s a similar situation in Saturn’s Moon Enceladus, which is spewing out water ice into space; there might be vast reserves of liquid water underneath its surface. You could imagine a habitable moon orbiting a gas giant in another star system, or maybe you can just let George Lucas imagine it for you and fill it with Ewoks.

The white dwarf G29-38 (Image Credit: NASA)
The white dwarf G29-38 (Image Credit: NASA)

Let’s look further afield. What about dying white dwarf stars? Even though their main sequence days are over, they’re still giving off a lot of energy, and will slowly cool down over the coming billions of years. Brown dwarfs could get in on this action as well. Even though they never had enough mass to ignite solar fusion, they’re still generating heat. This could provide a safe warm place for planets to harbor life.

It gets a little trickier in either of these systems. White and brown dwarfs would have very narrow habitable zones, maybe 1/100th the size of the one in our Solar System. And it might shift too quickly for life to get started or survive for very long. This is our view, what we know life to be with water as a solvent. But astrobiologists have found other liquids that might work well as solvents too.

Artist concept of Methane-Ethane lakes on Titan (Credit: Copyright 2008 Karl Kofoed).  Click for larger version.
Artist concept of Methane-Ethane lakes on Titan (Credit: Copyright 2008 Karl Kofoed). Click for larger version.

What about life forms that live in oceans of liquid methane on Titan, or creatures that use silicon or boron instead of carbon. It might just not be science fiction after all. It’s a vast Universe out there, stranger than we can imagine. Astronomers are looking for life wherever makes sense – wherever there’s liquid water. And if they don’t find any there, they’ll start looking places that don’t make sense.

What do you think? When we first find life, what will be its core building block? Silicon? Boron? or something even more exotic?

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Could A Planet Be as Big as a Star?

Could A Planet Be as Big as a Star?

How big do planets get? Can they get star sized?

Everybody wants the biggest stuff.

Soft drink sizes, SUV’s, baseball caps, hot dogs and truck nuts.

Astronomers mostly measure stars in terms of mass and use the Sun as a yard stick. This star is 3 solar masses, that star is 10 solar masses, and so on.

We’re pandering to those of you who want the most massive stuff as opposed to the most volumetric stuff. So if you want the biggest truck, but don’t care if it’s got the most truck atoms in one place, this might not be for you.

How massive can planets get, and where can I order a custom one more massive than a star?

It all depends on what your planet is made of. There are two flavors of planets, gas and rock.

Gas planets, like Saturn and Jupiter are pretty much made of the same stuff as our Sun.

Jupiter’s pretty big, but it’s actually only about 1/1000th the mass of our star. If you made it more massive. by crashing about 80 Jupiters together, you’d get the same amount of mass as the smallest possible red dwarf star.

And all that mass would compress and heat up the core and it would ignite as a star.

Artist's View of Extrasolar Planet HD 189733b
Artist’s View of Extrasolar Planet HD 189733b

Extrasolar planet astronomers have turned up some pretty massive gas planets. The most massive so far contains 28.7 times the mass of Jupiter.

That’s so massive it’s more like a brown dwarf.

But if you had a planet entirely made of rock, like the Earth. It would need to be much, much larger before its core would ignite in fusion.

It would need to be dozens of times the mass of our Sun.

Stars with 8-11 stellar masses can fuse silicon. So a rocky planet would need millions of times the mass of the Earth before it would have that kind of pressure and temperature.

So you could get a situation where you have more mass than the Sun in a rock flavored world, and it wouldn’t ignite as a star. It would get pretty warm though.

No star can burn iron. In fact, when stars develop iron in their core, that’s when they shut down suddenly and you get a supernova.

Feel free to collect all the iron in the Universe together and lump it into a ridiculously huge pile and no matter how long you stare at for, it’ll never boil or turn into a star.

It might turn into a black hole, though.

Artist's impression of Kepler-10c (foreground planet)
Artist’s impression of Kepler-10c (foreground planet)

The largest rocky planet ever discovered is Kepler 10c, with 17 times the mass of Earth.

Massive, but nowhere near the smallest star.

There’s new research that says that heavier elements blasted out of supernovae might collect within huge star forming nebulae, like gold in the eddies of a river. This metal could collect into actual stars. Perhaps 1 in 10,000 stars might be made of heavier elements, and not hydrogen and helium.

Metal stars.

So, it’s theoretically possible. There might be corners of the Universe where enough metal has collected together that you could end up with a star that’s made up of planety stuff. And that’s pretty amazing.

What do you think? If we found one of these giant metal stars, what should we call it?

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Wow! Water Ice Clouds Suspected In Brown Dwarf Beyond The Solar System

Artist's conception of brown dwarf WISE J085510.83-071442.5, which may host water ice clouds in its atmosphere. Credit: Rob Gizis (CUNY BMCC / YouTube (screenshot)

What are planetary atmospheres made of? Figuring out the answer to that question is a big step on the road to learning about habitability, assuming that life tends to flourish in atmospheres like our own.

While there is a debate about how indicative the presence of, say, oxygen or water is of life on Earth-like planets, astronomers do agree more study is required to learn about the atmospheres of planets beyond our solar system.

Which is why this latest find is so exciting — one astronomy team says it may have spotted water ice clouds in a brown dwarf (an object between the size of a planet and a star) that is relatively close to our solar system. The find is tentative and also in an object that likely does not host life, but it’s hoped that telescopes may get better at examining atmospheres in the future.

The object is called WISE J085510.83-071442.5, or W0855 for short. It’s the coldest brown dwarf ever detected, with an average temperature between 225 degrees Kelvin (-55 Fahrenheit, or -48 Celsius) and 265 Kelvin (17 Fahrenheit, or -8 Celsius.) It’s believed to be about three to 10 times the mass of Jupiter.

Astronomers looked at W0855 with an infrared mosaic imager on the 6.5-meter Magellan Baade telescope, which is located at Las Campanas Observatory in Chile. The team obtained 151 images across three nights in May 2014.

Astronomers plotted the brown dwarf on a color-magnitude chart, which is a variant of famous Hertzsprung-Russell diagram used to learn more about stars by comparing their absolute magnitude against their spectral types. “Color-Magnitude diagrams are a tool for investigating atmospheric properties of the brown dwarf population as well as testing model predictions,” the authors wrote in their paper.

Based on previous work on brown dwarf atmospheres, the team plotted W0855 and modelled it, discovering it fell into a range that made water ice clouds possible. It should be noted here that water ice is known to exist in all four gas giants of our own Solar System: Jupiter, Saturn, Uranus, and Neptune.

“Non-equilibrium chemistry or non-solar metallicity may change predictions,” the authors cautioned in their paper. “However, using currently available model approaches, this is the first candidate outside our own solar system to have direct evidence for water clouds.”

The research, led by the Carnegie Institution for Science’s Jacqueline Faherty, was published in Astrophysical Journal Letters. A preprint version of the paper is available on Arxiv.

Source: Carnegie Institution for Science

Can A ‘Planet-Like Object’ Start Its Life Blazing As Hot As A Star?

How WISE 70304-2705 could have evolved from a star to a "planet-like object". Credit: John Pinfield,

Nature once again shows us how hard it is to fit astronomical objects into categories. An examination of a so-far unique brown dwarf — an object that is a little too small to start nuclear fusion and be a star — shows that it could have been as hot as a star in the ancient past.

The object is one of a handful of brown dwarfs that are called “Y dwarfs”. This is the coolest kind of star or star-like object we know of. These objects have been observed at least as far back as 2008, although they were predicted by theory before.

A group of scientists observed the object, called WISE J0304-2705, with NASA’s space-based Wide-field Infrared Survey Explorer (WISE). Looking at the spectrum of light it had emitted, which shows the object’s composition, has scientists saying that what the brown dwarf is made of suggests it is rather old — billions of years old.

“Our measurements suggest that this Y dwarf may have a composition … or age characteristic of one of the galaxy’s older members,” stated David Pinfield at the University of Hertfordshire, who led the research.

“This would mean its temperature evolution could have been rather extreme – despite starting out at thousands of degrees, this exotic object is now barely hot enough to boil a cup of tea.”

Size comparison of stellar vs substellar objects. (Credit: NASA/JPL-Caltech/UCB).
Size comparison of stellar vs substellar objects. (Credit: NASA/JPL-Caltech/UCB).

While the object started out hot, its interior never was quite enough to fuse hydrogen. That led to the extreme cooling visible today.

Models suggest the object would have begun its life shining at 2,800 degrees Celsius (5,072 Fahrenheit), for a phase that would have lasted for 20 million years. In the next 100 million years, its temperature would have almost halved to 1,500 Celsius (2,730 Fahrenheit).

And it would have kept cooling, with a temperature of 1,000 Celsius (1,832 Fahrenheit) after a billion years, and after billions of more years, the temperature we see today — somewhere between 100 Celsius (212 Fahrenheit) and 150 Celsius (302 Fahrenheit).

The paper will be published shortly in the Monthly Notices of the Royal Astronomical Society. The research is available in preprint version on Arxiv. One limitation of the research is the small number of Y dwarfs discovered, only about 20, which means that more observations will be needed to see if other objects could have had this same evolution.

Source: Royal Astronomical Society

Surprise! Brown Dwarf Star Has Dusty Skies, Appearing Strangely Red

Artist's impression of brown dwarf ULAS J222711-004547, which has a very thick cloud layer of mineral dust. The dust is making the brown dwarf appear redder than its counterparts. Credit: Neil J. Cook, Centre for Astrophysics Research, University of Hertfordshire

Dust clouds on a brown dwarf or “failed star” are making it appear redder than its counterparts, new research reveals. Better studying this phenomenon could improve the weather forecast on these objects, which are larger than gas giant planets but not quite big enough to ignite nuclear fusion processes to become stars.

“These are not the type of clouds that we are used to seeing on Earth. The thick clouds on this particular brown dwarf are mostly made of mineral dust, like enstatite and corundum,” stated Federico Marocco, who led the research team and is an astrophysicist at the United Kingdom’s University of Hertfordshire.

Using the Very Large Telescope in Chile as well as data analysis, “not only have we been able to infer their presence, but we have also been able to estimate the size of the dust grains in the clouds,” he added.

The brown dwarf (known as ULAS J222711-004547) has an unusual concoction in its atmosphere of water vapor, methane, (likely) ammonia and these mineral particles. While scientists are only beginning to wrap their head around what’s going on in the atmosphere, they noted that the size of the dust grains can influence the color of the sky and make it turn redder.

Size comparison of stellar vs substellar objects. (Credit: NASA/JPL-Caltech/UCB).
Size comparison of stellar vs substellar objects. (Credit: NASA/JPL-Caltech/UCB).

“Being one of the reddest brown dwarfs ever observed, ULAS J222711-004547 makes an ideal target for multiple observations to understand how the weather is in such an extreme atmosphere,” stated Avril Day-Jones, an astrophysicist at the University of Hertfordshire who co-authored the paper.

“By studying the composition and variability in luminosity and colours of objects like this, we can understand how the weather works on brown dwarfs and how it links to other giant planets.”

You can read more about the research in the Monthly Notices of the Royal Astronomical Society or in preprint version on Arxiv.

And by the way, there’s been other exciting work lately in brown dwarfs; another group recently released the first weather map of another failed star (and we have some information on Universe Today on how that was done, too!) Also, there are other weird brown star atmospheres out there, as this 2013 find shows.

Source: Royal Astronomical Society

Behind the Scenes: The “Making Of” the First Brown Dwarf Surface Map

Two views of the first brown dwarf map
Two views of the brown dwarf map for Luhman 16B. Image credit: ESO/I. Crossfield

By now, you will probably have heard that astronomers have produced the first global weather map for a brown dwarf. (If you haven’t, you can find the story here.) May be you’ve even built the cube model or the origami balloon model of the surface of the brown dwarf Luhman 16B the researchers provided (here).

Since one of my hats is that of public information officer at the Max Planck Institute for Astronomy, where most of the map-making took place, I was involved in writing a press release about the result. But one aspect that I found particularly interesting didn’t get much coverage there. It’s that this particular bit of research is a good example of how fast-paced astronomy can be these days, and, more generally, it shows how astronomical research works. So here’s a behind-the-scenes look – a making-of, if you will – for the first brown dwarf surface map (see image on the right).

As in other sciences, if you want to be a successful astronomer, you need to do something new, and go beyond what’s been done before. That, after all, is what publishable new results are all about. Sometimes, such progress is driven by larger telescopes and more sensitive instruments becoming available. Sometimes, it’s about effort and patience, such as surveying a large number of objects and drawing conclusion from the data you’ve won.

Ingenuity plays a significant role. Think of the telescopes, instruments and analytical methods developed by astronomers as the tools in a constantly growing tool box. One way of obtaining new results is to combine these tools in new ways, or to apply them to new objects.

That’s why our opening scene is nothing special in astronomy: It shows Ian Crossfield, a post-doctoral researcher at the Max Planck Institute for Astronomy, and a number of colleagues (including institute director Thomas Henning) in early March 2013, discussing the possibility of applying one particular method of mapping stellar surfaces to a class of objects that had never been mapped in this way before.

The method is called Doppler imaging. It makes use of the fact that light from a rotating star is slightly shifted in frequency as the star rotates. As different parts of the stellar surfaces go by, whisked around by the star’s rotation, the frequency shifts vary slightly different depending on where the light-emitting region is located on the star. From these systematic variations, an approximate map of the stellar surface can be reconstructed, showing darker and brighter areas. Stars are much too distant for even the largest current telescopes to discern surface details, but in this way, a surface map can be reconstructed indirectly.

The method itself isn’t new. The basic concept was invented in the late 1950s, and the 1980s saw several applications to bright, slowly rotating stars, with astronomers using Doppler imaging to map those stars’ spots (dark patches on a stellar surface; the stellar analogue to Sun spots).

Crossfield and his colleagues were wondering: Could this method be applied to a brown dwarf – an intermediary between planet and star, more massive than a planet, but with insufficient mass for nuclear fusion to ignite in the object’s core, turning it into a star? Sadly, some quick calculations, taking into account what current telescopes and instruments can and cannot do as well as the properties of known brown dwarfs, showed that it wouldn’t work.

The available targets were too faint, and Doppler imaging needs lots of light: for one because you need to split the available light into the myriad colors of a spectrum, and also because you need to take many different rather short measurements – after all, you need to monitor how the subtle frequency shifts caused by the Doppler effect change over time.

So far, so ordinary. Most discussions of how to make observations of a completely new type probably come to the conclusion that it cannot be done – or cannot be done yet. But in this case, another driver of astronomical progress made an appearance: The discovery of new objects.

Artist's impression of the WISE satellite
Kevin Luhman discovered the brown dwarf pair in data from NASA’s Wide-field Infrared Survey Explorer (WISE; artist’s impression). Image: NASA/JPL-Caltech

On March 11, Kevin Luhman, an astronomer at Penn State University, announced a momentous discovery: Using data from NASA’s Wide-field Infrared Survey Explorer (WISE), he had identified a system of two brown dwarfs orbiting each other. Remarkably, this system was at a distance of a mere 6.5 light-years from Earth. Only the Alpha Centauri star system and Barnard’s star are closer to Earth than that. In fact, Barnard’s star was the last time an object was discovered to be that close to our Solar system – and that discovery was made in 1916.

Modern astronomers are not known for coming up with snappy names, and the new object, which was designated WISE J104915.57-531906.1, was no exception. To be fair, this is not meant to be a real name; it’s a combination of the discovery instrument WISE with the system’s coordinates in the sky. Later, the alternative designation “Luhman 16AB” for the system was proposed, as this was the 16th binary system discovered by Kevin Luhman, with A and B denoting the binary system’s two components.

These days, the Internet gives the astronomical community immediate access to new discoveries as soon as they are announced. Many, probably most astronomers begin their working day by browsing recent submissions to astro-ph, the astrophysical section of the arXiv, an international repository of scientific papers. With a few exceptions – some journals insist on exclusive publication rights for at least a while –, this is where, in most cases, astronomers will get their first glimpse of their colleagues’ latest research papers.

Luhman posted his paper “Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun” on astro-ph on March 11. For Crossfield and his colleagues at MPIA, this was a game-changer. Suddenly, here was a brown dwarf for which Doppler imaging could conceivably work, and yield the first ever surface map of a brown dwarf.

However, it would still take the light-gathering power of one of the largest telescopes in the world to make this happen, and observation time on such telescopes is in high demand. Crossfield and his colleagues decided they needed to apply one more test before they would apply. Any object suitable for Doppler imaging will flicker ever so slightly, growing slightly brighter and darker in turn as brighter or darker surface areas rotate into view. Did Luhman 16A or 16B flicker – in astronomer-speak: did one of them, or perhaps both, show high variability?

Astronomy comes with its own time scales. Communication via the Internet is fast. But if you have a new idea, then ordinarily, you can’t just wait for night to fall and point your telescope accordingly. You need to get an observation proposal accepted, and this process takes time – typically between half a year and a year between your proposal and the actual observations. Also, applying is anything but a formality. Large facilities, like the European Southern Observatory’s Very Large Telescopes, or space telescopes like the Hubble, typically receive applications for more than 5 times the amount of observing time that is actually available.

But there’s a short-cut – a way for particularly promising or time-critical observing projects to be completed much faster. It’s known as “Director’s Discretionary Time”, as the observatory director – or a deputy – are entitled to distribute this chunk of observing time at their discretion.

Image of the MPG/ESO 2.2 m telescope at ESO's La Silla observatory
To monitor Luhman 16A and B’s brightness flucutations, Beth Biller used the MPG/ESO 2.2. meter telescope at ESO’s La Silla Observatory. Image credit: ESO/José Francisco Salgado (josefrancisco.org)

On April 2, Beth Biller, another MPIA post-doc (she is now at the University of Edinburgh), applied for Director’s Discretionary Time on the MPG/ESO 2.2 m telescope at ESO’s La Silla observatory in Chile. The proposal was approved the same day.

Biller’s proposal was to study Luhman 16A and 16B with an instrument called GROND. The instrument had been developed to study the afterglows of powerful, distant explosions known as gamma ray bursts. With ordinary astronomical objects, astronomers can take their time. These objects will not change much over the few hours an astronomer makes observations, first using one filter to capture one range of wavelengths (think “light of one color”), then another filter for another wavelength range. (Astronomical images usually capture one range of wavelengths – one color – at a time. If you look at a color image, it’s usually the result of a series of observations, one color filter at a time.)

Gamma ray bursts and other transient phenomena are different. Their properties can change on a time scale of minutes, leaving no time for consecutive observations. That is why GROND allows for simultaneous observations of seven different colors.

Biller had proposed to use GROND’s unique capability for recording brightness variations for Luhman 16A and 16B in seven different colors simultaneously – a kind of measurement that had never been done before at this scale. The most simultaneous information researchers had gotten from a brown dwarf had been at two different wavelengths (work by Esther Buenzli, then at the University of Arizona’s Steward Observatory, and colleagues). Biller was going for seven. As slightly different wavelength regimes contain information about gas at slightly different colors, such measurements promised insight into the layer structure of these brown dwarfs – with different temperatures corresponding to different atmospheric layers at different heights.

For Crossfield and his colleagues – Biller among them –, such a measurement of brightness variations should also show whether or not one of the brown dwarfs was a good candidate for Doppler imaging.

Image of the TRAPPIST telescope in its dome.
The robotic telescope TRAPPIST, also at ESO’s La Silla observatory, was the first to find brightness fluctuations of the brown dwarf Luhman 16B.

As it turned out, they didn’t even have to wait that long. A group of astronomers around Michaël Gillon had pointed the small robotic telescope TRAPPIST, designed for detecting exoplanets by the brightness variations they cause when passing between their host star and an observer on Earth, to Luhman 16AB. The same day that Biller had applied for observing time, and her application been approved, the TRAPPIST group published a paper “Fast-evolving weather for the coolest of our two new substellar neighbours”, charting brightness variations for Luhman 16B.

This news caught Crossfield thousands of miles from home. Some astronomical observations do not require astronomers to leave their cozy offices – the proposal is sent to staff astronomers at one of the large telescopes, who make the observations once the conditions are right and send the data back via Internet. But other types of observations do require astronomers to travel to whatever telescope is being used – to Chile, say, to or to Hawaii.

When the brightness variations for Luhman 16B were announced, Crossfield was observing in Hawaii. He and his colleagues realized right away that, given the new results, Luhman 16B had moved from being a possible candidate for the Doppler imaging technique to being a promising one. On the flight from Hawaii back to Frankfurt, Crossfield quickly wrote an urgent observing proposal for Director’s Discretionary Time on CRIRES, a spectrograph installed on one of the 8 meter Very Large Telescopes (VLT) at ESO’s Paranal observatory in Chile, submitting his application on April 5. Five days later, the proposal was accepted.

View of the 8 meter telescope Antu
Antu, the first of the four 8 meter Unit Telescopes (UTs) of the Very Large Telescope (VLT) shortly after installation in 2000. Image: ESO

On May 5, the giant 8 meter mirror of Antu, one of the four Unit Telescopes of the Very Large Telescope, turned towards the Southern constellation Vela (the “Sail of the Ship”). The light it collected was funneled into CRIRES, a high-resolution infrared spectrograph that is cooled down to about -200 degrees Celsius (-330 Fahrenheit) for better sensitivity.

Three and two weeks earlier, respectively, Biller’s observations had yielded rich data about the variability of both the brown dwarfs in the intended seven different wavelength bands.

At this point, no more than two months had passed between the original idea and the observations. But paraphrasing Edison’s famous quip, observational astronomy is 1% observation and 99% evaluation, as the raw data are analyzed, corrected, compared with models and inferences made about the properties of the observed objects.

For Beth Biller’s multi-wavelength monitoring of brightness variations, this took about five months. In early September, Biller and 17 coauthors, Crossfield and numerous other MPIA colleagues among them, submitted their article to the Astrophysical Journal Letters (ApJL) after some revisions, it was accepted on October 17. From October 18 onward, the results were accessible online at astro-ph, and a month later they were published on the ApJL website.

In late September, Crossfield and his colleagues had finished their Doppler imaging analysis of the CRIRES data. Results of such an analysis are never 100% certain, but the astronomers had found the most probable structure of the surface of Luhman 16B: a pattern of brighter and darker spots; clouds made of iron and other minerals drifting on hydrogen gas.

As is usual in the field, the text they submitted to the journal Nature was sent out to a referee – a scientist, who remains anonymous,  and who gives recommendations to the journal’s editors whether or not a particular article should be published. Most of the time, even for an article the referee thinks should be published, he or she has some recommendations for improvement. After some revisions, Nature accepted the Crossfield et al. article in late December 2013.

With Nature, you are only allowed to publish the final, revised version on astro-ph or similar servers no less than 6 month after the publication in the journal. So while a number of colleagues will have heard about the brown dwarf map on January 9 at a session at the 223rd Meeting of the American Astronomical Society, in Washington, D.C., for the wider astronomical community, the online publication, on January 29, 2014, will have been the first glimpse of this new result. And you can bet that, seeing the brown dwarf map, a number of them will have started thinking about what else one could do. Stay tuned for the next generation of results.

And there you have it: 10 months of astronomical research, from idea to publication, resulting in the first surface map of a brown dwarf (Crossfield et al.) and the first seven-wavelength-bands-study of brightness variations of two brown dwarfs (Biller et al.). Taken together, the studies provide fascinating image of complex weather patterns on an object somewhere between a planet and a star the beginning of a new era for brown dwarf study, and an important step towards another goal: detailed surface maps of giant gas planets around other stars.

On a more personal note, this was my first ever press release to be picked up by the Weather Channel.

When Is a Star Not a Star?

Artist's impression of a Y-dwarf, the coldest known type of brown dwarf star. (NASA/JPL-Caltech)

When it’s a brown dwarf — but where do we draw the line?

Often called “failed stars,” brown dwarfs are curious cosmic creatures. They’re kind of like swollen, super-dense Jupiters, containing huge amounts of matter yet not quite enough to begin fusing hydrogen in their cores. Still, there has to be some sort of specific tipping point, and astronomers (being the scientists that they are) would like to know: when does a brown dwarf stop and a star begin?

Researchers from Georgia State University now have the answer.

From a press release issued Dec. 9 from the National Optical Astronomy Observatory (NOAO):

For most of their lives, stars obey a relationship referred to as the main sequence, a relation between luminosity and temperature – which is also a relationship between luminosity and radius. Stars behave like balloons in the sense that adding material to the star causes its radius to increase: in a star the material is the element hydrogen, rather than air which is added to a balloon. Brown dwarfs, on the other hand, are described by different physical laws (referred to as electron degeneracy pressure) than stars and have the opposite behavior. The inner layers of a brown dwarf work much like a spring mattress: adding additional weight on them causes them to shrink. Therefore brown dwarfs actually decrease in size with increasing mass.

Read more: The Secret Origin Story of Brown Dwarfs

As Dr. Sergio Dieterich, the lead author, explained, “In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed (see the attached illustration, based on a figure in the publication). We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs. “

Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”

The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication) Image credit: P. Marenfeld & NOAO/AURA/NSF.
The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication) Image credit: P. Marenfeld & NOAO/AURA/NSF.

“We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’.”

Dr. Todd Henry, RECONS Director

Aside from answering a fundamental question in stellar astrophysics about the cool end of the main sequence, the discovery has significant implications in the search for life in the universe. Because brown dwarfs cool on a time scale of only millions of years, planets around brown dwarfs are poor candidates for habitability, whereas very low mass stars provide constant warmth and a low ultraviolet radiation environment for billions of years. Knowing the temperature where the stars end and the brown dwarfs begin should help astronomers decide which objects are candidates for hosting habitable planets.

The data came from the SOAR (SOuthern Astrophysical Research) 4.1-m telescope and the SMARTS (Small and Moderate Aperture Research Telescope System) 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

Read more here.

Exploring Our Galaxy’s Ancient Brown Dwarfs

A brown dwarf from the thick-disk or halo is shown. Although astronomers observe these objects as they pass near to the solar system, they spend much of their time away from the busiest part of the Galaxy, and the Milky Way's disk can be seen in the background. Credit: John Pinfield

As the name implies, a brown dwarf is small… only about 7% the size of the Sun. As far as stellar senior citizens go, they’re cool. Zipping along through space at speeds of 100 to 200 kilometers per second, they may have formed back when our galaxy was young – perhaps 10 billion years ago. Now a team of astronomers headed by Dr. David Pinfield at the University of Hertfordshire has identified a pair of the oldest brown dwarfs known… a set of orbs which could be the harbinger of a huge amount of new, unseen objects.

Although we sometimes refer to them as stars, brown dwarfs are in a class of their own. Because they didn’t ignite in nuclear fusion, they don’t generate internal heat like an ordinary star. After they are formed, they continue to cool and fade as time passes. This process makes them very difficult to observe and the discovery of two very old brown dwarfs, with temperatures of 250-600 C is cause for astronomical excitement.

Just how did Pinfield’s team pick such tiny objects out of the vastness of space? The discovery was facilitated thanks to a survey made by the Wide-field Infrared Survey Explorer (WISE), a NASA observatory that scanned the mid-infrared sky from orbit in 2010 and 2011. The ancient objects are cataloged as WISE 0013+0634 and WISE 0833+0052, and they are located in the constellations of Pisces and Hydra. Because they are so elusive, they were also confirmed by large ground-based telescopes (Magellan, Gemini, VISTA and UKIRT).

However, identifying the pair wasn’t easy. Seeing through the eyes of infrared reveals a crowded space – one populated with reddened stars, distant background galaxies and pockets of nebulous gas and dust. Picking out such a small character from a stellar cast would be like finding one tiny pearl in the vastness of an ocean. But Pinfield’s researchers employed a new method which utilizes WISE’s capabilities. As it scanned the sky over and over again, it revealed the cool, brown dwarfs – picking up the faint signature that other searches had missed.

These two particular brown dwarfs are different from the other slow movers of their type. By studying their spectra, the astronomers have identified atmospheres almost entirely comprised of hydrogen. This sets them apart from younger stars which have an abundance of heavier elements. Does being lighter make them speedier? According to Pinfield, “Unlike in other walks of life, the galaxy’s oldest members move much faster than its younger population.”

Stars near to Sun are considered the “local volume” and are created with three overlapping populations – the thin disk, the thick disk and the halo. Each of these layers has a certain amount of age associated with it: the oldest being the thickest and its member stars move up and down at a higher rate of speed. The halo contains both disks, along with the initial materials which formed the very first stars. Thin disk objects abound in the local volume and account for about 97% of the local stars, while thick disk and halo objects are a meager 3%. Chances are, brown dwarfs belong to that smaller percentage which explains why these fast-moving thick-disk/halo objects are only now being revealed.

Just how many may await discovery? Scientists surmise there may be as many as 70 billion brown dwarfs in the galaxy’s thin disk, and the thick disk and halo take up significantly larger galactic volumes. Even at a tiny 3%, this means there could be an army of ancient brown dwarfs in the galaxy. “These two brown dwarfs may be the tip of an iceberg and are an intriguing piece of astronomical archaeology,” said Pinfield. “We have only been able to find these objects by searching for the faintest and coolest things possible with WISE. And by finding more of them we will gain insight into the earliest epoch of the history of the galaxy.”

Original Story Source: Royal Astronomical Society News Release. For further study: “A deep WISE search for very late type objects and the discovery of two halo/thick-disk T dwarfs: WISE 0013+0634 and WISE 0833+0052”, D. J. Pinfield et al, Monthly Notices of the Royal Astronomical Society, in press.