In 1994, a group of Spanish astrophysicists discovered the very first brown dwarf--a ball of gas in outer space that’s too big to be a planet, but too small to be a star. Twenty years later, over a thousand brown dwarfs have been discovered. And the researchers who study them are unlocking clues to how planets form. Delaware Public Media’s Eli Chen spoke to astrophysicists at a recent brown dwarf and exoplanet conference at the University of Delaware.
[audio:http://www.wdde.org/wp-content/uploads/2014/11/BROWNDWARFS.mp3|titles= Delaware Public Media Science reporter Eli Chen visits UD conference on brown dwarfs and exoplanets.]
We can learn a lot about how planets come into existence by studying brown dwarfs. They have Jupiter’s girth and contain much more mass, but not enough mass to burn nuclear energy like a star. Because they’re something in between, they’re worth looking at to answer what truly defines a planet or a star.
John Gizis, a professor of astrophysics at the University of Delaware, remembers when the first discovery of a brown dwarf was announced in the mid-90s.
“I was at that conference in Italy where it was unexpectedly announced," said Gizis. "In fact, my talk was bumped to make room for the announcement of the first brown dwarf and the first planet around a star, and it was super thrilling.”
Brown dwarfs and exoplanets, planets outside of our solar system, were coincidentally found around the same time. And today, they’ve have reached similar numbers - close to a couple thousand of each have been found. Both of them are also dim, cool objects compared to stars, which can make them hard to hunt for. Now that recent telescope technology can allow astronomers to detect features from their atmospheres, Emily Rice, a professor of astrophysics based at the City University of New York, says they can compare exoplanets and Brown Dwarfs to see what they might have in common.
“For the brown dwarfs, when we find ones that we think is going to have similar atmospheric properties, so they have similar observational properties to the exoplanets that we know about, we can use those as test beds and we can do more detailed observations, kind of atmospheric models on the brown dwarf and hope that we’re fixing them in a way that works for the exoplanets,” said Rice.
Late last month, Gizis brought together brown dwarf and planetary experts to share their research at a conference at the University of Delaware. And they even have a mascot named Lil’ BD--for brown dwarf---which Rice brought with her.
“He’s quiet, but he has a music video," said Rice. "Have I sent you the music video?”
She did. It's to the tune of Bruno Mars’ “Just the Way You Are” and Rice wrote the lyrics herself, including this snippet.
“Oh, nearby nearby, seem like stars but they’re barely shining. Their cores, their cores, so cold they’re not even trying, to fuse hydrogen, in any stable way.”
It’s best to think of brown dwarfs as failed stars. But whether or not they actually formed like stars is one of the biggest mysteries about these objects.
It's the area Sally Dodson-Robinson, an assistant professor at the University of Delaware, is studying and discussed at the conference.
"I talked about observational signposts for Brown Dwarfs formation," said Dodson-Robinson. "If you see a brown dwarf, how do you know if it formed like a star or formed like a planet?”
Much of the research supports the idea that brown dwarfs tend to form like stars, since many of them sit isolated in space. But since no one’s seen one actually form, scientists like Dodson-Robinson aren’t a hundred percent certain that they all form like stars, so they can’t rule out finding one that possibly formed like a planet.
“It would mean there is more than one way to make an object that looks like brown dwarfs," said Dodson-Robinson. "And so, at this point, people agree that there’s no reason to assume that they don’t form like stars, I would put it that way. It doesn’t mean there isn’t any possibility, we just haven’t found a reason to reject that explanation.”
Dodson-Robinson’s research mainly focuses on supermassive planets, planets with several times the mass of Jupiter. And John Gizis says that leads into another muddy area for brown dwarf researchers--whether small brown dwarfs, which might have 10 times the mass of Jupiter or less, should be considered their own class of celestial objects. It’s possible that after observing more brown dwarfs, they might find more distinct features associated with ones with lower masses.
“We also know that there are things that look like planets that are orbiting stars that are 5 times the mass of Jupiter. Whether there’s a difference or whether they should have different names for them isn’t clear, it depends on what you think is important," said Gizis.
And Rice says some of the names of these smaller brown dwarfs include--
“Super Jupiters, Sub Jupiters, Super Neptunes and Sub Neptunes, some people have tried to coin the term Planemo, for planetary mass object.”
But nothing’s really caught on.
While some brown dwarfs have been found to orbit stars, one of the big questions is whether planets can orbit brown dwarfs. In recent years, researchers have found discs around brown dwarfs. Planets usually form within discs, or rings, around a star.
Eric Mamajek, an astrophysicist at the University of Rochester in New York, recently found a brown dwarf with a disc full of what look like moons.
“The object I was talking about is J1407. We’ve discovered this disc of material around it that is essentially a giant ring system, that appears to be tens of kilometers in size, basically take saturn’s rings and make it much, much larger,” said Mamajek.
Mamajek says discs around Brown Dwarfs are hard to find because they’re faint, even with the best telescopes. He was able to find J1407 because it was passing front a star, which lit up the disc like an x-ray and made it easier to observe. His paper on the finding was just accepted into the journal Monthly Notices of the Royal Astronomical Society.
“One of the longer term questions that we’d like to know is if can there be objects orbiting brown dwarfs out there that could possibly be habitable?”
A tool that will help Mamajek and other astrophysicists, is the Hubble Telescope’s replacement, the James Webb Telescope. Brown Dwarf researchers use telescopes that can see in infrared wavelength, like the Spitzer telescopes, since colder objects in space are best seen in infrared. Neill Reid, a researcher with the space telescope institute, says the massive mirror size and sensitivity of James Webb will take Brown Dwarf research to the next level. And it’s infrared optimization will allow researchers to see all kinds of interesting atmospheric features.
“There are signatures of molecules that appear in that spectrum, there’s carbon dioxide, monoxide, methane, water, by getting spectra for the Brown Dwarfs, you can see how their atmospheric properties change as they get cooler and cooler,” said Reid.
The James Webb Telescope is slated to be launched in October 2018, and the hope is that these researchers will find more Brown Dwarfs or other objects that might support or challenge their theories.
“The observations right now are to keep finding more objects and we keep finding weirder objects and in science, when we find the oddballs, there’s something to learn from the oddballs,” said Mamajek.
In other words, to find the answers to what makes a Brown Dwarf or what makes a planet, we just need to keep looking.