Brown dwarfs may wreak havoc on orbits of nearby planets, causing desolation

Jul 10, 2014 by Nola Taylor Redd, Astrobio.net
An artist’s conception of the nearby brown dwarf WISE J085510.83, whose frosty temperatures are similar to those found at Earth’s North Pole. Credit: Penn State University/NASA/JPL-Caltech

Planets that move around their stars in circular orbits are a better bet for life than those in elongated orbits when it comes to creating habitable climate conditions, according to a new study.

That's because circular orbits tend to have more stable climates, while an overly elliptical orbit could send a planet into wildly fluctuating seasons and could even move it into and out of a star's habitable zone where liquid water can exist.

On average, the nearly 1,800 exoplanets discovered to date have orbits almost ten times as elongated as Earth's, likely resulting in temperature extremes. In some cases, interactions with other planets in the system could explain the stretched-out orbits. But in other systems, especially those with only one planet, the reason for the elliptical orbits can be challenging to explain.

"The fact that the average should be so high is a mystery we want to answer," said Alan Hulsebus, a doctorate student at Iowa State University and primary author of the paper.

He added:

"Did [the exoplanets] form this way by some unknown mechanism, or were they nudged into those [elliptical] orbits through interactions with another object? Was this a one-time action or something secular? If the latter, is that object still around and can we detect it?"

Hulsebus led a search of the 14 closest planet-hosting stars near Earth for signs of a brown dwarf, a massive object that could potentially shift the orbits of the planets.

The findings were published in the Astrophysical Journal and presented by Hulsebus at the 224th meeting of the American Astronomical Society in Boston, Massachusetts in June.

A tiny tug over time

The two leading models for planetary formation both suggest that planets start out in nearly circular orbits, which corresponds to an eccentricity measurement of 0. Earth's orbit is nearly circular at 0.02, which keeps the planet approximately the same distance from the Sun throughout the year.

An increased eccentricity means that the planet has a more elliptical, or stretched, orbit. Such planets may spend more time far from their stars, creating long, bitter winters. As they draw close to their stars, they could suffer scorching summers, particularly in the hemisphere pointed toward the star.

A planet could have a more eccentric orbit for a number of reasons. For example, collisions during the formation period could knock it out of its circular orbit.

Interactions with other planets could also change how they travel around their stars. Of the highly eccentric planets discovered, 78 percent of those with eccentricities greater than 0.5 have only one planet in the system, Hulsebus said. While the other planets could have been kicked out over the course of their evolution, Hulsebus and his team looked for a third option—the presence of a distant brown dwarf that could wreak havoc on the orbit of planets.

As a failed star which never accreted the necessary mass to start fusion in its core, a brown dwarf can be a few times heavier than Jupiter or reach masses up to 80 times as great. Because orbiting bodies travel more slowly the farther away they are from their stars, a distant brown dwarf may barely move across the sky while an interior planet races around its star. As a result, the two bodies would interact gravitationally at roughly the same time of the inner planet's year. The smaller planet experiences a gravitational tug that pulls it ever-so-slightly away from its star and closer to the brown dwarf. Over time, the process would stretch the orbit of the inner planet, making it steadily more elliptical.

The locations of brown dwarfs discovered by NASA’s Wide-field Infrared Survey Explorer and mapped by NASA’s Spitzer Space Telescope are shown as red circles. The red lines lead back to the sun. These brown dwarfs lie between 20 and 50 light-years from Earth. Credit: NASA/JPL-Caltech

"The brown dwarf perturbs the planet's orbit gravitationally, either on a short or long timescale, and by that induces eccentricity," said Smadar Naoz, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

Naoz, who was not involved in this paper, studies the dynamics of planetary systems.

Hulsebus and his team looked at all of the stars within approximately 50 light-years of Earth that are known to have planets, or that have debris disks that could indicate planetary formation. Of the 14 stars in that distance known to have planets, nine hosted at least one planet with an eccentricity greater than 0.1. The search was sensitive to objects that would lie just outside the Solar System's Kuiper belt, the region beyond Pluto.

Although the team is focused on brown dwarfs to explain elongated orbits, it has also been searching for objects around systems that travel along more stable, circular paths. Finding a distant brown dwarf around a planetary system with stable eccentricities could imply that something else is driving the instabilities.

"Even for those systems without eccentric planets, it is important to check for widely separated planets," Hulsebus said. "If they were discovered not to exist—or to only exist—around circular systems, that would be a noteworthy result."

Because brown dwarfs never started fusion, they don't generate the light that other stars produce. In optical wavelengths, scientists rely on light reflected from another star. But starlight overshadows the dimmer brown dwarf in orbit around it. A Jupiter-sized object that lies one thousand Astronomical Units from its star (one astronomical unit, or AU, is the distance Earth lies from the Sun) is a trillion times fainter.

"For every photon your camera collects from the planet, it is slammed by one trillion photons from the star," Hulsebus said.

Instead, the team used the InfraRed Array Camera on NASA's Spitzer Space Telescope to directly image the target stars in search of the signs of a faint brown dwarf. Instead of searching for reflected starlight, they detected the temperature of the brown dwarf as it cooled from its formation.

Of the 14 stars studied, 4 hosted objects with a brown-dwarf-like appearance. Follow-up observations will determine if they are, in fact, brown dwarfs. Three of the planetary systems have eccentricities of greater than 0.1.

For the remaining systems, the team was able to put upper limits on other possibly unseen objects and their orbits.

"We have not eliminated the possibility of perturbing objects in these systems, but we have restricted the parameters that they must have if they exist," Hulsebus said.

Edging out habitability

An artist’s concept of a free-floating brown dwarf. Credit: NASA/JPL-Caltech

Scientists tend to regard stable planets with constant temperatures to be better hosts for the evolution of life than those with temperatures that wildly fluctuate throughout the year.

A star's habitable zone is the region around it in which a planet can hold liquid water at its surface, another important ingredient for the development of life. A brown dwarf can affect whether or not an otherwise-habitable planet would remain in its habitable zone.

"If a brown dwarf induces a large eccentricity, so large that tidal forces from the star can shrink the planet's orbit, then it can move the planet in or out of the habitable zone," Naoz said.

Throughout the course of its orbit, the planet might duck in too close to the star, where temperatures would be too hot for it to maintain water, or travel to the outer edges of the , where would freeze.

The brown dwarf's size determines how significantly it will affect inner planets. The more massive the brown dwarf, and the closer it travels to a smaller planet, has an impact on how quickly and dramatically it will affect the planet's orbit.

"The best case scenario for maintaining life on a planet would be to have the changes occur slowly over a long time," Hulsebus said. "This would require a relatively small brown dwarf in a circular orbit, far from its host star."

Planets with highly eccentric orbits could move in and out of the habitable zone of their star, shown in green in this artist’s diagram. The planet shown in this diagram would suffer through a cold, bitter winter as it moved out beyond the planet’s habitable zone, causin water, an essential ingredient for life as we know it, to freeze. Credit: NASA/JPL-Caltech

The greatest wealth of extrasolar planets in recent years have been discovered by NASA's Kepler mission.

"Kepler has been stupendously successful at finding planets, and determining the frequency of brown dwarf companions around those systems would be very interesting," Hulsebus said.

The study of brown dwarfs is limited by the objects themselves. Hulsebus studied the closest planets to the Solar System because those were the most ideal. Brown dwarfs constantly cool, so older, smaller or more distant objects are more challenging to spot.

Because most of the stars that Kepler is studying lie so far from Earth, astronomers would only be able to spot brown dwarfs as close as about 4,500 AUs from their star. That distance is outside of the expected range of where a brown dwarf might have a significant effect on planetary companions, making the detection of brown dwarfs that could interfere with the of around Kepler impossible with present technology.

"More importantly, we wouldn't be able to see at this distance," Hulsebus said. "They would be too faint."

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Benni
1.6 / 5 (7) Jul 10, 2014
The "eccentricity cycle" of Earth's elliptical orbit around the sun occurs in a 100,000 year cycle varying from less elliptical to more elliptical over this cycle. It is the reason Ice Age cycles are so predictable, that is to say: "climate change".
Uncle Ira
4.2 / 5 (5) Jul 10, 2014
The "eccentricity cycle" of Earth's elliptical orbit around the sun occurs in a 100,000 year cycle varying from less elliptical to more elliptical over this cycle. It is the reason Ice Age cycles are so predictable, that is to say: "climate change".


You got a reason for believing that Bennie-Skippy? Or you just make it up last night because you think you might have the next new big science theory? I ask because this the first time I hear about this one.

Oh yeah Cher, I am not even going to ask you about the different equations or semi circle universes this time because we already know you are not going to be able to answer so you won't answer.
tonymsm
2.5 / 5 (2) Jul 11, 2014
Mr. Hulsebus' opinions are based on the accretion model, which cannot account for the eccentric, tilted, polar and/or retrograde orbits, retrograde spins or tilted axes of exoplanets - hence the mystery. It would produce circular orbits, needing something else - such as the influence of a distant brown dwarf - to elongate them. But what if it's wrong?
The theory that asteroids, rocky planets, gas giants and second generation stars all start out as lumps of shrapnel from supernovas can explain all the "anomalous" orbital characteristics of exoplanets - and even our variegated Solar System - by a "free" planet's path happening to meet the requirements for capture by a star, regardless of its relationship to the star's polar axis or rotation.
For a single captured planet an eccentric orbit is normal. But for two or more, their mutual gravitation(s) would modify their orbits over time to become more circular, minimizing the gravitational perturbations.
Torbjorn_Larsson_OM
5 / 5 (4) Jul 11, 2014
While this research got published, a new study robbed it of much of its strength. It turns out that in many cases eccentricity is rapidly lost, while in other cases it extends the habitable zone.

"Cold, stiff planets tend to resist the tidal stress and release energy very slowly. In fact, Henning and Hurford found that many of them actually generate less friction than previously thought. This may be especially true for planets farther from their stars. If these worlds are not crowded by other bodies, they may be stable in their eccentric orbits for a long time.

"In this case, the longer, non-circular orbits could increase the 'habitable zone,' because the tidal stress will remain an energy source for longer periods of time," said Hurford. "This is great for dim stars or ice worlds with subsurface oceans.""

[ http://scitechdai...survive/ ]
Torbjorn_Larsson_OM
4.2 / 5 (5) Jul 11, 2014
@Benni: What is understood as today's climate change is AGW, not the ice age cycles, where AGW is 1000s of times more rapid than the natural, non-anthropic cycles. A look at the data tells us that.

Milankovitch cycles driving ice ages are irrelevant to the eccentricities of 0.1+ discussed here, those cycles change Earth's orbital eccentricity of 0.02 with a mere +/- 0.01, or one order of magnitude less. [ http://en.wikiped...h_cycles ]

If you are interested in climate science, there are articles for that. Astrobiology is not the topic for AGW, since it is only relevant for us. Exoplanets have non-A GW heating as Earth had before the agrarian community. (Already farming seems to have started our effect on the climate...)
Torbjorn_Larsson_OM
4.2 / 5 (5) Jul 11, 2014
@tomysm: Planetary accretion is a fact, witnessed for example by the Tellus-Theia collision that made Earth and Moon. Earth's final 10 % mass came from that. It is also the accepted consensus, for that and many other reasons. "The currently accepted method by which the planets formed is known as accretion, ...". [ http://en.wikiped..._planets ]

But the planetary formation process has nothing to do with retrograde planets. And formation process would have to predict where the initial protoplanetary disk momentum ended up. Retrogrades are either disturbed planets (seen in exoplanet databases) or later captures (seen in moons among our gas giants).

Sure, if you have an alternative theory, publish it in peer review, have it predict as much as the older theories and win the competition. But until you do, personal incredulity is not science nor a useful guide.
Captain Stumpy
4 / 5 (4) Jul 12, 2014
It is the reason Ice Age cycles are so predictable, that is to say: "climate change".
@Benni
your post was off topic, but since you are here:

it is time to put your nuclear engineer logic and math to the test!
go to this site: http://dialogueso...nge.html

If you can PROVE, using the scientific method, that AGW is not real, then you will WIN $30,000.00 and fame (considering an overwhelming majority of scientists on the planet produce the science that supports it)

Since you are so intelligent and are capable of seeing how Milankovich cycles, etc can affect the weather, where meteorologists obviously never take stuff like that into consideration, then GO WIN YOUR MONEY
I DARE YOU
PROVE IT and the people here will have to accept it
or are you afraid of the public humiliation?
tonymsm
1 / 5 (2) Jul 13, 2014
@Torbjorn: Sorry, but the Tellus-Theia collision is just a theory (which also relies on accretion). It may be the accepted consensus; but that makes it "conventional", NOT fact. And what says a star's disk of cosmic junk is "protoplanetary"? Only the core accretion theory. Where's the proof? The Asteroid Belt hasn't accreted to a planet because smashing collisions - the basis of core accretion - don't happen. Two equal masses orbiting the Sun in the same direction and at the same distance have a high velocity through space but zero RELATIVE velocity, so instead we can see asteroids which have obviously accreted by gentle mutual gravitation.
"The currently accepted method..." Yes, but that doesn't make it right. Why do astronomers cling so desperately to a rule to which there are hundreds of exceptions? They have to accept that what was OK for our own backyard doesn't work when we start looking outside, and find a new theory, to which our Solar System is perhaps the only exception.
tonymsm
1 / 5 (3) Jul 13, 2014
@Torbjorn: Peer Review? Don't make me laugh! Look at its record:
It forced Galileo to recant his theory that the Earth revolves around the Sun... but he was right.
It ridiculed Immanuel Velikovsky... but if you make calculations based on the width of the Tibet Plateau and the 70km depth of the dent in the Moho below it, the neat, elliptical shape fits a rolling impact by a body the size of Venus.
It ended the career of Eric Laithwaite, because conventional physics couldn't explain the gyroscopic effects he showed... which might be a key to fast interplanetary travel.
It ruined the careers of the two men who discovered cold fusion... but the US Navy is still quietly working on it.
Most of the science articles I read have "balancing" comments by someone "who was not involved". It's amazing how many smell of sour grapes.
And as this is not my day job, if I wrote a paper, it wouldn't stand a snowball's chance in H... of getting past PR. That's why I'm here. Prove I'm wrong... if you can.
Captain Stumpy
4.2 / 5 (5) Jul 13, 2014
It forced Galileo to recant his theory that the Earth revolves around the Sun... but he was right
@tonymsm
this is not peer review, this is religious domination over science
It ridiculed Immanuel Velikovsky
no empirical evidence
It ruined the careers of the two men who discovered cold fusion
because the experiment was not repeatable, nor has it been proven in any way. in fact, most cold fusion research has failed miserably... why is that? where is the great e-cat or blacklight generators that were promised decades ago? The nutralino paper actually describes a perpetual motion machine. do you support perpetual motion machines?
Prove I'm wrong
if your posts contains logical fallacies or misrepresentations of facts, then it is proving itself incorrect. no work to do there.

Oh, now alchemist is going to think I am Torbjorn_Larsson_OM. LOL
Captain Stumpy
4 / 5 (4) Jul 13, 2014
Yes, but that doesn't make it right. Why do astronomers cling so desperately to a rule to which there are hundreds of exceptions?
@tonymsm
because no one has been able to provide another logical solution that also works within the laws of physics, which can be proven that also explains the situation.
to which our Solar System is perhaps the only exception
there are no exceptions to the law of physics. any model that is used to attempt to describe a situation in astophysics is based upon proven laws of physics and known material which has been empirically verified. Much like (to the chagrin of cantdrive) MHD and plasma physics. Modern MHD contains plasma physics within it and is far more accurate than EU acolytes think and it is proven, time and again.

so your point is invalid. sorry

peer review is the best system we have, otherwise AW or EU supporters will have scientific credibility, when their philosophy is known pseudoscience and debunked today by empirical data = proof
tonymsm
1 / 5 (3) Jul 14, 2014
@Captain Stumpy: NASA uses "slingshot" maneuvers to accelerate spacecraft on journeys to the outer Solar System (slingshots can also be used to slow a body down).
If a slingshot goes wrong, there are three possible outcomes:
(a) the body gets accelerated too much and flies off on an unintended track,
(b) it gets captured into (probably) an elliptical orbit around the body it intended to use to accelerate, or
(c) it gets too close to the body and gets pulled in, to eventually crash
The capture theory - alternate to core accretion - states that free planets from supernovae travel through space, their paths being modified perhaps numerous times by unintentional case (a) slingshots which accelerate or decelerate them and change their direction, until such time as they fall into case (b) or (c) with respect to a star.
There is nothing in the Laws of Physics to prevent a star such as the Sun capturing a motley crew of free planets in this way to create the Solar System.
tonymsm
1 / 5 (4) Jul 14, 2014
@Captain Stumpy: "no empirical evidence" I gave it to you: Measurements from Google Earth and data from the ESA GOCE satellite. Do the math, like I did, then tell me I'm wrong.
Wikipedia - like everyone else - says the Himalayas were created by uplift from the Indian plate being subducted beneath the Asian plate. That effect is real; but correlation does not prove causation - and there is NO WAY that any of the three tectonic processes - tension/compression, shear and subduction/uplift - singly or in combination, can produce a hollow ellipse (with another one inside it).
Captain Stumpy
5 / 5 (1) Jul 14, 2014
@Captain Stumpy: blah blah blah to create the Solar System.
@tonymsm
1- what is your point?
2- are you referencing a specific point I made?
3- Taking the comments I made above into consideration, I am not sure that this makes any sense.
Dr_toad
Jul 14, 2014
This comment has been removed by a moderator.