Researchers on this side of the Atlantic reported that double planets may be more common than we'd thought: a lot more common.
- Philae: Rich Data from a Bouncing Lander
- Double Planets Formed in Collisions
It's not likely that we'll see double planets orbiting each other much closer than that fictional pair. The Roche limit, or Roche radius, is a sort of tidal boundary.
If an object held together by gravity gets inside another object's Roche limit, the difference in gravitational attraction between the object's near and far sides pulls it apart.
NASA did a pretty good job of describing the Roche limit:
"The closer you are to a planet, the stronger is its gravitational pull on you. For a large moon, this means that the side closest to the planet is being pulled substantially more forcefully than the side facing away from the planet. Within a certain distance from the planet, that difference can be enough to pull the moon apart. The Roche limit is the minimum distance that a moon (or other large object) can be from a planet without being torn to bits.For Earth, the Roche limit is very roughly 10,000 miles. That doesn't mean that satellites and the International Space Station will be pulled apart. They're held together by the tensile strength of the metal or other materials they're made from.
"(For smaller objects, the difference in gravitational pull from one side to the other isn't enough to pull it apart.)
"If the planet and the orbiting body have the same density, that distance is about 2.5 times the radius of the planet."
(Cassini Solstice Mission, FAQ, What is the Roche limit?)
Some tiny natural satellites, like Jupiter's moon, Metis; and Saturn's Pan, are inside their planets' Roche limit; but their tensile strength holds them together.
The last time I checked, we didn't have an official definition for "double planet."
One rule-of-thumb guide for what's a double planet, and what's not, says that if a pair's center of mass for a pair is inside one of the planets: it's a planet and a moon.
If the center of mass is between the two, it's a double planet. By that definition, Earth and moon are a planet-moon system; and Pluto and Charon are a double planet.
CalcTool.org includes "Orbital period of a planet Based on Kepler's third law." I checked it out, giving values for Earth orbiting the Sun, and our moon's orbit, and got accurate numbers.
That's when I started having fun.
Two planets with Earth's mass and 12,000 mile semi-major axis would whip around each other once every 5.2455 hours. They'd be tidally locked, rotating on their axis once every orbit, like our moon. That would be a very short day.
Their semi-major axis has to be 33,075 miles for the pair to have a 24-hour day.
A reader of Alien vs. Editor, Caltech Jet Propulsion Laboratory's Planetquest/The Search for Another Earth's asked about climate on a 'double Earth' orbiting a red dwarf star.
The editor responded with a very informal discussion of climates on Earth-like double planets:
"Imagining the dance of double planets"The editor outlined two scenarios. In one, the planets' orbital plane was at right angles to their orbital plane with the star. The other assumed that their orbital plane was the same as the pair's orbit around their star.
Alien vs. Editor (May 31, 2012)
"...The planets would create tides on each other. The inherent flexibility of planetary materials will lock the pair into mutual synchronous rotation over time, with a fixed hemisphere on each component facing the other's fixed hemisphere. Over time, their rotation axes and the axis of their mutual orbit will all be parallel. It would take some careful computer modeling of specific scenarios to confirm that the double planet pair could actually 'hold together' as close to the red dwarf as they need to be to be in its habitable zone.
"Assuming the pair is stable, the gravitational influence of the pair on each other will be much stronger than the influence of the host M dwarf on the individuals. The star's tidal effect on the pair may keep the pair from settling into a circular orbit around each other. Then their mutual orbit would be forced into a slightly elliptical shape that is always changing. This depends on how close the planets are to each other and how close they are to the star...."
In both situations, someone standing on the 'planet-far-hemisphere' of either planet couldn't see the other world. On the 'planet-facing hemisphere,' the other planet would be an enormous globe, always in one part of the sky.
If the planets would probably have extreme seasons if they orbited each other in a plane that was at right angles to the pair's orbit around its star. If they were orbiting each other in the same plane that the pair orbited their star, they might have no seasons to speak of: unless the pair's orbit around the star was eccentric.
Near the equator, the planet-facing hemisphere of each world would be eclipsed once each day. That would probably keep the tropics of both a bit cooler than they'd be without the daily eclipse, and could affect global weather and ocean currents..
If the two worlds were far enough apart, higher latitudes would get the same sort of day/night cycle we experience on Earth. Except for a Brobdingnagian moon fixed in the sky.
(From Ittiz, via deviantart.com, used w/o permission.)
("Double Planet," by Ittiz. (2010))
The catalog of 1,800-plus planets circling other stars is growing. By one reasonable estimate, there are about 17,000,000,000 Earth-size planets in this galaxy.
Many of those will be too hot, too cold, or otherwise unsuited for life. But searching for life elsewhere in the universe isn't just 'science fiction' any more:
- "Cosmological aspects of planetary habitability"
Yu. A. Shchekinov, Department of Space Physics, SFU, Rostov on Don, Russia; M. Safonova, J. Murthy, Indian Institute of Astrophysics, Bangalore, India (April 2, 2014)
(Via Cornell University Library (June 26, 2014))
- "Exomoon habitability constrained by illumination and tidal heating"
René Heller, Rory Barnes, Astrobiology (September 8, 2012)
(Via Cornell University Library (June 26, 2014))
- "Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler"
Wesley A. Traub, Astrophysical Journal (September 21, 2011)
(Via Cornell University Library (June 26, 2014))
If we do find critters living on another planet, or in moons like Europa, Ganymede, and (maybe) Mimas, I'm pretty sure that some folks will be upset, or decide that the critters aren't real. I discussed faith, philosophy, and getting a grip about life in the universe, earlier this month. (November 7, 2014)
There was a controversy, not quite eight centuries back, about other worlds. Quite a few European scholars were ardent fans of Aristotle at the time. They apparently didn't like the newfangled notion that other worlds might exist: and said they can't, because Aristotle said so.
The Catholic Church stepped in, and ever since 1277, claiming that we have to be standing on the only world has been against the rules. Basically: God's God, Aristotle's not. (June 27, 2014; February 23, 2014)
I haven't run into an official 'Catholic' statement about life on other worlds, but I think the Pontifical Academy of Sciences' in "Study Week on Astrobiology" in 2009 says something about our insistence on living in the real world.
My interest in science isn't required by the Church, but thinking and curiosity are not sins. (November 21, 2014; August 1, 2014)
Some things aren't options for Catholics. For example, I must believe that we are creatures made from the stuff of this world and in the image of God: beings of matter of spirit. (Catechism of the Catholic Church, 362)
Being curious about where we came from and where we're going is designed into us. This curiosity isn't idle. We're "called to a personal relationship with God," and can learn something of God by studying God's creation. (Psalms 19:2; Catechism, 282-289, 299, 341)
We're also obliged to believe that God created and is creating a good and ordered universe: including us. What we're learning about this cosmos is, or should be, cause for greater admiration of God's greatness. (Genesis 1:27-28; Catechism, 283, 301)
God gave us brains, and expects us to use them. Science and technology, studying this astounding creation and applying that knowledge, is part of being human. (Wisdom 7:17; Catechism, 2292-2295)
Besides, part of our job is taking care of the physical world. That takes tech: used wisely, and that's another topic. (Catechism, 1, 373, 1730, 2375)
I won't claim that God couldn't get life started elsewhere, or that the Almighty is obliged to do so. God's God, I'm not.
If we have neighbors, we'll find them. Eventually. Or maybe they'll find us.
(From ESA/Rosetta/MPS for Osiris Team/LAM/IAA/SSO/INTA/UPM/DSP/IDA, via BBC News, used w/o permission.)
("Philae should tell us what comets are made from, and what happened at the dawn of the Solar System"
"Comet landing: UK team's data bonanza from Philae"Philae bounced away from its sunny landing spot, finally resting against a 'cliff,' and in partial shade. The bad news was that Philae's solar panels couldn't recharge its batteries, so the lander is now inactive.
Pallab Ghosh, BBC News (November 26, 2014)
"UK Researchers received 'rich' data from the Philae lander just before its power died.
"Scientists say they detected what might be complex carbon compounds on the surface of the comet the craft landed on two weeks ago.
"The results are from the Ptolemy instrument, which is a miniaturised on-board laboratory.
"The detection of carbon supports a view that comets may have brought key chemicals to Earth to kick-start life...."
The unexpected good news is that dust kicked up in its first bounce gave scientists a great deal of data about 67P/Churyumov–Gerasimenko's surface.
Philae's drill was supposed to send samples to Ptolemy, COSAC, and CIVA's visible-light microscope and an infrared spectrometer. (Wikipedia)
Ptolemy is an instrument measuring isotope ratios, COSAC (COmetary SAmpling and Composition) is a combined gas chromatograph and mass spectrometer; and CIVA's (Comet Nucleus Infrared and Visible Analyser) is a set of cameras.
Dropping power levels meant hard decisions for the Rosetta scientists. Program managers decided there was only enough power left to collect and analyze one of the two planned drill samples.
"...It is unclear whether the drill successfully managed to get a sample to COSAC.
"But mission planners did grant the UK team Philae's last ounce of strength to operate Ptolemy's oven, to heat up all the debris that had collected inside the instrument to 200C and analyse the gases that came off...."
(Pallab Ghosh, BBC News)
(From Dan Andrews, via BBC News, used w/o permission.)
(Philae's power levels dropping. As 67P/Churyumov–Gerasimenko approaches the sun, the lander may 'wake up' again.)
"...The team leader, Prof Ian Wright, told BBC News: 'We can say with absolute certainty that we saw a very large signal of what are basically organic (carbon) compounds.Professor Wright's "peaks" are on a graph of the Ptolemy instrument's data. I haven't found anything about which elements Ptolomy detected on comet 67P/ChuryumovGerasimenko.
" 'There is a rich signal there. It is not simple. It is not like there are two compounds; there are clearly a lot of things there - a lot of peaks. Sometimes a complicated compound can give a lot of peaks.'..."
"...In an exclusive interview with BBC News, Prof Wright explained that Ptolemy had gathered huge amounts of scientific data. Normally a quiet, understated man, he was marginally better at containing his enthusiasm than his co-worker and wife, Prof Monica Grady, who jumped for and then wept with joy and relief when Philae landed....
"...Prof Wright also explained that Philae's bouncy landing suited his experiment. Among Ptolemy's capabilities is the ability to analyse gases and particles around it, and so it was pre-programmed to sniff its environment shortly after landing...."
(Pallab Ghosh, BBC News)
My guess is that scientists are still analyzing what Philae transmitted before its batteries gave out.
I share Professor Wright's enthusiasm, but finding organic compounds on a comet is nothing new. We knew that a comet's nucleus is a mix of rock and dust, and more-or-less-volatile compounds like water ice, carbon dioxide, carbon monoxide, methane, and ammonia.
The last two, or four, are organic; since they contain carbon.
Somewhat arbitrarily, carbides, carbonates, simple oxides of carbon like carbon carbon monoxide and carbon dioxide, and cyanides are "inorganic:" sometimes. What's "organic" and what isn't depends on who you ask, since there isn't a single 'official' definition. (Wikipedia)
What's exciting about Philae's data is that it's the first taken directly from a comet's surface. Maybe it'll confirm what scientists thought we'd find: maybe not. Either way, we know more about comets now than we did month ago.
"Organic" doesn't mean "alive," although everything living on Earth is made of organic compounds: partly. Up to 70% of our bones are made of hydroxylapatite, a calcium compound containing calcium, oxygen, phosphorus: and no carbon.
According to one definition, something that's alive is a physical entity "having signaling and self-sustaining processes."
By that definition, rovers like Mars exploration rovers Spirit and Opportunity are close to being "alive." There will be spirited discussions when we build robots that make their own replacements, and that's yet another topic. Topics.
Getting back to 67P/ChuryumovGerasimenko, I agree with Professor Ian Wright:
"...'For years, I've been giving public lectures about what we plan to do. Now we have some data and it's: Wow! This is what scientists do this stuff for.'..."
(Pallab Ghosh, BBC News)
(From NASA/JPL-Caltech, via Space.com, used w/o permission.)
("Artist's concept depicting an imminent planetary collision around a pair of double stars."
"Binary Earth-Size Planets Possible Around Distant Stars"We know that a planet's moon can be a sizable fraction of the planet's mass. Earth's moon has about 1/81 as mass as Earth. That's more than any other satellite of a planet in the Solar system, but still low enough for our moon to be considered a satellite.
Charles Q. Choi, Space.com (November 21, 2014)
"Two Earth-size planets that orbit each other might exist around distant stars, researchers say.
The solar system has many examples of moons orbiting planets; Jupiter and Saturn both possess more than 60 satellites. However, these moons are usually much smaller than their planets — Earth is nearly four times wider than its moon and more than 80 times its mass.
Still, some moons are as large as planets. For instance, Ganymede, Jupiter's largest moon, is larger than Mercury, and three-quarters the diameter of Mars. Also, moons at times are nearly as large as their worlds; Pluto's largest moon, Charon, is about half the diameter of the dwarf planet itself. This raises the intriguing Possibility that planets of equal size could orbit each other...."
Charon has about 1/9th the mass of Pluto. Depending on who you read, Charon is a moon of Pluto, Charon and Pluto are a double planet, or both are dwarf planets.
My opinion is that we'll sort out nomenclature for 'stuff orbiting stars:' in another few decades. Or generations.
Asteroids occasionally come in pairs, like 69230 Hermes: two lumps of rock, each a few hundred meters across.
Writers have imagined double planets: with varying degrees of plausibility.
Scientists knew that orbits of double planets could be stable for billions of years, as long as the pair had formed at least half an astronomical unit away from its star. Closer to the star, tidal forces would be too great.
Two Earth-size planets could orbit each other: but scientists thought such pairs were unlikely, at best.
Until this month.
Three California Institute of Technology researchers, Keegan Ryan, Miki Nakajima, and David J. Stevenson, ran about two dozen collision and near-collision simulations of two planets like Earth.
Most simulations ended with one bigger planet, with or without rings; or with the planets escaping into separate orbits.
That's pretty much what most scientists expected. However —
"...about one-third of the simulations resulted in binary planets forming. These involved relatively slow, grazing collisions...."
(Charles Q. Choi, Space.com)
"...'Previously, the only expected outcomes of large-body impacts of this sort were escape or accretion — that is, either the two bodies do not stay together or they merge into one, occasionally with a disk of debris,' study co-author Keegan Ryan, an undergraduate student at the California Institute of Technology in Pasadena, told Space.com. 'Our findings suggest the possibility of another outcome — binary planets. The bodies stay mostly intact, but end in a bound orbit with one another.'..."The best explanation we have for how Earth's moon formed involves a collision with another planet, about the size of Mars. (September 27, 2013)
(Charles Q. Choi, Space.com)
This wasn't as freakishly unlikely as it might seem, since it looks like the Solar System's planets didn't start out in today's orbits. (September 27, 2013)
I'd be astonished if we learn that one out of three rocky planets circling other stars are doubles. Those simulations were simplified versions of reality, and may have been tweaked to improve the odds of double worlds.
But The Exosolar Planets Encylopedia catalogs 1,850 exoplanets circling 1,160 stars. That may not include the six added to the Caltech/NASA Exoplanet Archive last week. (Kepler-289 d, Kepler-430 b, Kepler-430 c, Kepler-431 b, Kepler-431 c, and Kepler-431 d/PH3 c)
We've gotten a big enough sample to estimate how many planets are in the Milky Way galaxy. Earlier this year, the Harvard-Smithsonian Center for Astrophysics' Francois Fressin said there are almost certainly around 17,000,000,000 planets about Earth's size out there. (April 18, 2014)
If even a small fraction of those were double planets, it would be a huge number.
How many might support life is — yet again another topic.
More about comets, moons, and distant worlds:
- " 'Philae ... Headed for History' "
(November 14, 2014)
- "Harpooning the 'Rubber Duck' Comet; Public Safety — and Space Aliens"
(November 7, 2014)
- "Earth's Wandering Poles, A Comet, a Wobbling Moon"
(October 24, 2014)
- "Kapteyn b, Habitable Zones, and Using Our Brains"
(July 18, 2014)
- "Life in the Universe, and Titan's Disappearing 'Island' "
(June 27, 2014)
- "Binary Planets"
Keegan Ryan, Miki Nakajima, David J. Stevenson; American Astronomical Society, DPS meeting #46, #201.02; abstract (November 2014)
- "46th Meeting of the American Astronomical Society’s Division for Planetary Sciences (DPS) with Historical Astronomy Division (HAD) Abstract Book"
(November 9-14, 2014)