NASA’s Kepler confirms 100+ exoplanets during its K2 mission
The largest haul of confirmed planets obtained since the space observatory transitioned to a different mode of observing includes a planetary system comprising four promising planets that could be rocky
An international team of astronomers led by the University of Arizona has discovered and confirmed a treasure trove of new worlds using NASA’s Kepler spacecraft on its K2 mission. Among the findings tallying 197 initial planet candidates, scientists have confirmed 104 planets outside our solar system. Among the confirmed is a planetary system comprising four promising planets that could be rocky.
The planets, all between 20 and 50 percent larger than Earth by diameter, are orbiting the M dwarf star K2-72, found 181 light years away in the direction of the Aquarius constellation. The star is less than half the size of the sun and less bright. The planets’ orbital periods range from five and a half to 24 days, and two of them may experience irradiation levels from their star comparable to those on Earth. Despite their tight orbits — closer than Mercury’s orbit around the sun — the possibility that life could arise on a planet around such a star cannot be ruled out, according to lead author Ian Crossfield, a Sagan Fellow at the University of Arizona’s Lunar and Planetary Laboratory.
The researchers achieved this extraordinary “roundup” of exoplanets by combining data with follow-up observations by earth-based telescopes including the North Gemini telescope and the W. M. Keck Observatory in Hawaii, the Automated Planet Finder of the University of California Observatories, and the Large Binocular Telescope operated by the University of Arizona. The discoveries are published online in the Astrophysical Journal Supplement Series.
Both Kepler and its K2 mission discover new planets by measuring the subtle dip in a star’s brightness caused by a planet passing in front of its star. In its initial mission, Kepler surveyed just one patch of sky in the northern hemisphere, measuring the frequency of planets whose size and temperature might be similar to Earth orbiting stars similar to our sun. In the spacecraft’s extended mission in 2013, it lost its ability to precisely stare at its original target area, but a brilliant fix created a second life for the telescope that is proving scientifically fruitful.
After the fix, Kepler started its K2 mission, which has provided an ecliptic field of view with greater opportunities for Earth-based observatories in both the northern and southern hemispheres. Additionally, the K2 mission is entirely community-driven with all targets proposed for by the scientific community.
Because it covers more of the sky, the K2 mission is capable of observing a larger fraction of cooler, smaller, red-dwarf type stars, and because such stars are much more common in the Milky Way than sun-like stars, nearby stars will predominantly be red dwarfs.
“An analogy would be to say that Kepler performed a demographic study, while the K2 mission focuses on the bright and nearby stars with different types of planets,” said Ian Crossfield. “The K2 mission allows us to increase the number of small, red stars by a factor of 20, significantly increasing the number of astronomical ‘movie stars’ that make the best systems for further study.”
To validate candidate planets identified by K2, the researchers obtained high-resolution images of the planet-hosting stars as well as high-resolution optical spectroscopy data. By dispersing the starlight as through a prism, the spectrographs allowed the researchers to infer the physical properties of a star — such as mass, radius and temperature — from which the properties of any planets orbiting it can be inferred.
These observations represent a natural stepping stone from the K2 mission to NASA’s other upcoming exoplanet missions such as the Transiting Exoplanet Survey Satellite and James Webb Space Telescope.
“This bountiful list of validated exoplanets from the K2 mission highlights the fact that the targeted examination of bright stars and nearby stars along the ecliptic is providing many interesting new planets,” said Steve Howell, project scientist for Kepler and K2 at NASA’s Ames Research Center in Moffett Field, California. “This allows the astronomical community ease of follow-up and characterization, and picks out a few gems for first study by the James Webb Space Telescope, which could perhaps provide information about their atmospheres.”
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Don’t get excited.For a planet to develop life, it must retain its water, which it can only do if it has a twin or massive natural satellite, like Moon, orbiting the planet close to its plane of ecliptic and therefore stabilizing its rotational axis against undue obliquity, which would expose the planet’s polar regions to 24/7 irradiation during the solstices. This has evidently happened with Venus and Mars, both of which have high deuterium/hydrogen ratios compared to Earth’s, indicative of loss of water through photodissociation. Not surprisingly, this critical consideration has been completely ignored by mainstream science, but it greatly reduces the probability of finding life elsewhere in the Universe.
@David Bennet Laing:
The stability (or lack thereof) of a rotational axis does not necessarily impact whether or not water can be maintained on a given planetary surface. Even if it IS a necessary/required component, the exoplanets under discussion here will have much of that stability provided courtesy of the star they so closely orbit, and should be able to maintain it thanks to the lack of giant planets to tug them out of line (unlike our own solar system). Any surface water of large enough amount will have tides similar to Earth’s solar tides, providing still further stability.
It is true that the retaining &/or gaining of an appreciable amount of water is absolutely the one facet of planetary development that we look for above all others when hunting for “Earth-like” or “life-sustaining” planets, simply because life occurs on Earth everywhere that molecule may be found. However, the literature suggests that a strong planetary magnetic field may be much more important than axial stability, since such a magnetic field generally prevents solar winds from dissociating surface & atmospheric water into its component H & O components, and further prevents stripping large amounts of atmosphere (especially the loose hydrogen) out into space.
Using your examples of Mars & Venus, both planets lack planetary-scale magnetic fields; they also lack the free hydrogen necessary to keep oxygen from bonding with absolutely everything else (ok, almost everything else) in their respective environments. The Barsoomian (a.k.a., “Martian”) surface is red because of the high iron oxide (rust) content, while the oxygen in Ishtar’s (Venus’) atmosphere is generally tied up in things like CO2 & SO2, with most of the rest similarly locked up in the crust. The remainder of Cytheria’s (also Venus’) atmospheric oxygen interacts with the help of other elements to transform what little hydrogen is left into several delightful things, e.g. the sulfuric acid clouds & precipitation that makes up what passes for Venusian “weather.”
In other words, the components needed to produce the oceans we may need for survival (let alone comfort) on Mars & Venus are either long since blasted into space (mostly the hydrogen), or else tied up in molecules that are chemically bonded so strongly as to be un-useful for life (mostly the oxygen), all thanks to vigorous interaction with the un-blunted solar wind. Pending the discovery of contrary evidence/examples, we suspect at this point that the same is likely to be true for most exoplanets which also lack a strong magnetic field.
All of that being said, there’s STILL no guarantee that water = life anywhere else in the universe, so the fact that stability may or may NOT be an issue really doesn’t change the situation much: until we DO find it, our sample size of life-bearing planets in the universe remains stuck at “1 per Universe.”
Thanks for the thoughtful discussion. My understanding, however, is that Venus is in slow, retrograde motion, and its obliquity is 177 degrees, not 3 degrees, as you allege. This would make a considerable difference to your argument. As far as photodissociation is concerned, the operative consideration here is that with high obliquity, one hemisphere is directed toward the primary for expended periods of time, not giving the water there the benefit of “cooling off” by turning out of the direct solar beam once a day, as is the case with Earth and it’s obliquity. This would favor high evaporation rates from the polar regions. There are two considerations here. First, under present conditions, photodissociation doesn’t take place until the water vapor rises high into the atmosphere, where the solar UV radiation is more intense, as it would do under the circumstances (24/7 irradiation). Second, at the time Venus lost its water, and Earth didn’t, neither one of these planets had a protective ozone layer, for which a life-generated oxygen-rich atmosphere is prerequisite, and that would have to wait, on Earth, at least, until the Great Oxygenation Event of some 2.3 billion Ma, by which time all reduced phases on Earth’s surface had been effectively oxidized. Thus, thermals would have been able to rise unimpeded through the atmospheres from the Sunward polar regions of both planets, and photodissociation would have been more likely under those circumstances, especially since the lack of an ozone layer would allow UV-C and UV-B to penetrate to lower levels of the existing atmospheres, thus being more effective at lower altitudes.
@David Bennett Laing
Well, I tried to leave an in-depth reply, but I seem to have misplaced it…
The gist of my reply is three-fold:
#1) The literature seems to indicate that lacking a significant planetary magnetic field is much worse for life than a lack of spin-axis stability. This is primarily due to A) the dissociation of water on the surface & in the atmosphere of such planets, thanks to stellar wind, and B) the resulting combination of free oxygen with the planetary crust and other hosts, preventing free hydrogen from recombining into water & allowing it to eventually escape to space.
#2) The exoplanets under discussion are close enough to their host star to have some of that same stability imparted by the star itself. Also, that system lacks the giant planets (present in our system) which would cause the tugs that would eventually render their respective axes unstable over time. Any large bodies of water present on the surface, if any (analogous to our own oceans, e.g.), would thus experience significant tides, contributing further toward axial stability.
#3) Neither the confirmed presence of water, nor that of axial stability guarantee the presence of life in such a place, even at what we might consider an ideal temperature/pressure/radiative balance. The fact is, until we DO find life somewhere else, we know of exactly ONE PLANET IN THE ENTIRE UNIVERSE that hosts life: our own Terra.
Smokey, here are my comments on your objections:
1. If the primary is close enough to the planet to exert a significant gravitational influence on its equatorial bulge (Earth is, as I understand it, at least twice as effective as Sun in this regard), then it is most likely closer to the primary than the so-called “Goldilocks zone,” so the point is moot.
2. Axial tilt is not necessary for water dissociation, but it is sufficient, and it is simpler than the magnetic mechanism you mention, which, of course, remains a possiblility. I prefer tilt on the grounds that it more closely conforms to Occam’s razor (it is a simpler explanation). What is your evidence for the statement that “lacking a significant planetary magnetic field is much worse for life than a lack of spin-axis stability?”
3. Tides, dramatic though they are to us, are measured in meters, not kilometers, and as such are insignificant in comparison with Earth’s equatorial bulge. Furthermore, they are fluid, and therefore less likely to affect the motions of a solid object, such as Earth, upon which they occur.
4. Again, the exposure of one water-covered hemisphere or the other to direct solar insolation for a protracted time over the solstices is a sufficient condition for the evaporation and photodissociation of water at 1 AU, especially in the absence of an ozone shield, which depends on life forces to generate the necessary oxygen to form it in the atmosphere, together with the resultant stratosphere (which is caused by the ozone layer).
5. The facts that Venus and Mars lack strong magnetic fields, but Earth has a strong one are compelling reasons to speculate that Earth’s field might have worked to prevent dissociation and sweeping away of hydrogen by the solar ionic wind, but it needs to be shown what importance it may have relative to obliquity considerations.
6. We appear to agree on the disposition of oxygen on Mars and Venus in the absence of hydrogen to combine with it.
7. As I have argued before, your assumption that the solar wind and the possession by a planet of a strong magnetic field are responsible for the absence or presence of water on the planet needs to be proven as more or less effective than obliquity of rotation. Certainly, it could be a factor, but so could obliquity. Unfortunately, as you point out, we don’t yet have a sufficient planetary sample on which to make that determination. Until we do, it seems only responsible to consider both possibilities.
I appreciate the discussion, DBL! You list a number of relevant ideas, & I’d like to start off by pointing out that none of my responses are an effort to say “YOU’RE WRONG,” because future exploration may find that your hypothesis is valid under a number of conditions, possibly many. I’m simply presenting the reason(s) why axial tilt isn’t often to be found in the scientific discussion of whether or not a planet may be habitable or not.
Point by point, then:
#1) a. In this case, if our solar system lacked not only the Moon, but a Jupiter & Saturn as well, then we’d still be fine because the Sun’s pull would be strong enough to cancel out the tugs from the other planets (for the most part), even out here at 1AU. By the same token, it is the pull of Sol on one pole combined with an amplifying pull on the other pole by one or more of the massive outer planets that leads to Mars’ significant axial wobble (sometimes tilting less than Earth’s sometimes twice as much over a period of mere thousands of years), lacking as it does a large moon to cancel out those effects. However, Venus also lacks a large moon, and its axis is more stable than Earth’s. This is because the Sun’s gravity swamps the influence of the outer planets at Venus’ distance… which is MUCH further out that the two exoplanets under discussion.
Also, we aren’t talking about a pair of random exoplanets, but two particular ones orbiting one particular star. We know the dimensions of this system, and that they do not (we think) possess the same types of de-stabilizing planetary bodies we do in our outer solar system. We further see that the exoplanets in question ARE in their so-called “habitable zone” around a star, which is quite a bit smaller than the Sun. Thus, they should (key word!) be axially stable even if they are not tidally locked (1:1 spin:rev) to their host star.
#2) Occam’s Razor is a fine tool when properly used. The key assumption to your hypothesis seems to be that a low, stable axial tilt necessarily contributes in some significant way to the retention of planetary water, and vice versa. [See #5 & #7 for more] However, photo-dissociation of water generally only occurs when insolation (“instellation” for exoplanets?) is so high as to be intolerable for most life (i.e., at a distance inside the “habitable zone”), and the times when it does occur within the HZ can be mitigated or prevented with a sufficient strong (i.e. Earth-like) magnetic field, according to the current literature. Thus, even granting axial tilt as a potential factor, Occam’s Razor can actually be said to suggest the opposite of what you have been asserting… specifically that axial tilt/stability may be much less important than the presence of a significant magnetic field.
Once again, a demonstrative example exists here in our neighborhood, for even with a large natural satellite for stability, Earth’s rotation has a >23deg. axial tilt which further nutates (or “precesses”) around the sky every ~26,000ka. Venus, despite lacking any natural satellites, has an axial tilt of less than 3deg., with a precession so slow as to be hard to measure accurately. Further, Venus’ gravity is nearly that of Earth, and its atmosphere is >90 times as thick, providing a “cap” on molecular (Brownian) motion to space that Earth lacks. Despite this, Venus is the planet lacking in H2O, not Earth.
#3) a. Earth’s equatorial bulge has everything to do with its 24hr rotation, and little to do with external forces (cf. Jupiter w/an extreme bulge); Venus’ lack of a bulge owes largely to its slow rotation (solar day ~2,800hrs, sidereal day ~2x longer — a little more than one Venusian year!). Of the two, Venus’ rotation is more stable than Earth’s. Also, the exoplanets we’re discussing may or may not have these bulges, as we’re not yet sure of their spin-orbit ratios at this time. Therefor any discussion of such a bulge’s effect on their rotation is probably premature, but barring something measured in minutes to hours (very fast spin), it’s probably not going to contribute significantly.
b. When I mentioned the Earth’s tides (specifically the ocean tides) it was because the effect of that water “sloshing” about the planet tends to help keep the Earth’s center of mass stable about its rotational axis. That motion would not vanish with the vanishing of the Moon, although as you accurately point out, the sloshing would be ~2/3 less.
That said, it is the pull of Earth’s tides (water & crust) on the Moon which are accelerating Luna into an ever-higher orbit while simultaneously slowing our planet’s rotation (& lessening our equatorial bulge in the process). In fact, the Moon will have drifted far enough from Earth to enter its own distinct solar orbit (thank YOU, Jupiter & Saturn!) long before the Earth becomes tidally locked facing it — causing a whole new round of debate in the IAU in the process over how to define a planet, no doubt! ^_~
#4) Again, one considers the relative tilts of Earth & Venus (at >23 degrees & <3 degrees, respectively) as a counter-argument; Earth is the one with the water, despite having a much less dense atmosphere & poles which experience unending sunlight for a significant portion of the year.
#5) Experiments validating the solar wind = H2O loss principle have already been done, and in fact are ongoing at Mars as we speak. It was actually a bit of a surprise given those previous experiments how much atmosphere Mars (& to a lesser extent, Venus) still has left! Some of that mystery has been dispelled by the currently-orbiting MAVEN space craft, which found that the solar wind can induce local-to-regional magnetic fields due to interaction with the Martian crust. It also found that induction of such fields occurs more often during times of high solar wind. In other words, solar storms are still eroding the Martian atmosphere, just less than expected because they induce localized magnetic fields which mitigate the expected loss of atmosphere. By the same token, periods of solar quiescence are found to be the times when those fields are least active, and the loss of atmosphere proceeds apace. [See also #7]
#6) Yessir! There's a reason we call it "oxidation," regardless of the substance under discussion (wood, iron, copper, etc.). =D
#7) To be clear, the issue is not the evaporation of water, but its dissociation, i.e., the receipt of sufficient energy to cause that water molecule to break into its component H2s & Os. Thus, while a planet in its HZ may in fact experience a great deal more melting &/or evaporation at its poles with a significant/unstable axial tilt, such a tilt doesn’t necessarily provide enough energy for large amounts of O to break away completely from its partner H2. Looked at from the other side of your hypothesis, a planet receiving enough insolation at the poles to not just evaporate but begin chemically breaking up water into H2 & O will likely be receiving quite enough insolation to be thought of as “too close/hot to be habitable,” regardless of its axial tilt &/or stability. (Although, even Mercury has water ice in its shadowed craters, so…*shrug*)
Anyhow, this is the heart of the current skepticism regarding axial tilt as a potential primary mechanism for the dissociation & subsequent loss of planetary water. These points are found in the scientific literature, and generally verified by observations to date. When combined with the fact that every star-ward-facing pole has an equal & opposite space-ward pole — which spends an equal amount of time re-combining/re-freezing the H2s & Os back into water/ice — axial tilt just doesn’t seem a viable option.
You’re welcome to continue to disagree; I understand & accept your points, despite not subscribing to them for the reasons mentioned. Cheers!
In point #1, “Earth” should read “Moon.”
I’ll repost this in a more appropriate place!
Thanks for the thoughtful discussion. My understanding, however, is that Venus is in slow, retrograde motion, and its obliquity is 177 degrees, not 3 degrees, as you allege. This would make a considerable difference to your argument. As far as photodissociation is concerned, the operative consideration here is that with high obliquity, one hemisphere is directed toward the primary for expended periods of time, not giving the water there the benefit of “cooling off” by turning out of the direct solar beam once a day, as is the case with Earth and it’s obliquity. This would favor high evaporation rates from the polar regions. There are two considerations here. First, under present conditions, photodissociation doesn’t take place until the water vapor rises high into the atmosphere, where the solar UV radiation is more intense, as it would do under the circumstances (24/7 irradiation). Second, at the time Venus lost its water, and Earth didn’t, neither one of these planets had a protective ozone layer, for which a life-generated oxygen-rich atmosphere is prerequisite, and that would have to wait, on Earth, at least, until the Great Oxygenation Event of some 2.3 billion Ma, by which time all reduced phases on Earth’s surface had been effectively oxidized. Thus, thermals would have been able to rise unimpeded through the atmospheres from the Sunward polar regions of both planets, and photodissociation would have been more likely under those circumstances, especially since the lack of an ozone layer would allow UV-C and UV-B to penetrate to lower levels of the existing atmospheres, thus being more effective at lower altitudes.
“My understanding, however, is that Venus is in slow, retrograde motion, and its obliquity is 177 degrees, not 3 degrees, as you allege.”
This depends on whether one uses the “right-hand rule” — in other words, the principle that ALL planets rotate in a prograde fashion (West to East) — or instead uses North relative to Earth / the solar system as a whole (most planets/dwarf planets are oriented as is Earth), with rotational spin making no difference to the determination of N vs. S. The latter results in planetary bodies possibly having either a prograde (W to E) or retrograde (E to W) rotation.
Using the Earth/Sol System as a system-wide standard for “North,” Venus’ rotation is considered retrograde (E to W) about an axis less than 3 deg. tilted from its orbital plane, with Ishtar Terra — a large continental mass analogous to Asia on Earth — considered to be in the Northern Hemisphere of the planet (as with Asia on Earth). However, via the “right-hand rule” — where geographic North is dependent upon direction of rotation — Venus’ rotation must be considered “prograde” (W to E), forcing the axis to be reckoned as tilted >177 deg. with respect to its orbit. Ishtar Terra would thus be in Venus’ Southern Hemisphere, despite being on the same side of the planet/solar system as Asia is on Earth. In other words, using the latter method, Venus’ rotational poles ARE flipped relative to the Earth’s, but the rotation is no longer considered “retrograde.” (All of this is mentioned on Wikipedia, among other places.)
Both methods of reference have their uses & are valid in their own regimes. The former is the one NASA & the USGS use, which is why I used it.
“This would make a considerable difference to your argument.”
Not at all, since a “>177 degree tilt” still puts the rotational axis within 3 degrees of perpendicularity relative to Venus’ orbital plane (smallest value in the solar system), and the precession of seasons is still negligible compared to the Earth’s ~26,000yr wobble (misstated earlier by me: that should have read either ~26,000yr, or ~26ka; ~26,000ka is just wrong!). In other words, whether you consider the planet “right-side up” or “upside down,” a tilt of less than 3 degrees or greater than 177 degrees makes no difference in this context: in either case the Cytherian polar regions are rendered tiny compared to Terra’s (Arctic Circle north of 66N, vs. the Venusian equivalent north of 87N), the incident angle of sunlight received on those regions is much lower than on the Terran poles (~2.7deg. vs ~23 deg.), and the “wobble” is too small to be confidently measured, despite the state of axial ‘flip-itude’.
“As far as photodissociation is concerned, the operative consideration here is that with high obliquity, one hemisphere is directed toward the primary for expended periods of time, not giving the water there the benefit of “cooling off” by turning out of the direct solar beam once a day, as is the case with Earth and it’s obliquity.“ (emphasis mine)
Mmmm, Earth’s obliquity is a little higher than you seem to be giving it credit. You may be surprised just how much of Earth’s polar surface area spends 60+ days in sunlight out of every year. I just did some research and was surprised myself: did you know latitudes pole-ward of 75 deg. get more than 2400hrs of uninterrupted daylight? Even Venus’ slow, retrograde rotation results in only ~1400hrs from sunrise to sunset (assuming one could SEE the sun), followed directly by ~1400hrs of night. Thus, if upwards of 6% of the Earth’s surface spends upwards of 10% of their year (~850hrs & up) in continuous sunlight, and if that same area sees a fairly high average angle of incidence for that same sunlight, one would then tend to expect to see a measurable solar impact on the loss of Earth’s water/hydrogen to space due to axial tilt: to date, we have not.
This is why I’m still comfortable saying that “your hypothesis is reasonable,” even though I disagree with it. It has a plausible mechanism, even if the evidence to date confirming such an effect has not yet been observed; that may simply be a function of time &/or effort. It is only this lack of verifying evidence which persuades me to consider it less important than other factors, and I remain open to being persuaded.
OK, let me see if I can be persuasive. In the first place, it seems to me to be rather contrived to assume that two different rules applied to the revolution, rotation, and obliquity of the Solar System’s planets. If they all originated by the same mechanism (all their orbits are essentially coplanar, and they all revolve in the same direction), then Venus’s slow retrograde rotation can only be seen as the inevitable consequence of its rotational axis having turned through 177 degrees from the plane of the ecliptic. If this is so, it’s highly unlikely that it turned in one swell foop, so it must have spent some time at or around 90 degrees, in which position, it would be subject to high irradiation from Sun. Obliquity has a lot to do with the intensity of radiation. With only a 23.4 degree tilt, Earth’s polar regions get only a very low Sun angle of 24/7 irradiation of the polar regions during the solstices, which is insufficient to promote much evaporation and photodissociation. The high Sun angle in effect during obliquities greater than say 45 degrees is a very different story, especially if it were to happen repeatedly, as it probably did. All in all, I think the scenario that I outlined has far fewer problems than the other.
And speaking of the origins of the Solar System, the problems with the old Kant-Laplace theory, notably that Sun only possesses less than 1% of the rotational inertia of the system, can be readily explained if the planets all formed BEFORE Sun commenced thermonuclear “burning.” George H. Darwin and Henri Poincare pointed out that a spinning object, like proto-Sun in the center of the collapsing solar nebula, MUST at some point, dependent only on its density and spin rate, flatten out and eject a portion of itself from its equator, thus slowing its rotation. The ejected matter would become the proto-planet Neptune, and it would spiral outward through the collapsing cloud disk, while accumulating dust that fell upon it, accelerating its rotation until it, too, spun out a natural satellite, which lacked the mass to do the same. This same process would have repeated several times during the ongoing dust cloud’s collapse, for proto-Sun, proto-planets, and proto-moons until the Solar System as we know it was created. It was only after this process was complete that Sun became large enough to commence thermonuclear “burninng.” This is a classic case of the developers of a theory (Kant and Laplace and followers) not considering all the constraints affecting the model in question. For more on this idea, see my introductory college Earth science textbook, “The Earth System,” WmCBrown, 1991, pp. 465-469.
“In the first place, it seems to me to be rather contrived to assume that two different rules applied to the revolution, rotation, and obliquity of the Solar System’s planets.”
Whoops, stop right there: terminology alert! There are NOT two different “contrived” rules that apply arbitrarily (my word) to the obliquity of the planets.
There ARE two different rules used to determine which direction “North” is, BUT one only uses one of them at a time for all of the planets at once. As such, if one calls Venus’ rotation “retrograde,” then one must also allow that its North pole points the same general direction of our North pole. In other words, Venus’ rotation is only “retrograde” if its spin is “backwards” from all the other similarly aligned N-S planetary axes. Likewise, if one uses the “right-hand rule” to determine that North on Venus is inclined 177.4 deg. from its orbit, then its rotation is necessarily “prograde;” its rotation cannot be called “retrograde” because that very motion is what determined North in the first place.
In short, the term “retrograde rotation” and an axial tilt of 177.4 deg. are incompatible terms when applied simultaneously to Venus. Moving on!
“If [the planets] all originated by the same mechanism (all their orbits are essentially coplanar, and they all revolve in the same direction), then Venus’s slow retrograde rotation can only be seen as the inevitable consequence of its rotational axis having turned through 177 degrees from the plane of the ecliptic.”
Not necessarily (more on this below), but for now let’s accept that at face value, and go from there. ^_^
“If this is so, it’s highly unlikely that it turned in one swell foop, so it must have spent some time at or around 90 degrees, in which position, it would be subject to high irradiation from Sun.”
Ahhhh, now I think I see the key element of the idea you’re presenting! Actually, a single swell foop (I love this term, BTW) is precisely what many scientists currently think did happen, and not a gradual migration over time: “The best, current, ideas still favor some dramatic event that occurred while Venus (and Uranus for that matter) were being formed. … One of these [large planetesimals] may have collided with the Earth, dredging up material that later solidified into our Moon. … Venus may have experienced an encounter with one of these large bodies in which, unlike for the Earth, the material didn’t form a separate moon, but was absorbed into the body of Venus. … The result is that the new spin direction and speed for Venus was seriously altered from its initial state which could have been very Earth-like. Today, the result of that last, ancient collision is Venus with a retrograde rotation.” [ http://www.astronomycafe.net/qadir/q50.html ]
There is another semi-popular theory which also has math & observational evidence to back it up: the idea that Venus’ axis has ALWAYS been pointed the way it is now, and that only its rotational rate has changed:
“Now Alexandre Correira and Jacques Laskar suggest that Venus may not have flipped at all. They propose instead that its rotation slowed to a standstill and then reversed direction. … Regardless of whether it flipped or not, it is bound to settle into one of four stable rotation states, two in either direction. The researchers add that Venus would be more stable in one of the two retrograde rotational states. So in essence, it was just a question of time before Venus started spinning the wrong way.” [http://www.scientificamerican.com/article/why-venus-spins-the-wrong/]
Both theories are mentioned here [http://www.universetoday.com/36123/axis-of-venus/], and there are even some folks who “combine” the two hypotheses, saying that a massive impact(s) struck Venus just so as to negate the previous rotation, leaving us with the contrary/retrograde rotation we see today.
However the idea of a slow drift of the pole through the full ~180 deg. sweep is NOT currently thought likely, primarily for two reasons: 1) spinning objects in space like to wobble (nutate), but like Weebles(TM) and spinning gyroscopes, they DON’T like to flip over — spontaneously or gradually — except in special cases; and 2) the wobble of Venus’ orbit can be measured, but it’s so small that such measurements are within the error bars — meaning Venus shows no contemporary signs of any inclination (aha!) to “tip over” even a little bit, let alone in the large-scale way your hypothesis requires. Because there’s no such predisposition observed, there’s no reason to think it has had any reason to move prior to this point in time either, since objects in motion/at rest tend to stay in motion/at rest.
That doesn’t make such motion impossible: one might argue that perhaps Venus DID slowly tip over in the past, and then a massive impact stopped that motion in the position we see it now. If so, that would allow for your theory to work as described. However as you mentioned in the quoted section above, we’re postulating that Venus formed like all the other planets, not in some weird way that applies only to that one body. In that case, we’re simultaneously postulating an original orientation similar to all of the other planets as well.
Like Venus though, not one of the other planets (Uranus included) show any sign of this gradual motion of the rotational axis over large swathes of the sky (currently stable, despite being “on its side”). Thus your scenario needs at least TWO large impulses in order to work, and it further requires those events to be far apart in time… long enough at least for high obliquity to do its presumed damage. This is in direct contrast with the scenarios listed previously which only require one large (set of) impact event(s in a short time frame, e.g., the Late Heavy Bombardment) to produce the situation we see now. As Occam’s Razor favors the simpler of the possibilities available (all other things being equal), a single large event — or no “event” at all — appears more likely than multiple, similarly large events which are also conveniently spaced in time.
So the bottom line here is that of all the possible ways Venus’ axial tilt may have ended up as it stands today, scientists don’t currently think it got there gradually over time. Occam’s Razor is NOT a proof, so things could possibly have happened your way as well (I know of no records left by anyone present who could tell us for sure). All it can show us is which option(s) seem more likely from a logical standpoint, all other variables being equal… which, excitingly, they never seem to be!
(Sorry for all the links, thank you Anthony W. & the Mod Squad!)
I don’t really see the logic in this. Prograde rotation simply turns into retrograde rotation when North becomes South because the obliquity has exceeded 90 degrees. Whether you “define” North and South one way or another is irrelevant and only confuses the issue. “Retrograde” and “obliquity of 177.4 degrees are fully compatible with each other.
One swell foop. Why invoke unlikely processes? Impacts are very rare occurrences, and the Great Bombardment was likely caused mainly by fissioned material from proto-Sun and proto-Venus falling back onto the planet during its formation. Absent such “contrived” concepts as impacts, the likelihood is that Venus’s rotational axis underwent slow, repetitive swings rather than any sudden readjustment. In my opinion, impacts have been overused and dredged up in order to explain obscure concepts that remain obscure because of compartmentalized thinking, a plague of modern science.
As for Correira and Laskar’s “explanation,” that seems to me to be overly contrived; arm waving in order to explain something that they don’t want to recognize, an attempt to be clever, as it were.
According to Space Daily, “…recent studies suggest that occasionally — at intervals of a few tens of millions of years — Mars’ obliquity may swing from 0 degrees all the way up to 60 degrees. (Thus Mars sometimes joins Uranus and Pluto as the only planets that “lie on their side”.) At present — by sheer chance — Mars is about halfway through one of its obliquity cycles, at a tilt of about 25 degrees.” This suggests that obliquities of 45 and 60 degrees are not all that unusual in the Solar System.
I know of no such constraints on obliquity as you allege. As far as I’m aware, a top can assume any obliquity, depending on the forces applied, and I’ve seen them do just that. I don’t think that Occam’s razor applies to unusual event,s such as sudden tippings, as much as to the more usual situation of gradually shifting obliquity.
It seems to me that you are relying overly on the musings and pronouncements of others, as if they had some kind of sanction, such as publication in peer-reviewed journals. The latter have been useful in mediating scientific discourse, but the glut of idiosyncratic idea-spinning is so great nowadays, especially with all sorts of computer modeling thrown in, that they have become rather meaningless in the real world. Far better, I think to stay open to all the pertinent evidence and make up your own mind on the way things are, with the judicious help of Occam’s razor.
I apologize for not being able to properly explain how the only way one gets a 177+ deg axial tilt for Venus is through prograde rotation by definition. I’m not sure how else I can put it.
OF COURSE the word used doesn’t change the fact that Venus rotates contrary to the rest of our system, nor am I trying to say it does. However, I’ve already given multiple possible reasons for why that motion might be occuring without invoking a slow rolling tilt (of Venus’ axis) of almost 180 deg. that you’ve provided NO explanation for, nor any explanation as to why it has since stopped. I’ve also told you WHY few people subscribe to this theory, namely that there’s no such motion observed now; if it was there before, why isn’t it now, as it is, e.g., on Mars?
Since you bring them up, Mars’ swings in obliquity are well understood as resulting mostly from the contrary-wise tugs of the Sun & the outer planets, combined with its complete lack of a large stabilizing satellite. Also (completely unlike Venus), we can see the Martian poles moving measurably — as well as the fact that they oscillate through a constrained range, rather than just continuing on through a complete axial flip. The fact we can directly observe this motion (& further, calculate the influences of the rest of the system upon it in advance using our “meaningless in the real world” models) is how all of those numbers you present in your quote were found in the first place. It is the complete lack of such observable numbers for Venus that, once again, mean it’s either not moving now when it was before (implying some large event to provide the starting/stopping impetus), or else it never did move before, as Correira & Laskar (et al.) allege.
“As for Correira and Laskar’s “explanation,” that seems to me to be overly contrived; arm waving in order to explain something that they don’t want to recognize, an attempt to be clever, as it were.”
Again, if Venus’ poles moved before, why aren’t they moving now? It’s the simple question that is only resolved by either giant impacts, or else (as they posit) a complete lack of such movement in the first place. This is simple logic, not “arm-waving:” if it’s not moving now, then the only way it was moving before is because something pushed it, then stopped it, unless it was never moving to begin with.
If I may be direct with you, this is all logical deduction based on direct & indirect observations, along with the falsifiable calculations based upon them, and is not to be confused with either “idiosyncratic idea-spinning” or “over-reliance” on others’ “musings & pronouncements,” as you put it. I certainly hope I’ve refrained from similar dismissive &/or belittling language in my responses to you, as I personally haven’t intended any such thing. My apologies if it’s come across that way; my only thought was to inform and to exchange information & ideas.
Thanks for the dialogue, but I think it’s best I bow out now. I hope we can chat again another time more amicably, whether or not we agree on the topic at hand.
Smokey, I have looked at a range of available evidence within the planetary system, and drawn the most reasonable conclusions I could, based on the evidence at hand. Answering your various objections has been, as usual, a useful exercise for me, as it has served to hone my presentation, and it has helped to support my contention that I am correct in my interpretation. Thank you for your participation.