Sunday, December 18, 2016
Tuesday, December 13, 2016
Habitable Planets around White Dwarfs
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| The white dwarf G29-38 (NASA) |
This last question is particularly interesting because White Dwarfs are the remains of a Red Giants. Red Giants are the last phase of fusion based main sequence stars. Between White Dwarf, Red Giant and main sequence phases, a star changes so drastically that it is unlikely planets close to the star would survive into the next phase. Some very interesting things need to happen for a planet to form within the Habitable Zone of a White Dwarf.
With main sequence stars, the Habitable Zone slows moves away from the star because the star slowly gets hotter. A planet that starts out within the Habitable Zone of a young main sequence stars may not remain within the Habitable Zone for the full length of that star's life-cycle.[001] [002] Oddly enough, White Dwarfs have the exact opposite problem. White Dwarfs cool down as they age.[003] Habitable Zones around White Dwarfs will shrink until being too small for any sizable planet to exist within it.
The Habitable Zone around a White Dwarf is very small compared to that of Sun. The Habitable Zone around Sun is roughly between the orbits of Venus and Mars. The Habitable Zone around a White Dwarf is much closer than even the orbit of Mercury around Sun.[003] It seems a planet that close to a White Dwarf cannot exist without some special events.
How do planets get to the Habitable Zone?
A planet could have existed in the previous solar system during the main sequence star phase, but much further out; so far out that it may not have been previously habitable. When the main sequence star expands to become a Red Giant, then explodes to leave a White Dwarf, the planet would have to move from the outer reaches of the solar system to a stable close orbit. This sounds incredible, but apparently it is possible since planets have been discovered around Neutron Stars, which go thru even more violence.[003]Another possible scenario is it the matter ejected from the exploding Red Giant, or other remaining debris within the solar system somehow creates a new accretion disk around the newly formed White Dwarf, from which new planets could form.[003]
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| Water Cycle |
Even if either of these scenarios do happen, a water related challenge presents itself. A lot water must somehow remain or be (re)introduced on these special planets. Water is likely stripped from any existing planet that moves so close to the White Dwarf.[004] Water is also unlikely to be available on any planet that forms so close to any star, White Dwarf or otherwise. Maybe these planets could gather new water via the same processes as Earth, possibly "delivered to by a barrage of comets."[003]
If a planet is lucky enough to form around a White Dwarf, what's that White Dwarf going to look like in the sky? White Dwarfs have a lot of mass, but they are very small in size. White Dwarfs are about the same size as Earth.[005] By my rough calculations, the Habitable Zone around a White Dwarf is about 5 times the distance of Earth to the Moon. So, I image the White Dwarf would appear several times smaller in the sky as the Earth appears to the Moon. Maintaining habitability of planet around a White Dwarf might be a bit like trying to keep warm outside on a freezing night next to a slowly fading campfire.
Primary reference:
A Loeb, D Maoz, Monthly Notices of the Royal Astronomical Society: Letters, Volume 432, Issue 1, p.11-15, arXiv:1301.4994 [astro-ph.EP], Detecting bio-markers in habitable-zone earths transiting white dwarfs
Article Series:
- Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
- Small stars may have stable Habitable Zones, but habitable planets might not be common there
- Habitable Planets around White Dwarfs
- Habitable Worlds Around Binary Star Systems might not match Sci-fi
- How many Earth-like planets are orbiting Sun-like stars?
- First round of life in the Universe might have been possible extremely early
- Factors a planet needs for suitability of life; perhaps
- "Goldilocks zone of metallicity" on a galactic scale
- Maybe we are the first
Sunday, December 11, 2016
Saturday, December 10, 2016
Thursday, December 08, 2016
Wednesday, December 07, 2016
Monday, December 05, 2016
Small stars may have stable Habitable Zones, but habitable planets might not be common there
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| Protoplanetary accretion disk around a new star |
Habitable Zones appear to be more stable and longer lasting around small stars, such as Red Dwarfs.[001] However, what are the chances of habitable planets appearing in these small zones around these small stars? It turns out that the chances may not be good.
According to the study A Decreased Probability Of Habitable Planet Formation Around Low-Mass Stars, multiple simulations suggest that low-mass stars are unlikely to have terrestrial planets of sufficient size within their Habitable Zones. This is due to a several factors. That's not to say it is impossible nor improbable; just not as common as previous thought.[002]
Other factors
Besides Habitable Zones, another factor to consider is the Habitable Planet Mass Limit. There is evidence that suggests that plate tectonic activity on a global level is a necessary factor for supporting life on a planet similar to Earth. Planets must be of a particular size in order to allow for global tectonics.[002] The lack or presence of global tectonics seems to be a factor in the differences between Venus and Earth. Though Venus seems to be large enough, its surface heals too quickly to allow for global tectonics.[003] The examples within our own solar system suggest that even when planets are large enough, there is no guarantee they will have global tectonics.Another factor is the Protoplanetary Disk. During the planet formation phase (accretion), there has to be enough material within the disk of matter that forms around very young stars (Protoplanetary Disk) in order produce larger planets. Though there are a lot of unknowns regarding this factor, the "ratio of disk mass to stellar mass is roughly constant with stellar mass". Also, planets seem to form much faster around small stars for various reasons. With less time to form and less mass within the Protoplanetary Disk, planets around low-mass stars may typically be much smaller. A second issue with fast forming planets is that they are much less likely to have enough time to collect enough water to support life.[002]
Exceptions?
Nothing is absolute. Gliese 581 is a Red Dwarf that has a number of large planets, and also has a debris disk that appears to have tens times amount of comet debris than our own Solar System. This suggests low-mass stars can have habitable planets. That said, Gliese 581 may be an outlier. Other factors are obviously involved that need further study.Primary reference:
S. N. Raymond, J. Scalo and V. S. Meadows, The Astrophysical Journal 669 (Nov., 2007) 606–614, arXiv:0707.1711 [astro-ph], A Decreased Probability of Habitable Planet Formation around Low-Mass Stars
Response:
Hacker News
Article Series:
- Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
- Small stars may have stable Habitable Zones, but habitable planets might not be common there
- Habitable Planets around White Dwarfs
- Habitable Worlds Around Binary Star Systems might not match Sci-fi
- How many Earth-like planets are orbiting Sun-like stars?
- First round of life in the Universe might have been possible extremely early
- Factors a planet needs for suitability of life; perhaps
- "Goldilocks zone of metallicity" on a galactic scale
- Maybe we are the first
Friday, December 02, 2016
Saturday, November 26, 2016
Friday, November 25, 2016
Thursday, November 24, 2016
Wednesday, November 23, 2016
Tuesday, November 22, 2016
Saturday, November 19, 2016
Friday, November 18, 2016
Tuesday, November 15, 2016
Monday, November 14, 2016
Thursday, November 10, 2016
Wednesday, November 09, 2016
Saturday, November 05, 2016
Saturday, October 29, 2016
Second time in Dallas this year
I've been to Dallas, TX twice this year, now. Both trips were for business. The first trip was for SOLIDWORKS World 2016. I'm always very busy for each SOLIDWORKS World convention, so there's not much time to explore on my own. This time around I was attending ASME Y Committee meetings. I was able to do a bit more exploring, but not much.
While wondering around, I found this eyeball. Even many locals don't seem to know that this thing exists and that it is in Downtown.
There were a number of good restaurants that were enjoyed on various days, such as Wild Salsa, TruFire Kitchen & Bar and recent local favorite Hard Eight.
On thing to note is that it's not easy getting around the Dallas area without getting dinged for tolls. In the week I was there, I racked up almost $20 in tolls by just driving around between places I needed to be. That is a bit annoying, especially since I have my navigation app set to avoid tolls.
The weather was great, particularly so far along into Fall. I didn't need a jacket or long sleeve shirt at any time.
Due to a sudden opening in my schedule, I did find some time to visit Arlington Museum of Art. I found it moderately interesting, but overall it was a bit underwhelming. The most interesting works I found there were by Charmaine Locke, but not much was available to view.
While wondering around, I found this eyeball. Even many locals don't seem to know that this thing exists and that it is in Downtown.
I also visited the Reunion Tower again to get some great photos of points of interest in Dallas.
On thing to note is that it's not easy getting around the Dallas area without getting dinged for tolls. In the week I was there, I racked up almost $20 in tolls by just driving around between places I needed to be. That is a bit annoying, especially since I have my navigation app set to avoid tolls.
The weather was great, particularly so far along into Fall. I didn't need a jacket or long sleeve shirt at any time.
Due to a sudden opening in my schedule, I did find some time to visit Arlington Museum of Art. I found it moderately interesting, but overall it was a bit underwhelming. The most interesting works I found there were by Charmaine Locke, but not much was available to view.
Saturday, October 22, 2016
Thursday, October 20, 2016
Wednesday, October 19, 2016
Monday, October 17, 2016
Friday, October 14, 2016
Friday, October 07, 2016
Sunday, October 02, 2016
Bad day for a couple of Subaru owners
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Death-defying u-turn
The first instance happened this morning near the bottom of a shallow hill on a two-way, two-lane road with rapidly moving traffic. A maroon-color Outback rapidly pulls over to the shoulder. The Outback is full of people and luggage, including stuff tied to the roof rack. Cars immediately following the Outback braked to avoid a collision, with a couple of them swerving briefly into the lane of opposing traffic. Dust was kicked up a dozen feet into the air. But, that's not all! The lady driving the Outback activates her left turn blinker just as Allie and I approach in our car. All of a sudden the lady veers back into the road at a sharp angle, with her front left fender heading straight for our front right fender. Allie honked and swerved into the opposing traffic lane to avoid the collision! The lady barely stopped in time to miss us. Fortunately there was a gap in the traffic on the opposite lane into which we could veer. Just as we passed, she continued her sharp turn into the lane and across into the opposite lane. She cut out in front of the car that was immediately behind us.Then she proceeds to make a full u-turn right in front of on-coming traffic in the opposing lane. Traffic moving in both directions had to slam on their brakes to avoid being hit by this lady. I'm sure she left a phantom intersection in her wake. When I looked back, I saw a field of brake lights as we continued on. After a few cuss words were uttered by Allie, I stated, "that the lady was driving like Clark Griswold". "Yeah, [more expletives], Griswold!" Allie appreciatively replies.
Bang, crumble, bye-bye bike
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| Roof-racked bicycle might've looked something like this before being mangled beyond recognition. |
The expensive looking bicycle was no longer a bicycle. I mean, there was still kinda two wheels, but it was a mangled mess. The roof rack was also a mangled mess. The roof rack being attached to the roof of the Subaru likely suggests that the Subaru's roof was also damaged to some extent.
At this point, the driver still doesn't exit his car. I think he was stunned. He finally pulls into a nearby parking spot. He still waits a considerable amount of time to get out. Maybe he was surveying the damage to the roof of the Subaru from the inside? When he finally gets out, he just stares at the mangled mess where his bicycle and roof rack formerly existed.
In what I thought to be a quiet voice, I made a comment, half to myself, half to Toebzilla, "That's an expensive mistake". Despite the distance between us, the driver, who appeared to be of the more laid-back early-twenties variety, looks over at me and just yells "Fuck!". I could do no more than to reply with a consoling "Yup!" and continue on my way with Toebzilla.
It's been one of those days.
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