Its Trouble...

Tuesday, December 13, 2016

Habitable Planets around White Dwarfs

Out of all the odd scenarios that might be possible in our Universe, the thought that there might be habitable planets around White Dwarfs is one of the stranger ideas, in my opinion. Imagine what it would be like to look up to see a White Dwarf dominating the daytime sky. How close would the planet have to be to the star to receive enough light and energy to support life? How would a planet find itself within the Habitable Zone around a White Dwarf?

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]

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:

Sunday, December 11, 2016

A #dog.

via Instagram http://ift.tt/2hnluoN

Saturday, December 10, 2016

Special big snowflake

via Instagram http://ift.tt/2hb19oD

Thursday, December 08, 2016

Fall 2016 Trip to Silicon Valley

Wednesday, December 07, 2016

Facebook asks me to review Earth. Ah, Planet Earth. Where do I start?

Monday, December 05, 2016

Small stars may have stable Habitable Zones, but habitable planets might not be common there

Protoplanetary accretion disk around a new star

The previous article Limited lifespan of Habitable Zones around other stars (September 2016) covered the topic of Habitable Zones for planets around other solar systems. However, Habitable Zones are only one of considerations for finding other stars with habitable planets. Given the lack of data right now, sometimes science has to fall back on simulations. These simulations do not replace actual observations. Instead, they offer clues as to how we may be proceed in our search for hard data.

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: