As far as I can tell, “some people” are French astronomer Jacques Laskar and associated researchers. Laskar is pretty much out on his own with regard to this theory, which is based on some computational modeling of chaotic systems and playing with parameters.
Although the Earth’s magnetic field deflects incoming charged particles from the Sun (solar wind) it is the thick mass of atmosphere that protect inhabitants from the powerful ionizing radiation that would be harmful to life. There is no question that the magnetic field helps the Earth retain atmosphere, as it does deflect the bulk of the solar wind around the Earth. The extent to which the atmosphere would be depleted is unknown, though. Current theories of Earth’s history indicate that the Earth would have a thick atmosphere like that of Venus if not for the presence of life binding up much of the carbons and preventing a runaway greenhouse effect.
As for terraforming, not only do we have no practical experience with engineering project on this scale by orders of magnitudes, but we also don’t really know enough about the possible celestial bodies (Venus, Mars, Europa, Titan) to even make educated guesses about what processes would have to be run and controlled in order to reach a state habitable to humans, nor if such conditions could be maintained in a stable equilibrium. The amount of energy necessary to produce a reducing atmosphere similar to Earth’s is enormous, even if we assume that we can obtain the basic constituents (oxygen, nitrogen) from smaller planetoids. By the time we have anything like the capability to do this, it will likely be far easier to merely build large enclosed solar orbiting constructs from extraplanetary material which can be more easily regulated rather than trying to force an open environmental system to maintain Earth-like conditions under equilibrium.
I can’t find a cite; but years ago I read that the Moon would hold atmosphere about a 100,000 years; Mars is bigger than that. And yes; terraforming the Moon has been seriously suggested. As far as the atmosphere leaking away; in cases like the Moon or Mars where it can almost-but-not-quite hold on to the air, I understand that we could theoretically surround the planet/moon with orbiting “shrouds”. Very large, very thin films, rather like solar sails that would bounce the occasional escaping atom atmosphere back towards the planet. That would extend the life of the biosphere pretty much indefinitely.
IMHO, the answer to the OP is basically, “Depends; how much effort do you want invested, and how much artificial maintenance of the environment is allowed?”.
By the time we have the technology to seriously try to terraform planets, I expect most of the effort involved will be in telling the fleets of replicating robotic constructors to to the job. I don’t think that they’ll worry too much about how difficult or not the job is; only the time scale.
Note that short of us starting such a project and then civilization collapsing, we will have technology far in advance of our own to do the job with. Because if we start out with technology that would take thousands of years to do the job, we’ll advance to much better technology long before the job is done. So I think the argument that “It would take millennia to do the job with near future technology” really matters much.
Here’s a start - Venus, in addition to water, needs an increase in rotational rate. Let’s move one of Jupiter’s watery moons by slinging parts off with a solar powered rail gun and using that where needed. Slam the remaining parts of that moon into Venus at the correct angle to spin it up to a 24 hour day. The prize is: another Earth. Could do this with Earth’s moon, too.
Not done with Jupiter tho - all that hydrogen is just too tempting to leave alone. Obtain a mostly metallic asteroid, smelt that sucker down into (lengthy) wires, and create a gigantic commutator to wrap around Jupiter to turn it into a rotator, making the whole thing into an electric motor - that metallic hydrogen core ought to be quite conductive, apply power and spin it up faster and faster, until it blows apart from centripetal force, and collect the pieces.
Is there an object in the asteroid belt that we could drag to Mars to act as a moon, capable of stabilizing Mars’s wobble and driving its core to produce a magnetic field?
I think most of the stuff in the asteriod belt is rocky, so not too useful for adding ecosphere (air, water).
You need to go at least to Juipiter, probably farther, to get ice. The ideal would be to send robot fleets to the kuiper belt etc. to find chunky iceteroids. They would attach their fusion jets to one side and keep nudginging it until it’s orbit slowed down and it fell inward to the sun. The jet units could process some of the ice to recover deuterium or tritium to create the heat to use more of the water as reaction mass. Even if you consume half the ice getting the item to target, it’s still free water and methane. Maybe you could blast the ice to fragments just before impact to lessen the size of an impact crater. Maybe you want a nice deep hole to fill with liquid water. Still doesn’t lessen the problem that it gets about half the sunshine of earth so you will need some serious greenhouse gases.
Interesting topic to which I’ve devoted some thought & research. Perhaps a better question might be whether we’d be wiser to learn to adapt our way of life to what the other orbital bodies had to offer already?
Venus is just plain too damn hot, even hotter than Mercury due to its extreme greenhouse effect. Also, isn’t its extremely dense & corrosive atmosphere just a little too much to tackle, at least without switching places w/ Mars & letting the 2 acclimate in time?
Mars’ core has cooled & solidified & no longer generating a protective magnetic field, which is one of the main reasons, besides its relatively weak gravity, it’s lost most of its atmosphere & water, blown away gradually over time by the solar wind. I believe we stand a much better chance of living underground on Mars than on the surface. Water is still available in very limited quantity & probably enough oxygen trapped in oxides available to support a limited population. Redirecting asteroids or lowering scoops from orbital craft into gas giants’ atmospheres might provide any deficiencies of basic necessary elements.
Europa has potential, but as mentioned previously, the intense magnetic fields & resultant radiation would require living subsurface, although water does seem to be in great abundance.
Since Jupiter already generates more energy than it receives from the Sun, and just hasn’t got quite enough mass to have lit its own nuclear fires, perhaps we might be able to give it enough of an impetus by sinking a massive thermonuclear “ignitor” into its metallic hydrogen core… the resultant shock just might be enough to trigger a “second sun” & we’d have a miniature “solar system” with a number of possibly habitable “planets” like Europa & Ganymede orbiting & able to support something closer to our form of life with enough heat generated by the new “sun”. Being so small, Jupiter-Sun would “burn” very slowly.
But, consider my original point - If we can’t learn to tend to our own “native” home, how arrogant can we be, looking for others to “consume”?