I would like to know if besides these planets, astronomers have discovered a gas planet like Jupiter in this system and if on these planets could exist life without a comet protector like Jupiter does for the Earth.
No such planet has been announced as having been discovered. The paper only shows evidence for the 7 (really 6 because the 7th can't be officially confirmed with only 1 observation) terrestrial planets and does not make the case for any other planets. The paper doesn't indicate that more planets could exist, but does remark that there are large error bars on some of their data, leaving room for uncertainty.
Ultimately, I think we can cross off a gas giant existing in this system for a few reasons.
- We don't see transit timing variations due to a gas giant. A gas giant would have noticeable gravitational influences on the inner 7 planets and we would see this influence by the small variations in the orbital periods of the inner planets. The authors of the paper did see transit timing variations, but they were able to explain all of the variations as due to the gravitational influences of the 7 planets only. They never had to invoke an 8th unseen planet to explain what they say.
- We don't observe such a (easily-detectable) transit. There are only two reasons we wouldn't see the transit of this planet. Either the gas giant is orbiting in a different plane than the rest of the planets (which are all remarkably close to the same plane, as pointed out by the authors), or else the orbital period is so long, it was never captured by any of the previous observations (which spans a number of years). Neither situation seems likely to occur.
- The mass of the central star is only 8% of the Sun's mass. Smaller stars tend to form smaller planets. Its really hard for gas giants to form around small stars, primarily due to lack of material. From what we understand about planet formation, the chances of a gas giant even being able to form around this star is pretty small.
Of course, only continued observation will really be able to convince us that a gas giant doesn't exist.
On these planets could exist life without a comet protector like Jupiter does for the Earth?
This is a great question. I think the answer is, we can't be sure. Jupiter does do a great job shepherding comets and protecting the Earth. Possibly this system is rife with comets that constantly bombard the planets. However, that's just one small part of the puzzle. Our Moon does a phenomenal job in protecting us as well.
I think, when it comes to these planets, your primary concern for whether life can exist or not is the central star. It is a low-mass, ultra-cool dwarf star. These stars tend to be very volatile, much more so than our generally quiescent Sun. This means these planets are likely to receive much more radiation and be hit by many more solar storms than we are. Furthermore, these planets are so close to TRAPPIST-1 that they're all tidally locked - one face is always towards the star and one is always away. This could make one side inhospitably hot and the other inhospitably cold. The climate/weather on such a planet would likely be unsuitable for life (but who knows for sure). The tidal locking could potentially be good though as it means only the outward-facing side would generally be hit by comets, thus protecting any life on the Sun facing side (assuming its not too hot on that side).
As the paper on the first three planets discovered around the star states, even though firm constraints have not yet been put on the masses of the planets,
The results of planetary thermal evolution models — and the intense extreme ultraviolet (1−1,000 Å) emission of low-mass stars18 during their early lives — make it unlikely that such small planets would have thick envelopes of hydrogen and/or helium gases.
There is no evidence for planets beyond TRAPPIST-1h.
The evolutionary history of the system is unclear. It is thought that stars like TRAPPIST-1 - termed "ultracool dwarfs" - could have rocky planets around them, but they would have to have formed beyond the frost line, in the region where volatiles exist. They would then have migrated inwards, falling into orbital resonances. Any putative gas giant would need to have an orbital history consistent with such an evolution.
Astronomers have not observed any other objects in the system - exomoons or exocomets included - so we do not have a good idea of what small bodies may exist in the system, and thus how they might impact life on the planets.
The team used the transit-timing variation (TTV) method to detect the planets. Essentially, it looks for perturbations in transits of planets to figure out if there are other planets in the system. Models can then be created that attempt to reproduce the results. They found that a 6-planet model with the data for 6 planets; the seventh planet - with poorly constrained data - can still be consistently included.
However, there are instability issues. Over one million years, they determined that the system has a 25% chance of an instability; over one billion years, there is only an 8.1% chance it will survive with little or no changes. In other words, the systems is not particularly stable over long periods of time, and it remains it be seen how a gas giant could play into that.
If there is a gas giant, it could interact with the planets and might further throw the system into havoc, meaning that it would have been hard to survive even 500 million years, the age of the system. Add to that the fact that the planets likely formed beyond the frost line and so would have been near where the gas giant formed, and you have a recipe for disaster.
However, the authors note that there are poor constraints on many orbital parameters and masses, and it is possible that one or more extra planets could stabilize the system. However, they haven't seen anything else, which is troubling - and a gas giant would have a good chance of showing up via the TTV method.