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Suppose a team of astronauts have travelled to another solar system, and are currently living in a craft which is in Medium Earth Orbit of a planet which has oceans, continents, and complex life. Assume that among them are people of every relevant field of science, and onboard there is all equipment necessary for your answer.

Essentially, what I want to know is; how much information about this planet can these people accquire while in orbit, without actually setting foot on it or even entering the atmosphere? When I say "information", I mean virtually everything of scientific value - mass, atmospheric composition, topography, temperature, life, you name it.

If this question is broad or opinion-based, I apologize. Please let me know and I'll try my best to amend it accordingly. If you need further clarification, just ask.

EDIT: More clarification, in response to Molot's comment. For the purposes of the question, imagine that the technology they'll be using to find out about the planet is equivalent to modern technology, despite their means of transport being obviously not so.

As I said; "assume . . . onboard there is all equipment necessary". However, as regards "telling what there is to find or what is needed", I'll compose a short list here of the topics the team are interested in, so answerers can let me know what's possible in these fields.

• Mass of the planet and distribution of that mass


• Internal composition of the planet (materials in mantle and core and size of layers)


• Tectonic activity


• Topography


• Composition and density of atmosphere; breathability to humans


• Weather patterns, prevailing winds/currents, wind speed etc.


• Temperature of the air, oceans etc., temperature of the planet's interior


• Presence of life, nature, chemistry and abundance of that life; possibly even observation of individual organisms (See this question: Can I monitor animal movements from space?)


• Biomes


• Ph of land, oceans etc.


• Magnetic field

That's all I can think of right now, I might add to the list later on. Essentially, I'd like to know what the scientists can and can't find out about these subjects, or whether they're possible to study from orbit at all. Hopefully this helps.

• Mass of the planet and distribution of that mass: Mass yes, they are orbiting the planet; they can calculate the mass of the planet with great precision.


• Internal composition of the planet (materials in mantle and core and size of layers): spectral analysis of the incandescent lava will tell them that the outer layers consis mostly of aluminosilicates. Spectral analysis of the atmosphere will tell them that the oceans are made of water. The presence of a strong-ish magnetic field will tell them that there must be a ferromagnetic nucleus.


• Tectonic activity: No, not really. If they spend enough time, they will notice the effects of large earthquakes, so they will know that the planet is tectonically active, but any precise measurement will have to be done on the ground.


• Topography: Yes, obviously, they can make exquisitely detailed maps, down to sub-meter scale.


• Composition and density of atmosphere; breathability to humans: Yes, clearly. Spectral analysis will give them the composition with great precision. Analysis of winds will give them the density; they know the temperature, so they can compute pressure.


• Weather patterns, prevailing winds/currents, wind speed etc. Mostly yes. Weather patterns in general (rain, wind, cold, hot) are plainly observable. Strong winds are measurable. Tropical cyclones will be seen and analyzed, giving them a good idea of the major ocean currents. Small details are observable only from the ground; for example, while they can make good guess on how much water falls on the ground, they can tell precisely only if they actually come down and measure it.


• Temperature of the air, oceans etc., temperature of the planet's interior: Temperature of the air yes, we are using satellites for that purpose. Temperature of the ocean, yes, but with much less precision. It's meaningless to speak of the temperature of the interior; but they can observe volcanic eruptions and they can measure the temperature of the lava.


• Presence of life, nature, chemistry and abundance of that life: Yes, obviously. Clorophyll is very easily noticed (it's green), and the large amount of oxygen in the atmosphere can only be sustained by biological processes. Not to mention the large and easily noticeable chalk cliffs. The presence of chlorophyll will immediatly let them understand that life is carbon-based. Large animals are visible, if they are willing to spend the money on the kind of telescopes required.


• Biomes: Forests, deserts, pairies, tundra, taiga, etc. are plainly visible. The changing colors of deciduous forests cannot be missed. Large individual trees are visible with the right optics.


• Ph of land, oceans etc. Not exactly, but with enough precision to know that they won't get chemical burns if they bathe in the ocean.


• Magnetic field: Yes, obviously. They are flying in it.

Whatever can be sensed by satellites orbiting our planet can, in principle, be sensed orbiting around another planet. Just to list a few:

• Mass: since the mass of the orbiting satellite is in principle well known, it is possible to measure not only the total mass of the planet, but also local variation of gravity, thus mapping the internal density distribution of the planet itself.


• Surface and below surface topology: visual inspection and radar scanning can measure the surface topology with a good level of detail. We have already managed to do it on Venus. We can also scan below the surface, to detect buried structures.
  
  


• Atmospheric composition: by measuring absorption spectra, it is possible to determine the gases present in the atmosphere, and by observing the dynamics of the atmosphere itself it is also possible to investigate its physics.


• Magnetic field


• Presence of life: some years ago a NASA probe, using Earth for a gravitational kick, measured the planet searching for signature of life. It found them! It is possible to detect the fingerprint of, among others, oxygen and chlorophyll. Not to mention night lights, of course.

If the spy movies are to be believed, they would also be able to read license plates from orbit. Applying such visual observation technology to more scientific purposes, they could catalog the major life forms, study migration patterns, map the land masses and ocean currents.

Over time they could determine the seasons and track the snowline, determine storm patterns and build a model of planetary weather patterns.

More directly, they should sample the vacuum around their station for remnants of bacteria thrown up out of the atmosphere. If they are lucky and find some, a great deal could be learned concerning the planet's fundamental biology.

Assuming that by "without actually setting foot on it or even entering the atmosphere" your scientists cannot deploy landers to the surface of this planet, and are restricted to using orbital satellites only, there is quite a lot that they would be able to learn about this planet. Remote sensing is a booming field of research right Now, so who even knows what will be possible in the future?

The first thing you need to know about remote sensing techniques is the trade-off between spectral resolution and spatial resolution. Hyperspectral imaging can measure on wavelength increments on the scale of nanometers. However, the trade-off is that your pixel size becomes huge in comparison what is possible with monochromatic imaging. As an example, the HiRISE imagery of Mars is monochromatic with a ground resolution of 25×25 cm. Landsat and Sentinel imagery of Earth has a resolution of 10×10 m or coarser, depending on the sensor band you're looking at. The specifications of orbiters with hyperspectral imaging suites escape me at the moment, but I have worked with hyperspectral image cubes of Mars and Earth both in courses for my Bsc and Msc degrees, so I know they exist. I am on my phone right now, so looking up references will have to wait for later.

The second trade-off is another between wavelength and ground resolution: longer wavelengths have coarser maximum ground resolutions.

There is a third kind of resolution that is important when using remote sensing platforms: temporal resolution. In other words: the amount of time that passes before the same patch of surface area is sensed a second (or third, or fourth... you get the idea) time.

Now, all that background information is nice and all, but what does this mean for your scientists? What can they study, using only sensors in the electromagnetic spectrum that are in orbit around the planet's? Like I said, quite a lot. The actual limits of what is possible will be determined by the spatial, spectral, and temporal resolutions of your scientists' observation platforms, but here's a few things we have studied here on Earth, using satellite imagery:

• Ocean temperature


• Land surface temperature


• Weather patterns


• Ocean currents


• Mineralogic composition of surface bedrock


• We've constructed detailed elevation maps


• The evolution of glaciers and ice caps


• The presence and health of vegetation


• The presence of near-surface groundwater

Combining any of these leads to more advanced insights and better understanding of what to study next. And that is not even touching on what you might learn by simply looking for (large) animals on your images.

Maybe you could rephrase the question "what can't they find out from orbit"
The moon missions brought back surface material which has given unexpected results not available from orbit so that's not going to happen if they can not get samples.
Thinking along those lines would most likely give better answer's to your question.