Outward

The order, and why it is this one

Join anywhere. Each stop carries the route with it, so the foot of every question here offers the next one and the way back to this page.

1. 01Where does the solar system end?There are three defensible answers, and the outermost lies hundreds to thousands of times further out than the innermost. One is where the planets stop, one is where the Sun’s wind stops, and one is where the Sun’s gravity stops holding on. Spacecraft have passed the first and two have crossed the second. The third is tens of thousands of years away at Voyager 1’s speed.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
2. 02Is there another planet in the solar system?A key part of the case for an undiscovered planet several times the mass of Earth is the alignment of a small number of distant objects. Whether that alignment is real depends on how the surveys that found them were pointed, and that question has been argued both ways in print.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Uncertain. Meaningful scientific, historical or philosophical disagreement remains.Read
3. 03How many planets are there?Thousands of planets have been confirmed around other stars, and the detected ones are a biased sample of a much larger population. Estimating the true number means correcting for what the methods cannot see, which is where the interesting arguments are.Shallowsdepth, meaning: Interesting questions with substantial evidence and real uncertainty remaining.Known. Strongly supported by evidence and broadly established.Read
4. 04Is anyone else out there?In thirty years the question moved from “do other planets exist?” to “which of the thousands we have found should we look at?” No detection of life beyond Earth has been confirmed.Deepdepth, meaning: Major unanswered questions.Unknown. Humanity does not currently have a satisfactory answer.Read
5. 05Where does the observable universe end?The universe has a finite age, so only light from within a finite distance has had time to arrive. That distance is not fourteen billion light years, because space expanded while the light was in transit, and the horizon is a limit on observation rather than on existence.Deepdepth, meaning: Major unanswered questions.Known. Strongly supported by evidence and broadly established.Read
6. 06Is the universe infinite?Space appears flat to the precision anyone can measure, which is consistent with infinite extent and also with a very large finite volume. The distinction is not one that light can carry: no signal from beyond the horizon has arrived yet, and because the expansion is accelerating, from far enough away none ever will.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Unknown. Humanity does not currently have a satisfactory answer.Read