The group is preparing a mysterious '2017 Seventeen Project', and Chapter 1 revolves around the theme 'Alone'. In his trailer video, Hoshi is dialing up someone on an old-school landline phone. You see the words "The Line for reaching to the beloved ones" and "Even though you are being alone, remember we are on your side and don't be afraid of all the fears you have."

'Prime Number' is also an ongoing motif in Seventeen's teaser series as fans have to enter the prime number seen in each member's trailers on the group's official website in order to be able to access their teaser images. Hoshi's prime number is the largest so far as it is 2232111.

Check out the teasers and stay tuned for more trailers from Seventeen!

SEE ALSO: Seventeen donates 1 Billion KRW (3,000) to help wildfire victims