In case you have support for this system, You can download Solaris 8 iso image as patch 10356262.
You can use more fresh Solaris version for boot for recovery root password, but:
v240 can not run Solaris 11.
Solaris 10 not available for download now...
Genaral procedure for recover root password:
- Boot system from install media at single mode.
- Mount disk ( It may require additional steps in case root disk mirrored)
- Edit <mountpoint of disk>/etc/shadow and put hash of new password for root.
- unmount disk
- Boot from original disk and check.
You should create backup of you system before any modification.
Some non-standart way:
Solaris 8 - is very old system. Check it on well-know vulnerability.
In case you have readable copy of /etc/shadow file - You can try run john for brute force root password.
In case you not have support, not have backup, not know root password - are you sure that this system realy required ?